2024
Effect of Duration and Delay on the Identifiability of VR Motion
Mark Miller, Vivek Nair, Eugy Han, Cyan DeVeaux, Christian Rack, Rui Wang, Brandon Huang, Marc Latoschik, James F. O'Brien, Jeremy N. Bailenson
SePAR 2024
Social virtual reality is an emerging medium of communication. In this medium, a user's avatar (virtual representation) is controlled by the tracked motion of the user's headset ... [more] Social virtual reality is an emerging medium of communication. In this medium, a user's avatar (virtual representation) is controlled by the tracked motion of the user's headset and hand controllers. This tracked motion is a rich data stream that can leak characteristics of the user or can be effectively matched to previously-identified data to identify a user. To better understand the boundaries of motion data identifiability, we investigate how varying training data duration and train-test delay affects the accuracy at which a machine learning model can correctly classify user motion in a supervised learning task simulating re-identification. The dataset we use has a unique combination of a large number of participants, long duration per session, large number of sessions, and a long time span over which sessions were conducted. We find that training data duration and train-test delay affect identifiability; that minimal train-test delay leads to very high accuracy; and that train-test delay should be controlled in future experiments. [less]
Effect of Data Degradation on Motion Re-Identification
Vivek Nair, Mark Roman Miller, Rui Wang, Brandon Huang, Christian Rack, Marc Latoschik, James F. O'Brien
SePAR 2024
The use of virtual and augmented reality devices is increasing, but these sensor-rich devices pose risks to privacy. The ability to track a user’s motion and infer the identity ... [more] The use of virtual and augmented reality devices is increasing, but these sensor-rich devices pose risks to privacy. The ability to track a user’s motion and infer the identity or characteristics of the user poses a privacy risk that has received significant attention. Existing deep-network-based defenses against this risk, however, require significant amounts of training data and have not yet been shown to generalize beyond specific applications. In this work, we study the effect of signal degradation on identifiability, specifically through added noise, reduced framerate, reduced precision, and reduced dimensionality of the data. Our experiment shows that state-of-the-art identification attacks still achieve near-perfect accuracy for each of these degradations. This negative result demonstrates the difficulty of anonymizing this motion data and gives some justification to the existing data- and compute-intensive deep-network based methods. [less]
Truth in Motion: The Unprecedented Risks and Opportunities of Extended Reality Motion Data
Vivek Nair, Louis Rosenberg, James F. O'Brien, Dawn Song
IEEE S&P
Motion tracking "telemetry" data lies at the core of nearly all modern extended reality (XR) and metaverse experiences. While generally presumed innocuous, recent studies ... [more] Motion tracking "telemetry" data lies at the core of nearly all modern extended reality (XR) and metaverse experiences. While generally presumed innocuous, recent studies have demonstrated that motion data actually has the potential to profile and deanonymize XR users, posing a significant threat to security and privacy in the metaverse. [less]
Deep Motion Masking for Secure, Usable, and Scalable Real-Time Anonymization of Ecological Virtual Reality Motion Data
Vivek Nair, Wenbo Guo, James F. O'Brien, Louis Rosenberg, Dawn Song
IEEE VR3D
Virtual reality (VR) and "metaverse" systems have recently seen a resurgence in interest and investment as major technology companies continue to enter the space. However ... [more] Virtual reality (VR) and "metaverse" systems have recently seen a resurgence in interest and investment as major technology companies continue to enter the space. However, recent studies have demonstrated that the motion tracking "telemetry" data used by nearly all VR applications is as uniquely identifiable as a fingerprint scan, raising significant privacy concerns surrounding metaverse technologies. In this paper, we propose a new "deep motion masking" approach that scalably facilitates the real-time anonymization of VR telemetry data. Through a large-scale user study (N=182), we demonstrate that our method is significantly more usable and private than existing VR anonymity systems [less]
Inferring Private Personal Attributes of Virtual Reality Users from Ecologically Valid Head and Hand Motion Data
Vivek Nair, Christian Rack, Wenbo Guo, Rui Wang, Shuixian Li, Brandon Huang, Atticus Cull, James F. O'Brien, Marc Latoschik, Louis Rosenberg, Dawn Song
IEEE VR3D
Motion tracking "telemetry" data lies at the core of nearly all modern virtual reality (VR) and metaverse experiences. While generally presumed innocuous, recent studies have ... [more] Motion tracking "telemetry" data lies at the core of nearly all modern virtual reality (VR) and metaverse experiences. While generally presumed innocuous, recent studies have demonstrated that motion data actually has the potential to uniquely identify VR users. In this study, we go a step further, showing that a variety of private user information can be inferred just by analyzing motion data recorded from VR devices. We conducted a large-scale survey of VR users (N=1,006) with dozens of questions ranging from background and demographics to behavioral patterns and health information. We then obtained VR motion samples of each user playing the game "Beat Saber," and attempted to infer their survey responses using just their head and hand motion patterns. Using simple machine learning models, over 40 personal attributes could be accurately and consistently inferred from VR motion data alone. Despite this significant observed leakage, there remains limited awareness of the privacy implications of VR motion data, highlighting the pressing need for privacy-preserving mechanisms in multi-user VR applications. [less]
Berkeley Open Extended Reality Recordings 2023 (BOXRR-23): 4.7 Million Motion Capture Recordings from 105,000 XR Users
Vivek Nair, Wenbo Guo, Rui Wang, James F. O'Brien, Louis Rosenberg, Dawn Song
IEEE VR 2024
Extended reality (XR) devices such as the Meta Quest and Apple Vision Pro have seen a recent surge in attention, with motion tracking "telemetry" data lying at the core of nearly ... [more] Extended reality (XR) devices such as the Meta Quest and Apple Vision Pro have seen a recent surge in attention, with motion tracking "telemetry" data lying at the core of nearly all XR and metaverse experiences. Researchers are just beginning to understand the implications of this data for security, privacy, usability, and more, but currently lack large-scale human motion datasets to study. The BOXRR-23 dataset contains 4,717,215 motion capture recordings, voluntarily submitted by 105,852 XR device users from over 50 countries. BOXRR-23 is over 200 times larger than the largest existing motion capture research dataset and uses a new, highly efficient and purpose-built XR Open Recording (XROR) file format. [less]
2023
Unique Identification of 50,000+ Virtual Reality Users from Head and Hand Motion Data
Vivek Nair, Wenbo Guo, Justus Mattern, Rui Wang, James F. O'Brien, Louis Rosenberg, Dawn Song
USENIX Security 23
With the recent explosive growth of interest and investment in virtual reality (VR) and the "metaverse," public attention has rightly shifted toward the unique security and ... [more] With the recent explosive growth of interest and investment in virtual reality (VR) and the "metaverse," public attention has rightly shifted toward the unique security and privacy threats that these platforms may pose. While it has long been known that people reveal information about themselves via their motion, the extent to which this makes an individual globally identifiable within virtual reality has not yet been widely understood. In this study, we show that a large number of real VR users (N=55,541) can be uniquely and reliably identified across multiple sessions using just their head and hand motion relative to virtual objects. After training a classification model on 5 minutes of data per person, a user can be uniquely identified amongst the entire pool of 55,541 with 94.33% accuracy from 100 seconds of motion, and with 73.20% accuracy from just 10 seconds of motion. This work is the first to truly demonstrate the extent to which biomechanics may serve as a unique identifier in VR, on par with widely used strong biometrics like facial or fingerprint recognition. [less]
Exploring the Privacy Risks of Adversarial VR Game Design
Vivek Nair, Gonzalo Munilla Garrido, Dawn Song, James F. O'Brien
PoPETS 2023
Thirty study participants playtested an innocent-looking "escape room" game in virtual reality (VR). Behind the scenes, an adversarial program had accurately inferred over ... [more] Thirty study participants playtested an innocent-looking "escape room" game in virtual reality (VR). Behind the scenes, an adversarial program had accurately inferred over 25 personal data attributes, from anthropometrics like height and wingspan to demographics like age and gender, within just a few minutes of gameplay. As notoriously data-hungry companies become increasingly involved in VR development, this experimental scenario may soon represent a typical VR user experience. While virtual telepresence applications (and the so-called "metaverse") have recently received increased attention and investment from major tech firms, these environments remain relatively under-studied from a security and privacy standpoint. In this work, we illustrate how VR attackers can covertly ascertain dozens of personal data attributes from seemingly-anonymous users of popular metaverse applications like VRChat. These attackers can be as simple as other VR users without special privilege, and the potential scale and scope of this data collection far exceed what is feasible within traditional mobile and web applications. We aim to shed light on the unique privacy risks of the metaverse, and provide the first holistic framework for understanding intrusive data harvesting attacks in these emerging VR ecosystems. [less]
KBody: Balanced monocular whole-body estimation
Nikolaos Zioulis, James F. O'Brien
CVFAD 2023
KBody is a method for fitting a low-dimensional body model to an image. It follows a predict-and-optimize approach, relying on data-driven model estimates for the constraints ... [more] KBody is a method for fitting a low-dimensional body model to an image. It follows a predict-and-optimize approach, relying on data-driven model estimates for the constraints that will be used to solve for the body's parameters. Compared to other approaches, it introduces virtual joints to identify higher quality correspondences and disentangles the optimization between the pose and shape parameters to achieve a more balanced result in terms of pose and shape capturing capacity, as well as pixel alignment. [less]
KBody: Towards general, robust, and aligned monocular whole-body estimation
Nikolaos Zioulis, James F. O'Brien
RHOBIN 2023
KBody is a method for fitting a low-dimensional body model to an image. It follows a predict-and-optimize approach, relying on data-driven model estimates for the constraints ... [more] KBody is a method for fitting a low-dimensional body model to an image. It follows a predict-and-optimize approach, relying on data-driven model estimates for the constraints that will be used to solve for the body's parameters. Acknowledging the importance of high quality correspondences, it leverages ``virtual joints" to improve fitting performance, disentangles the optimization between the pose and shape parameters, and integrates asymmetric distance fields to strike a balance in terms of pose and shape capturing capacity, as well as pixel alignment. We also show that generative model inversion offers a strong appearance prior that can be used to complete partial human images and used as a building block for generalized and robust monocular body fitting. Project page: https://klothed.github.io/KBody. [less]
Results of the 2023 Census of Beat Saber Users: Virtual Reality Gaming Population Insights and Factors Affecting Virtual Reality E-Sports Performance
Vivek Nair, Viktor Radulov, James F. O'Brien
Survey
The emergence of affordable standalone virtual reality (VR) devices has allowed VR technology to reach mass-market adoption in recent years, driven primarily by the popularity ... [more] The emergence of affordable standalone virtual reality (VR) devices has allowed VR technology to reach mass-market adoption in recent years, driven primarily by the popularity of VR gaming applications such as Beat Saber. However, despite being the top-grossing VR application to date and the most popular VR e-sport, the population of over 6 million Beat Saber users has not yet been widely stud- ied. In this report, we present a large-scale comprehensive survey of Beat Saber players (N=1,006) that sheds light on several important aspects of this population, including their background, biometrics, demographics, health information, behavioral patterns, and technical device specifications. We further provide insights into the emerging field of VR e-sports by analyzing correlations between responses and an authoritative measure of in-game performance. [less]
2022
Monocular Facial Performance Capture Via Deep Expression Matching
Stephen Bailey, Jérémy Riviere, Morten Mikkelsen, James F. O'Brien
SCA 2022
Facial performance capture is the process of automatically animating a digital face according to a captured performance of an actor. Recent developments in this area have ... [more] Facial performance capture is the process of automatically animating a digital face according to a captured performance of an actor. Recent developments in this area have focused on high-quality results using expensive head-scanning equipment and camera rigs. These methods produce impressive animations that accurately capture subtle details in an actor’s performance. However, these methods are accessible only to content creators with relatively large budgets. Current methods using inexpensive recording equipment generally produce lower quality output that is unsuitable for many applications. In this paper, we present a facial performance capture method that does not require facial scans and instead animates an artist-created model using standard blend-shapes. Furthermore, our method gives artists high-level control over animations through a workflow similar to existing commercial solutions. Given a recording, our approach matches keyframes of the video with corresponding expressions from an animated library of poses. A Gaussian process model then computes the full animation by interpolating from the set of matched keyframes. Our expression-matching method computes a low-dimensional latent code from an image that represents a facial expression while factoring out the facial identity. Images depicting similar facial expressions are identified by their proximity in the latent space. In our results, we demonstrate the fidelity of our expression-matching method. We also compare animations generated with our approach to animations generated with commercially available software. [less]
2021
This photograph has been altered: Testing the effectiveness of image forensic labeling on news image credibility
Cuihua Shen, Mona Kasra, James F. O'Brien
Misinformation Review
Despite the ubiquity of images and videos in online news environments, much of the existing research on misinformation and its correction is solely focused on textual misinformation ... [more] Despite the ubiquity of images and videos in online news environments, much of the existing research on misinformation and its correction is solely focused on textual misinformation, and little is known about how ordinary users evaluate fake or manipulated images and the most effective ways to label and correct such falsities. We designed a visual forensic label of image authenticity, Picture-O-Meter, and tested the label’s efficacy in relation to its source and placement in an experiment with 2440 participants. Our findings demonstrate that, despite human beings’ general inability to detect manipulated images on their own, image forensic labels are an effective tool for counteracting visual misinformation. [less]
2020
Fast and Deep Facial Deformations
Stephen Bailey, Dalton Omens, Paul Dilorenzo, James F. O'Brien
SIGGRAPH 2020
Film-quality characters typically display highly complex and expressive facial deformation. The underlying rigs used to animate the deformations of a character’s face ... [more] Film-quality characters typically display highly complex and expressive facial deformation. The underlying rigs used to animate the deformations of a character’s face are often computationally expensive, requiring high-end hardware to deform the mesh at interactive rates. In this paper, we present a method using convolutional neural networks for approximating the mesh deformations of characters’ faces. For the models we tested, our approximation runs up to 17 times faster than the original facial rig while still maintaining a high level of fidelity to the original rig. We also propose an extension to the approximation for handling high-frequency deformations such as fine skin wrinkles. While the implementation of the original animation rig depends on an extensive set of proprietary libraries making it difficult to install outside of an in-house development environment, our fast approximation relies on the widely available and easily deployed TensorFlow libraries. In addition to allowing high frame rate evaluation on modest hardware and in a wide range of computing environments, the large speed increase also enables interactive inverse kinematics on the animation rig. We demonstrate our approach and its applicability through interactive character posing and real-time facial performance capture. [less]
2019
Fake images: The effects of source, intermediary, and digital media literacy on contextual assessment of imag credibility online
Cuihua Shen, Mona Kasra, Wenjing Pan, Grace A. Bassett, Yining Malloch, James F. O'Brien
New Media and Society
Fake or manipulated images propagated through the Web and social media have the capacity to deceive, emotionally distress, and influence public opinions and actions. Yet ... [more] Fake or manipulated images propagated through the Web and social media have the capacity to deceive, emotionally distress, and influence public opinions and actions. Yet few studies have examined how individuals evaluate the authenticity of images that accompany online stories. This article details a 6-batch large-scale online experiment using Amazon Mechanical Turk that probes how people evaluate image credibility across online platforms. In each batch, participants were randomly assigned to 1 of 28 news-source mockups featuring a forged image, and they evaluated the credibility of the images based on several features. We found that participants’ Internet skills, photo-editing experience, and social media use were significant predictors of image credibility evaluation, while most social and heuristic cues of online credibility (e.g. source trustworthiness, bandwagon, intermediary trustworthiness) had no significant impact. Viewers’ attitude toward a depicted issue also positively influenced their credibility evaluation. [less]
2018
Fast and Deep Deformation Approximations
Stephen Bailey, Dave Otte, Paul Dilorenzo, James F. O'Brien
SIGGRAPH 2018
Character rigs are procedural systems that compute the shape of an animated character for a given pose. They can be highly complex and must account for bulges, wrinkles, and ... [more] Character rigs are procedural systems that compute the shape of an animated character for a given pose. They can be highly complex and must account for bulges, wrinkles, and other aspects of a character's appearance. When comparing film-quality character rigs with those designed for real-time applications, there is typically a substantial and readily apparent difference in the quality of the mesh deformations. Real-time rigs are limited by a computational budget and often trade realism for performance. Rigs for film do not have this same limitation, and character riggers can make the rig as complicated as necessary to achieve realistic deformations. However, increasing the rig complexity slows rig evaluation, and the animators working with it can become less efficient and may experience frustration. In this paper, we present a method to reduce the time required to compute mesh deformations for film-quality rigs, allowing better interactivity during animation authoring and use in real-time games and applications. Our approach learns the deformations from an existing rig by splitting the mesh deformation into linear and nonlinear portions. The linear deformations are computed directly from the transformations of the rig's underlying skeleton. We use deep learning methods to approximate the remaining nonlinear portion. In the examples we show from production rigs used to animate lead characters, our approach reduces the computational time spent on evaluating deformations by a factor of 5x-10x. This significant savings allows us to run the complex, film-quality rigs in real-time even when using a CPU-only implementation on a mobile device. [less]
Approximate svBRDF Estimation From Mobile Phone Video
Rachel A. Albert, Dorian Yao Chan, Dan B Goldman, James F. O'Brien
EGSR 2018
We describe a new technique for obtaining a spatially varying BRDF (svBRDF) of a flat object using printed fiducial markers and a cell phone capable of continuous flash video ... [more] We describe a new technique for obtaining a spatially varying BRDF (svBRDF) of a flat object using printed fiducial markers and a cell phone capable of continuous flash video. Our homography-based video frame alignment method does not require the fiducial markers to be visible in every frame, thereby enabling us to capture larger areas at a closer distance and higher resolution than in previous work. Pixels in the resulting panorama are fit with a BRDF based on a recursive subdivision algorithm, utilizing all the light and view positions obtained from the video. We show the versatility of our method by capturing a variety of materials with both one and two camera input streams and rendering our results on 3D objects under complex illumination. [less]
Seeing Is Believing: How People Fail to Identify Fake Images on the Web
Mona Kasra, Cuihua Shen, James F. O'Brien
CHI 2018
The growing ease with which digital images can be convincingly manipulated and widely distributed on the Internet makes viewers increasingly susceptible to visual misinformation ... [more] The growing ease with which digital images can be convincingly manipulated and widely distributed on the Internet makes viewers increasingly susceptible to visual misinformation and deception. In situations where ill-intentioned individuals seek to deliberately mislead and influence viewers through fake online images, the harmful consequences could be substantial. We describe an exploratory study of how individuals react, respond to, and evaluate the authenticity of images that accompany online stories in Internet-enabled communications channels. Our preliminary findings support the assertion that people perform poorly at detecting skillful image manipulation, and that they often fail to question the authenticity of images even when primed regarding image forgery through discussion. We found that viewers make credibility evaluation based mainly on non-image cues rather than the content depicted. Moreover, our study revealed that in cases where context leads to suspicion, viewers apply post-hoc analysis to support their suspicions regarding the authenticity of the image. [less]
2017
Simulation of Subseismic Joint and Fault Networks Using a Heuristic Mechanical Model
Paul Gillespie, Giulio Casini, Hayley Iben, James F. O'Brien
SSRD 2017
Flow simulations of fractured and faulted reservoirs require representation of subseismic structures about which subsurface data are limited. We describe a method for ... [more] Flow simulations of fractured and faulted reservoirs require representation of subseismic structures about which subsurface data are limited. We describe a method for simulating fracture growth that is mechanically based but heuristic, allowing for realistic modelling of fracture networks with reasonable run times. The method takes a triangulated meshed surface as input, together with an initial stress field. Fractures initiate and grow based on the stress field, and the growing fractures relieve the stress in the mesh. We show that a wide range of bedding-plane joint networks can be modelled simply by varying the distribution and anisotropy of the initial stress field. The results are in good qualitative agreement with natural joint patterns. We then apply the method to a set of parallel veins and demonstrate how the variations in thickness of the veins can be represented. Lastly, we apply the method to the simulation of normal fault patterns on salt domes. We derive the stress field on the bedding surface using the horizon curvature. The modelled fault network shows both radial and concentric faults. The new method provides an effective means of modelling joint and fault networks that can be imported to the flow simulator. [less]
2016
Seeing Is Believing: Do People Fail to Identify Fake Images on the Web?
Mona Kasra, Cuihua Shen, James F. O'Brien
AoIR 2016
Images have historically been perceived as photographic proof of the depicted events. However, the growing ease with which digital images can be convincingly manipulated ... [more] Images have historically been perceived as photographic proof of the depicted events. However, the growing ease with which digital images can be convincingly manipulated and then widely distributed on the Internet makes viewers increasingly susceptible to visual misinformation and deception. In situations where ill-intentioned individuals seek to deliberately mislead and influence viewers through forged online images, the harmful consequences could be substantial on both personal and social levels. This sort paper, describes preliminary work on an exploratory study of how individuals react, respond to, and evaluate the authenticity of images that accompany online stories in Internet-enabled communications channels (social networking site, blogs, email). Our preliminary findings support the assertion that people perform poorly at detecting skillful image manipulation, and that they often fail to question the authenticity of images even when primed regarding image forgery through discussion. We found that viewers make credibility evaluation based mainly on non-image cues rather that the content depicted. Moreover, our study revealed that in cases where context leads to suspicion, viewers apply post hoc analysis to support their suspicions regarding the authenticity of the image. [less]
Repurposing Hand Animation for Interactive Applications
Stephen Bailey, Martin Watt, James F. O'Brien
SCA 2016
In this paper we describe a method for automatically animating interactive characters based on an existing corpus of key-framed hand-animation. The method learns separate ... [more] In this paper we describe a method for automatically animating interactive characters based on an existing corpus of key-framed hand-animation. The method learns separate low-dimensional embeddings for subsets of the hand-animation corresponding to different semantic labels. These embeddings use the Gaussian Process Latent Variable Model to map high-dimensional rig control parameters to a three-dimensional latent space. By using a particle model to move within one of these latent spaces, the method can generate novel animations corresponding to the space's semantic label. Bridges link each pose in one latent space that is similar to a pose in another space. Animations corresponding to a transitions between semantic labels are generated by creating animation paths that move though one latent space and traverse a bridge into another. We demonstrate this method by using it to interactively animate a character as it plays a simple game with the user. The character is from a previously produced animated film and the data we use for training is the data that was used to animate the character in the film. The animated motion from the film represents an enormous investment of skillful work. Our method allows this work to be repurposed and reused for interactively animating the familiar character from the film. [less]
2015
Interactive Detailed Cutting of Thin Sheets
Pierre-Luc Manteaux, Wei-Lun Sun, Francois Faure, Marie-Paule Cani, James F. O'Brien
MIG 2015
In this paper we propose a method for the interactive detailed cutting of deformable thin sheets. Our method builds on the ability of frame-based simulation to solve for ... [more] In this paper we propose a method for the interactive detailed cutting of deformable thin sheets. Our method builds on the ability of frame-based simulation to solve for dynamics using very few control frames while embedding highly detailed geometry - here an adaptive mesh that accurately represents the cut boundaries. Our solution relies on a non-manifold grid to compute shape functions that faithfully adapt to the topological changes occurring while cutting. New frames are dynamically inserted to describe new regions. We provide incremental mechanisms for updating simulation data, enabling us to achieve interactive rates. We illustrate our method with examples inspired by the traditional Kirigami artform. [less]
View-Dependent Adaptive Cloth Simulation with Buckling Compensation
Woojong Koh, Rahul Narain, James F. O'Brien
TVCG 2015
This paper describes a method for view-dependent cloth simulation using dynamically adaptive mesh refinement and coarsening. Given a prescribed camera motion, the method ... [more] This paper describes a method for view-dependent cloth simulation using dynamically adaptive mesh refinement and coarsening. Given a prescribed camera motion, the method adjusts the criteria controlling refinement to account for visibility and apparent size in the camera's view. Objectionable dynamic artifacts are avoided by anticipative refinement and smoothed coarsening, while locking in extremely coarsened regions is inhibited by modifying the material model to compensate for unresolved sub-element buckling. This approach preserves the appearance of detailed cloth throughout the animation while avoiding the wasted effort of simulating details that would not be discernible to the viewer. The computational savings realized by this method increase as scene complexity grows. The approach produces a 2x speed-up for a single character and more than 4x for a small group as compared to view-independent adaptive simulations, and respectively 5x and 9x speed-ups as compared to non-adaptive simulations. [less]
Optimal Presentation of Imagery with Focus Cues on Multi-Plane Displays
Rahul Narain, Rachel A. Albert, Abdullah Bulbul, Gregory J. Ward, Marty Banks, James F. O'Brien
SIGGRAPH 2015
We present a technique for displaying three-dimensional imagery of general scenes with nearly correct focus cues on multi-plane displays. These displays present an additive ... [more] We present a technique for displaying three-dimensional imagery of general scenes with nearly correct focus cues on multi-plane displays. These displays present an additive combination of images at a discrete set of optical distances, allowing the viewer to focus at different distances in the simulated scene. Our proposed technique extends the capabilities of multi-plane displays to general scenes with occlusions and non-Lambertian effects by using a model of defocus in the eye of the viewer. Requiring no explicit knowledge of the scene geometry, our technique uses an optimization algorithm to compute the images to be displayed on the presentation planes so that the retinal images when accommodating to different distances match the corresponding retinal images of the input scene as closely as possible. We demonstrate the utility of the technique using imagery acquired from both synthetic and real-world scenes, and analyze the system's characteristics including bounds on achievable resolution. [less]
Resampling Adaptive Cloth Simulations onto Fixed-Topology Meshes
George Brown, Armin Samii, James F. O'Brien, Rahul Narain
SCA 2015 Poster
We describe a method for converting an adaptively remeshed simulation of cloth into an animated mesh with fixed topology. The topology of the mesh may be specified by the ... [more] We describe a method for converting an adaptively remeshed simulation of cloth into an animated mesh with fixed topology. The topology of the mesh may be specified by the user or computed automatically. In the latter case, we present a method for computing the optimal output mesh, that is, a mesh with spatially varying resolution which is fine enough to resolve all the detail present in the animation. This technique allows adaptive simulations to be easily used in applications that expect fixed-topology animated meshes. [less]
2014
Exposing Photo Manipulation from Shading and Shadows
Eric Kee, James F. O'Brien, Hany Farid
TOG 2014
We describe a method for detecting physical inconsistencies in lighting from the shading and shadows in an image. This method imposes a multitude of shading- and shadow-based ... [more] We describe a method for detecting physical inconsistencies in lighting from the shading and shadows in an image. This method imposes a multitude of shading- and shadow-based constraints on the projected location of a distant point light source. The consistency of a collection of such constraints is posed as a linear programming problem. A feasible solution indicates that the combination of shading and shadows is physically consistent, while a failure to find a solution provides evidence of photo tampering. [less]
Eyeglasses-free Display: Towards Correcting Visual Aberrations with Computational Light Field Displays
Fu-Chung Huang, Gordon Wetzstein, Brian A. Barsky, Ramesh Raskar
ACM SIGGRAPH 2014
Millions of people worldwide need glasses or contact lenses to see or read properly. We introduce a computational display technology that predistorts the presented content ... [more] Millions of people worldwide need glasses or contact lenses to see or read properly. We introduce a computational display technology that predistorts the presented content for an observer, so that the target image is perceived without the need for eyewear. By designing optics in concert with prefiltering algorithms, the proposed display architecture achieves significantly higher resolution and contrast than prior approaches to vision-correcting image display. We demonstrate that inexpensive light field displays driven by efficient implementations of 4D prefiltering algorithms can produce the desired vision-corrected imagery, even for higher-order aberrations that are difficult to be corrected with glasses. The proposed computational display architecture is evaluated in simulation and with a low-cost prototype device. [less]
Adaptive Tearing and Cracking of Thin Sheets
Tobias Pfaff, Rahul Narain, Juan Miguel de Joya, James F. O'Brien
SIGGRAPH 2014
This paper presents a method for adaptive fracture propagation in thin sheets. A high-quality triangle mesh is dynamically restructured to adaptively maintain detail ... [more] This paper presents a method for adaptive fracture propagation in thin sheets. A high-quality triangle mesh is dynamically restructured to adaptively maintain detail wherever it is required by the simulation. These requirements include refining where cracks are likely to either start or advance. Refinement ensures that the stress distribution around the crack tip is well resolved, which is vital for creating highly detailed, realistic crack paths. The dynamic meshing framework allows subsequent coarsening once areas are no longer likely to produce cracking. This coarsening allows efficient simulation by reducing the total number of active nodes and by preventing the formation of thin slivers around the crack path. A local reprojection scheme and a substepping fracture process help to ensure stability and prevent a loss of plasticity during remeshing. By including bending and stretching plasticity models, the method is able to simulate a large range of materials with very different fracture behaviors. [less]
Self-Refining Games using Player Analytics
Matt Stanton, Ben Humberston, Brandon Kase, James F. O'Brien, Kayvon Fatahalian, Adrien Treuille
SIGGRAPH 2014
Data-driven simulation demands good training data drawn from a vast space of possible simulations. While fully sampling these large spaces is infeasible, we observe that ... [more] Data-driven simulation demands good training data drawn from a vast space of possible simulations. While fully sampling these large spaces is infeasible, we observe that in practical applications, such as gameplay, users explore only a vanishingly small subset of the dynamical state space. In this paper we present a sampling approach that takes advantage of this observation by concentrating precomputation around the states that users are most likely to encounter. We demonstrate our technique in a prototype self-refining game whose dynamics improve with play, ultimately providing realistically rendered, rich fluid dynamics in real time on a mobile device. Our results show that our analytics-driven training approach yields lower model error and fewer visual artifacts than a heuristic training strategy. [less]
View-Dependent Adaptive Cloth Simulation
Woojong Koh, Rahul Narain, James F. O'Brien
SCA 2014
This paper describes a method for view-dependent cloth simulation using dynamically adaptive mesh refinement and coarsening. Given a prescribed camera motion, the method ... [more] This paper describes a method for view-dependent cloth simulation using dynamically adaptive mesh refinement and coarsening. Given a prescribed camera motion, the method adjusts the criteria controlling refinement to account for visibility and apparent size in the camera's view. Objectionable dynamic artifacts are avoided by anticipative refinement and smoothed coarsening. This approach preserves the appearance of detailed cloth throughout the animation while avoiding the wasted effort of simulating details that would not be discernible to the viewer. The computational savings realized by this method increase as scene complexity grows, producing a 2x speed-up for a single character and more than 4x for a small group. [less]
Can 3D Shape be Estimated from Focus Cues Alone?
Rachel A. Albert, Abdullah Bulbul, Rahul Narain, James F. O'Brien, Martin S. Banks
VSS 2014
Focus cues—blur and accommodation—have generally been regarded as very coarse, ordinal cues to depth. This assessment has been largely determined by the inability to display ... [more] Focus cues—blur and accommodation—have generally been regarded as very coarse, ordinal cues to depth. This assessment has been largely determined by the inability to display these cues correctly with conventional displays. For example, when a 3D shape is displayed with sharp rendering (i.e., pinhole camera), the expected blur variation is not present and accommodation does not have an appropriate effect on the retinal image. When a 3D shape with rendered blur (i.e., camera with non-pinhole aperture) is displayed, the viewer's accommodation does not have the appropriate retinal effect. We asked whether the information provided by correct blur and accommodation can be used to determine shape. We conducted a shape-discrimination experiment in which subjects indicated whether a hinge stimulus was concave or convex. The stimuli were presented monocularly in a unique volumetric display that allows us to present correct or nearly correct focus cues. The hinge was textured using a back-projection technique, so the stimuli contained no useful shape cues except blur and accommodation. We used four rendering methods that vary in the validity of focus information. Two single-plane methods mimicked a conventional display and two volumetric methods mimicked natural viewing. A pinhole camera model was used in one single-plane condition, so image sharpness was independent of depth. In the other single-plane condition, natural blur was rendered thereby creating an appropriate blur gradient. In one volumetric condition, a linear blending rule was used to assign intensity to image planes. In the other volumetric condition, an optimized blending rule was used that creates a closer approximation to real-world viewing. Subject performance was at chance in the single-plane conditions. Performance improved substantially when in the volumetric conditions, slightly better in the optimized-blending condition. This is direct evidence that 3D shape judgments can be made from the information contained in blur and accommodation alone. [less]
Correct blur and accommodation information is a reliable cue to depth ordering.
Marina Zannoli, Rachel A. Albert, Abdullah Bulbul, Rahul Narain, James F. O'Brien, Martin Banks
VSS 2014
Marshall et al. (1996) showed that blur could in principle also be used to determine depth ordering of two surfaces across an occlusion boundary from the correlation between ... [more] Marshall et al. (1996) showed that blur could in principle also be used to determine depth ordering of two surfaces across an occlusion boundary from the correlation between the boundaryâ€TMs blur and the blur of the two surfaces. They tested this experimentally by presenting stimuli on a conventional display and manipulating rendered blur. This approximates the retinal image formed by surfaces at different depths and an occlusion boundary, but only when the viewer accommodates to the display screen. Accommodation to other distances creates incorrect blur. Viewers' judgments of depth ordering were inconsistent: they generally judged the sharper surface as nearer than the blurrier one regardless of boundary blur. We asked if more consistent performance occurs when accommodation has the appropriate effect on the retinal image. We used a volumetric display to present nearly correct focus cues. Images were displayed on four image planes at focal distances from 1.4-3.2 diopters. Viewers indicated the nearer of two textured surfaces separated by a sinusoidal boundary. The stimuli were presented either on one plane as in previous experiments or on two planes (separated either by 0.6 or by 1.2 diopters) such that focus cues are nearly correct. Viewers first fixated and accommodated to a cross on one of the planes. The stimulus was then presented either for 200ms, too short for accommodative change, or for 4s, allowing accommodative change. Responses were much more accurate in the two-plane condition than in the single-plane condition, which shows that appropriate blur can be used to determine depth ordering across an occlusion boundary. Responses were also more accurate with the longer presentations, which shows that accommodation aids depth-order determination. Thus, correct blur and accommodation information across an occlusion boundary yields more accurate depth-ordering judgments than indicated by previous work. [less]
The Perception of Surface Material from Disparity and Focus Cues
Martin Banks, Abdullah Bulbul, Rachel Albert, Rahul Narain, James F. O'Brien, Gregory Ward
VSS 2014
The visual properties of surfaces reveal many things including a floor's cleanliness and a car's age. These judgments of material are based on the spread of light reflected ... [more] The visual properties of surfaces reveal many things including a floor's cleanliness and a car's age. These judgments of material are based on the spread of light reflected from a surface. The bidirectional reflectance distribution function (BRDF) quantifies the pattern of spread and how it depends on the direction of incident light, surface shape, and surface material. Two extremes are Lambertian and mirrored surfaces, which respectively have uniform and delta-function BRDFs. Most surfaces have more complicated BRDFs and we examined many of them using the Ward model as an approximation for real surfaces. Reflections are generally view dependent. This dependence creates a difference between the binocular disparities of a reflection and the surface itself. It also creates focus differences between the reflection and physical surface. In simulations we examined how material type affects retinal images. We calculated point-spread functions (PSFs) for reflections off different materials as a function of the eye's focus state. When surface roughness is zero, the reflection PSF changes dramatically with focus state. With greater roughness, the PSF change is reduced until there is no effect of focus state with sufficiently rough surfaces. The reflection PSF also has a dramatic effect on the ability to estimate disparity. We next examined people's ability to distinguish surface markings from reflections and to identify different types of material. We used a unique volumetric display that allows us to present nearly correct focus cues along with more traditional depth cues such as disparity. With binocular viewing, we observed a clear effect of the disparity of reflections on these judgments. We also found that disparity provided less useful information with rougher materials. With monocular viewing, we observed a small but consistent effect of the reflection's focal distance on judgments of markings vs. reflections and on identification of material. [less]
2013
Fast Simulation of Mass-Spring Systems
Tiantian Liu, Adam Bargteil, James F. O'Brien, Ladislav Kavan
SIGGRAPH Asia 2013
We describe a scheme for time integration of mass-spring systems that makes use of a solver based on block coordinate descent. This scheme provides a fast solution for classical ... [more] We describe a scheme for time integration of mass-spring systems that makes use of a solver based on block coordinate descent. This scheme provides a fast solution for classical linear (Hookean) springs. We express the widely used implicit Euler method as an energy minimization problem and introduce spring directions as auxiliary unknown variables. The system is globally linear in the node positions, and the non-linear terms involving the directions are strictly local. Because the global linear system does not depend on run-time state, the matrix can be pre-factored, allowing for very fast iterations. Our method converges to the same final result as would be obtained by solving the standard form of implicit Euler using Newton's method. Although the asymptotic convergence of Newton's method is faster than ours, the initial ratio of work to error reduction with our method is much faster than Newton's. For real-time visual applications, where speed and stability are more important than precision, we obtain visually acceptable results at a total cost per timestep that is only a fraction of that required for a single Newton iteration. When higher accuracy is required, our algorithm can be used to compute a good starting point for subsequent Newton's iteration. [less]
Exposing Photo Manipulation with Inconsistent Shadows
Eric Kee, James F. O'Brien, Hany Farid
TOG 2013
We describe a geometric technique to detect physically inconsistent arrangements of shadows in an image. This technique combines multiple constraints from cast and attached ... [more] We describe a geometric technique to detect physically inconsistent arrangements of shadows in an image. This technique combines multiple constraints from cast and attached shadows to constrain the projected location of a point light source. The consistency of the shadows is posed as a linear programming problem. A feasible solution indicates that the collection of shadows is physically plausible, while a failure to find a solution provides evidence of photo tampering. [less]
Folding and Crumpling Adaptive Sheets
Rahul Narain, Tobias Pfaff, James F. O'Brien
SIGGRAPH 2013
We present a technique for simulating plastic deformation in sheets of thin materials, such as crumpled paper, dented metal, and wrinkled cloth. Our simulation uses a framework ... [more] We present a technique for simulating plastic deformation in sheets of thin materials, such as crumpled paper, dented metal, and wrinkled cloth. Our simulation uses a framework of adaptive mesh refinement to dynamically align mesh edges with folds and creases. This framework allows efficient modeling of sharp features and avoids bend locking that would be otherwise caused by stiff in-plane behavior. By using an explicit plastic embedding space we prevent remeshing from causing shape diffusion. We include several examples demonstrating that the resulting method realistically simulates the behavior of thin sheets as they fold and crumple. [less]
Near-exhaustive Precomputation of Secondary Cloth Effects
Doyub Kim, Woojong Koh, Rahul Narain, Kayvon Fatahalian, Adrien Treuille, James F. O'Brien
SIGGRAPH 2013
The central argument against data-driven methods in computer graphics rests on the curse of dimensionality: it is intractable to precompute "everything" about a complex ... [more] The central argument against data-driven methods in computer graphics rests on the curse of dimensionality: it is intractable to precompute "everything" about a complex space. In this paper, we challenge that assumption by using several thousand CPU-hours to perform a massive exploration of the space of secondary clothing effects on a character animated through a large motion graph. Our system continually explores the phase space of cloth dynamics, incrementally constructing a secondary cloth motion graph that captures the dynamics of the system. We find that it is possible to sample the dynamical space to a low visual error tolerance and that secondary motion graphs containing tens of gigabytes of raw mesh data can be compressed down to only tens of megabytes. These results allow us to capture the effect of high-resolution, off-line cloth simulation for a rich space of character motion and deliver it efficiently as part of an interactive application. [less]
Type-Constrained Direct Fitting of Quadric Surfaces
James Andrews, Carlo H. Séquin
CAD 2013
We present a catalog of type-specific, direct quadric fitting methods: Given a selection of a point cloud or triangle mesh, and a desired quadric type (e.g. cone, ellipsoid, paraboloid ... [more] We present a catalog of type-specific, direct quadric fitting methods: Given a selection of a point cloud or triangle mesh, and a desired quadric type (e.g. cone, ellipsoid, paraboloid, etc), our methods recover a best-fit surface of the given type to the given data. Type-specific quadric fitting methods are scattered throughout the literature; here we present a thorough, practical collection in one place. We add new methods to handle neglected quadric types, such as non-circular cones and general rotationally symmetric quadrics. We improve upon existing methods for ellipsoid- and hyperboloid-specific fitting. Our catalog handles a wide range of quadric types with just two high-level fitting strategies, making it simpler to understand and implement. [less]
Simulating Liquids and Solid-Liquid Interactions with Lagrangian Meshes
Pascal Clausen, Martin Wicke, Jonathan Shewchuk, James F. O'Brien
TOG 2013
This paper describes a Lagrangian finite element method that simulates the behavior of liquids and solids in a unified framework. Local mesh improvement operations maintain ... [more] This paper describes a Lagrangian finite element method that simulates the behavior of liquids and solids in a unified framework. Local mesh improvement operations maintain a high-quality tetrahedral discretization even as the mesh is advected by fluid flow. We conserve volume and momentum, locally and globally, by assigning each element an independent rest volume and adjusting it to correct for deviations during remeshing and collisions. Incompressibility is enforced with per-node pressure values, and extra degrees of freedom are selectively inserted to prevent pressure locking. Topological changes in the domain are explicitly treated with local mesh splitting and merging. Our method models surface tension with an implicit formulation based on surface energies computed on the boundary of the volume mesh. With this method we can model elastic, plastic, and liquid materials in a single mesh, with no need for explicit coupling. We also model heat diffusion and thermoelastic effects, which allow us to simulate phase changes. We demonstrate these capabilities in several fluid simulations at scales from millimeters to meters, including simulations of melting caused by external or thermoelastic heating. [less]
Generalized, Basis-Independent Kinematic Surface Fitting
James Andrews, Carlo H. Séquin
JCAD 2013
Kinematic surfaces form a general class of surfaces, including surfaces of revolution, helices, spirals, and more. Standard methods for fitting such surfaces are either specialized ... [more] Kinematic surfaces form a general class of surfaces, including surfaces of revolution, helices, spirals, and more. Standard methods for fitting such surfaces are either specialized to a small subset of these surface types (either focusing exclusively on cylinders or exclusively on surfaces of revolution) or otherwise are basis-dependent (leading to scale-dependent results). Previous work has suggested re-scaling data to a fixed size bounding box to avoid the basis-dependence issues. We show that this method fails on some simple, common cases such as a box or a cone with small noise. We propose instead adapting a well-studied approximate maximum-likelihood method to the kinematic surface fitting problem, which solves the basis-dependence issue. Because this technique is not designed for a specific type of kinematic surface, it also opens the door to the possibility of new variants of kinematic surfaces, such as affinely-scaled surfaces of revolution. [less]
Interactive Albedo Editing in Path-Traced Volumetric Materials
Miloš Hašan, Ravi Ramamoorthi
TOG 2013
Materials such as clothing or carpets, or complex assemblies of small leaves, flower petals or mosses, do not fit well into either BRDF or BSSRDF models. Their appearance is ... [more] Materials such as clothing or carpets, or complex assemblies of small leaves, flower petals or mosses, do not fit well into either BRDF or BSSRDF models. Their appearance is a complex combination of reflection, transmission, scattering, shadowing and inter-reflection. This complexity can be handled by simulating the full volumetric light transport within these materials by Monte Carlo algorithms, but there is no easy way to construct the necessary distributions of local material properties that would lead to the desired global appearance. In this paper, we consider one way to alleviate the problem: an editing algorithm that enables a material designer to set the local (single-scattering) albedo coefficients interactively, and see an immediate update of the emergent appearance in the image. This is a difficult problem, since the function from materials to pixel values is neither linear nor low-order polynomial. We combine the following two ideas to achieve high-dimensional heterogeneous edits: precomputing the homogeneous mapping of albedo to intensity, and a large Jacobian matrix, which encodes the derivatives of each image pixel with respect to each albedo coefficient. Combining these two datasets leads to an interactive editing algorithm with a very good visual match to a fully path-traced ground truth. [less]
2012
Adaptive Anisotropic Remeshing for Cloth Simulation
Rahul Narain, Armin Samii, James F. O'Brien
SIGGRAPH Asia 2012
We present a technique for cloth simulation that dynamically refines and coarsens triangle meshes so that they automatically conform to the geometric and dynamic detail ... [more] We present a technique for cloth simulation that dynamically refines and coarsens triangle meshes so that they automatically conform to the geometric and dynamic detail of the simulated cloth. Our technique produces anisotropic meshes that adapt to surface curvature and velocity gradients, allowing efficient modeling of wrinkles and waves. By anticipating buckling and wrinkle formation, our technique preserves fine-scale dynamic behavior. Our algorithm for adaptive anisotropic remeshing is simple to implement, takes up only a small fraction of the total simulation time, and provides substantial computational speedup without compromising the fidelity of the simulation. We also introduce a novel technique for strain limiting by posing it as a nonlinear optimization problem. This formulation works for arbitrary non-uniform and anisotropic meshes, and converges more rapidly than existing solvers based on Jacobi or Gauss-Seidel iterations. [less]
Correcting for Optical Aberrations using Multilayer Displays
Fu-Chung Huang, Douglas Lanman, Brian A. Barsky, Ramesh Raskar
SIGGRAPH ASIA 2012
Optical aberrations of the human eye are currently corrected using eyeglasses, contact lenses, or surgery. We describe a fourth option: modifying the composition of displayed ... [more] Optical aberrations of the human eye are currently corrected using eyeglasses, contact lenses, or surgery. We describe a fourth option: modifying the composition of displayed content such that the perceived image appears in focus, after passing through an eye with known optical defects. Prior approaches synthesize pre-filtered images by deconvolving the content by the point spread function of the aberrated eye. Such methods have not led to practical applications, due to severely reduced contrast and ringing artifacts. We address these limitations by introducing multilayer pre-filtering, implemented using stacks of semi-transparent, light-emitting layers. By optimizing the layer positions and the partition of spatial frequencies between layers, contrast is improved and ringing artifacts are eliminated. We assess design constraints for multilayer displays; autostereoscopic light field displays are identified as a preferred, thin form factor architecture, allowing synthetic layers to be displaced in response to viewer movement and refractive errors. We assess the benefits of multilayer pre-filtering versus prior light field pre-distortion methods, showing pre-filtering works within the constraints of current display resolutions. We conclude by analyzing benefits and limitations using a prototype multilayer LCD. [less]
Updated Sparse Cholesky Factors for Corotational Elastodynamics
Florian Hecht, Yeon Jin Lee, Jonathan Shewchuk, James F. O'Brien
TOG 2012
We present warp-canceling corotation, a nonlinear finite element formulation for elastodynamic simulation that achieves fast performance by making only partial or ... [more] We present warp-canceling corotation, a nonlinear finite element formulation for elastodynamic simulation that achieves fast performance by making only partial or delayed changes to the simulation’s linearized system matrices. Coupled with an algorithm for incremental updates to a sparse Cholesky factorization, the method realizes the stability and scalability of a sparse direct method without the need for expensive refactorization at each time step. This finite element formulation combines the widely used corotational method with stiffness warping so that changes in the per-element rotations are initially approximated by inexpensive per-node rotations. When the errors of this approximation grow too large, the per-element rotations are selectively corrected by updating parts of the matrix chosen according to locally measured errors. These changes to the system matrix are propagated to its Cholesky factor by incremental updates that are much faster than refactoring the matrix from scratch. A nested dissection ordering of the system matrix gives rise to a hierarchical factorization in which changes to the system matrix cause limited, well-structured changes to the Cholesky factor. We show examples of simulations that demonstrate that the proposed formulation produces results that are visually comparable to those produced by a standard corotational formulation. Because our method requires computing only partial updates of the Cholesky factor, it is substantially faster than full refactorization and outperforms widely used iterative methods such as preconditioned conjugate gradients. Our method supports a controlled trade-off between accuracy and speed, and unlike most iterative methods its performance does not slow for stiffer materials but rather it actually improves. [less]
Interactive Inverse 3D Modeling
James Andrews, Hailin Jin, Carlo H. Séquin
CAD 2012
"Interactive Inverse 3D Modeling" is a user-guided approach to shape construction and redesign that extracts well-structured, parameterized, procedural descriptions ... [more] "Interactive Inverse 3D Modeling" is a user-guided approach to shape construction and redesign that extracts well-structured, parameterized, procedural descriptions from unstructured, hierarchically flat input data, such as point clouds, boundary representation meshes, or even multiple pictorial views of a given inspirational prototype. This approach combines traditional "forward" 3D modeling tools with a system of user-guided extraction modules and optimization routines. With a few cursor strokes users can express their preferences of the type of modeling primitives to be used in a particular area of the given prototype to be approximated, and they can also select the degree of parameterization associated with each modeling routine. The results are then pliable, structured descriptions that are well suited to implement the particular design modifications intended by the user. [less]
Exposing Digital Forgeries in Ballistic Motion
Valentina Conotter, James F. O'Brien, Hany Farid
TIFS 2012
We describe a geometric technique to detect physically implausible trajectories of objects in video sequences. This technique explicitly models the three-dimensional ... [more] We describe a geometric technique to detect physically implausible trajectories of objects in video sequences. This technique explicitly models the three-dimensional ballistic motion of objects in free-flight and the two-dimensional projection of the trajectory into the image plane of a static or moving camera. Deviations from this model provide evidence of manipulation. The technique assumes that the object's trajectory is substantially influenced only by gravity, that the image of the object's center of mass can be determined from the images, and requires that any camera motion can be estimated from background elements. The computational requirements of the algorithm are modest, and any detected inconsistencies can be illustrated in an intuitive, geometric fashion. We demonstrate the efficacy of this analysis on videos of our own creation and on videos obtained from video-sharing websites. [less]
Exposing Photo Manipulation with Inconsistent Reflections
James F. O'Brien, Hany Farid
TOG 2012
The advent of sophisticated photo editing software has made it increasingly easier to manipulate digital images. Often visual inspection cannot definitively distinguish ... [more] The advent of sophisticated photo editing software has made it increasingly easier to manipulate digital images. Often visual inspection cannot definitively distinguish the resulting forgeries from authentic photographs. In response, forensic techniques have emerged to detect geometric or statistical inconsistencies that result from specific forms of photo manipulation. In this paper we describe a new forensic technique that focuses on geometric inconsistencies that arise when fake reflections are inserted into a photograph or when a photograph containing reflections is manipulated. This analysis employs basic rules of reflective geometry and linear perspective projection, makes minimal assumptions about the scene geometry, and only requires the user to identify corresponding points on an object and its reflection. The analysis is also insensitive to common image editing operations such as resampling, color manipulations, and lossy compression. We demonstrate this technique with both visually plausible forgeries of our own creation and commercially produced forgeries. [less]
2011
Data-Driven Elastic Models for Cloth: Modeling and Measurement
Huamin Wang, Ravi Ramamoorthi, James F. O'Brien
SIGGRAPH 2011
Cloth often has complicated nonlinear, anisotropic elastic behavior due to its woven pattern and fiber properties. However, most current cloth simulation techniques simply ... [more] Cloth often has complicated nonlinear, anisotropic elastic behavior due to its woven pattern and fiber properties. However, most current cloth simulation techniques simply use linear and isotropic elastic models with manually selected stiffness parameters. Such simple simulations do not allow differentiating the behavior of distinct cloth materials such as silk or denim, and they cannot model most materials with fidelity to their real-world counterparts. In this paper, we present a data-driven technique to more realistically animate cloth. We propose a piecewise linear elastic model that is a good approximation to nonlinear, anisotropic stretching and bending behaviors of various materials. We develop new measurement techniques for studying the elastic deformations for both stretching and bending in real cloth samples. Our setup is easy and inexpensive to construct, and the parameters of our model can be fit to observed data with a well-posed optimization procedure. We have measured a database of ten different cloth materials, each of which exhibits distinctive elastic behaviors. These measurements can be used in most cloth simulation systems to create natural and realistic clothing wrinkles and shapes, for a range of different materials. [less]
Perceptually Based Tone Mapping for Low-Light Conditions
Adam Kirk, James F. O'Brien
SIGGRAPH 2011
In this paper we present a perceptually based algorithm for modeling the color shift that occurs for human viewers in low-light scenes. Known as the Purkinje effect, this ... [more] In this paper we present a perceptually based algorithm for modeling the color shift that occurs for human viewers in low-light scenes. Known as the Purkinje effect, this color shift occurs as the eye transitions from photopic, cone-mediated vision in well-lit scenes to scotopic, rod-mediated vision in dark scenes. At intermediate light levels vision is mesopic with both the rods and cones active. Although the rods have a spectral response distinct from the cones, they still share the same neural pathways. As light levels decrease and the rods become increasingly active they cause a perceived shift in color. We model this process so that we can compute perceived colors for mesopic and scotopic scenes from spectral image data. We also describe how the effect can be approximated from standard high dynamic range RGB images. Once we have determined rod and cone responses, we map them to RGB values that can be displayed on a standard monitor to elicit the intended color perception when viewed photopically. Our method focuses on computing the color shift associated with low-light conditions and leverages current HDR techniques to control the image’s dynamic range. We include results generated from both spectral and RGB input images. [less]
Interactive Furniture Layout Using Interior Design Guidelines
Paul Merrell, Eric Schkufza, Zeyang Li, Maneesh Agrawala, Vladlen Koltun
SIGGRAPH 2011
We present an interactive furniture layout system that assists users by suggesting furniture arrangements that are based on interior design guidelines. Our system incorporates ... [more] We present an interactive furniture layout system that assists users by suggesting furniture arrangements that are based on interior design guidelines. Our system incorporates the layout guidelines as terms in a density function and generates layout suggestions by rapidly sampling the density function using a hardware-accelerated Monte Carlo sampler. Our results demonstrate that the suggestion generation functionality measurably increases the quality of furniture arrangements produced by participants with no prior training in interior design. [less]
On the Duality of Forward and Inverse Light Transport
Manmohan Chandraker, Jiamin Bai, Tian-Tsong Ng, Ravi Ramamoorthi
PAMI 2011
Inverse light transport seeks to undo global illumination effects, such as interreflections, that pervade images of most scenes. This paper presents the theoretical and ... [more] Inverse light transport seeks to undo global illumination effects, such as interreflections, that pervade images of most scenes. This paper presents the theoretical and computational foundations for inverse light transport as a dual of forward rendering. Mathematically, this duality is established through the existence of underlying Neumann series expansions. Physically, it can be shown that each term of our inverse series cancels an interreflection bounce, just as the forward series adds them. While the convergence properties of the forward series are well-known, we show that the oscillatory convergence of the inverse series leads to more interesting conditions on material reflectance. Conceptually, the inverse problem requires the inversion of a large light transport matrix, which is impractical for realistic resolutions using standard techniques. A natural consequence of our theoretical framework is a suite of fast computational algorithms for light transport inversion -- analogous to finite element radiosity, Monte Carlo and wavelet-based methods in forward rendering -- that rely at most on matrix-vector multiplications. We demonstrate two practical applications, namely, separation of individual bounces of the light transport and fast projector radiometric compensation to display images free of global illumination artifacts in real-world environments. [less]
Interactive Extraction and Re-Design of Sweep Geometries
James Andrews, Pushkar P. Joshi, Carlo H. Séquin
CGI 2011
We introduce two interactive extraction modules that can fit the parameters of generalized sweeps to large, unstructured meshes for immediate, high-level, detail-preserving ... [more] We introduce two interactive extraction modules that can fit the parameters of generalized sweeps to large, unstructured meshes for immediate, high-level, detail-preserving modification. These modules represent two extremes in a spectrum of parameterized shapes: rotational sweeps defined by a few global parameters, and progressive sweeps forming generalized cylinders with many slowly varying local parameters. Both modules are initialized and controlled by the user drawing a few strokes onto the displayed original model. We demonstrate the system on various shapes, ranging from clean, mechanical geometries to organic forms with intricate surface details. [less]
Bringing Clothing into Desired Configurations with Limited Perception
Marco Cusumano-Towner, Arjun Singh, Stephen Miller, James F. O'Brien, Pieter Abbeel
ICRA 2011
We consider the problem of autonomously bringing an article of clothing into a desired configuration using a general-purpose two-armed robot. We propose a hidden Markov ... [more] We consider the problem of autonomously bringing an article of clothing into a desired configuration using a general-purpose two-armed robot. We propose a hidden Markov model (HMM) for estimating the identity of the article and tracking the article's configuration throughout a specific sequence of manipulations and observations. At the end of this sequence, the article's configuration is known, though not necessarily desired. The estimated identity and configuration of the article are then used to plan a second sequence of manipulations that brings the article into the desired configuration. We propose a relaxation of a strain-limiting finite element model for cloth simulation that can be solved via convex optimization; this serves as the basis of the transition and observation models of the HMM. The observation model uses simple perceptual cues consisting of the height of the article when held by a single gripper and the silhouette of the article when held by two grippers. The model accurately estimates the identity and configuration of clothing articles, enabling our procedure to autonomously bring a variety of articles into desired configurations that are useful for other tasks, such as folding. [less]
Modeling and Perception of Deformable One-Dimensional Objects
Shervin Javdani, Sameep Tandon, Jie Tang, James F. O'Brien, Pieter Abbeel
ICRA 2011
Recent advances in the modeling of deformable one-dimensional objects (DOOs) such as surgical suture, rope, and hair show significant promise for improving the simulation ... [more] Recent advances in the modeling of deformable one-dimensional objects (DOOs) such as surgical suture, rope, and hair show significant promise for improving the simulation, perception, and manipulation of such objects. An important application of these tasks lies in the area of medical robotics, where robotic surgical assistants have the potential to greatly reduce surgeon fatigue and human error by improving the accuracy, speed, and robustness of surgical tasks such as suturing. However, different types of DOOs exhibit a variety of bending and twisting behaviors that are highly dependent on material properties. This paper proposes an approach for fitting simulation models of DOOs to observed data. Our approach learns an energy function such that observed DOOs configurations lie in local energy minima. Our experiments on a variety of DOOs show that models fitted to different types of DOOs using our approach enable accurate prediction of future configurations. Additionally, we explore the application of our learned model to the perception of DOOs. [less]
2010
Computer generation of ribbed sculptures
James Hamlin, Carlo H. Séquin
JMA 2010
Charles Perry's monumental sculpture Solstice is analysed and its generative geometrical logic based on a twisted toroidal sweep is captured in a computer programme with ... [more] Charles Perry's monumental sculpture Solstice is analysed and its generative geometrical logic based on a twisted toroidal sweep is captured in a computer programme with interactively adjustable control parameters. This programme is then used to generate other models of ribbed sculptures based on one or more interlinked torus knots. From this family of sculptures related to Perry's Solstice we derive a broader paradigm for the generation of "ribbed" sculptures. It is based on one or two simple, mathematically defined "guide rails", which are then populated with a dense set of thinner "ribs" to create lightweight, transparent surfaces. With this broadened concept and a few suitably modified and parameterized programmes we can emulate many other ribbed sculptures by Charles Perry and also create new sculpture designs and mathematical visualization models that profit from the semi-transparent look of these structures. [less]
Multi-Resolution Isotropic Strain Limiting
Huamin Wang, James F. O'Brien, Ravi Ramamoorthi
SIGGRAPH Asia 2010
In this paper we describe a fast strain-limiting method that allows stiff, incompliant materials to be simulated efficiently. Unlike prior approaches, which act on springs ... [more] In this paper we describe a fast strain-limiting method that allows stiff, incompliant materials to be simulated efficiently. Unlike prior approaches, which act on springs or individual strain components, this method acts on the strain tensors in a coordinate-invariant fashion allowing isotropic behavior. Our method applies to both two- and three-dimensional strains, and only requires computing the singular value decomposition of the deformation gradient, either a small 2x2 or 3x3 matrix, for each element. We demonstrate its use with triangular and tetrahedral linear-basis elements. For triangulated surfaces in three-dimensional space, we also describe a complementary edge-angle-limiting method to limit out-of-plane bending. All of the limits are enforced through an iterative, non-linear, Gauss-Seidel-like constraint procedure. To accelerate convergence, we propose a novel multi-resolution algorithm that enforces fitted limits at each level of a non-conforming hierarchy. Compared with other constraint-based techniques, our isotropic multi-resolution strain-limiting method is straightforward to implement, efficient to use, and applicable to a wide range of shell and solid materials. [less]
Symmetrical Embeddings of Regular Maps R5.13 and R5.6
Carlo H. Séquin
This report is a documentation of my trial-and-error design process to find a symmetrical embedding of the regular map R5.13 on a genus-5 2-manifold. It documents the non-linear ... [more] This report is a documentation of my trial-and-error design process to find a symmetrical embedding of the regular map R5.13 on a genus-5 2-manifold. It documents the non-linear way in which my mind homed-in on a valid solution and then refined that solution to obtain a satisfactory geometrical model. This design-thinking log may serve as a case study for a design approach that switches back and forth between doodling with physical materials, computer-aided template and model construction, and verification of the results on tangible visualization models. Lessons learned on R5.13 were subsequently applied to solve the embedding of the regular map R5.6. [less]
Personalized Photograph Ranking and Selection System
Che-Hua Yeh, Yuan-Chen Ho, Brian A. Barsky, Ming Ouhyoung
ACM MM 2010
In this paper, we propose a novel personalized ranking system for amateur photographs. Although some of the features used in our system are similar to previous work, new features ... [more] In this paper, we propose a novel personalized ranking system for amateur photographs. Although some of the features used in our system are similar to previous work, new features, such as texture, RGB color, portrait (through face detection), and black-and-white, are included for individual preferences. Our goal of automatically ranking photographs is not intended for award-wining professional photographs but for photographs taken by amateurs, especially when individual preference is taken into account. The performance of our system in terms of precision-recall diagram and binary classification accuracy (93%) is close to the best results to date for both overall system and individual features. Two personalized ranking user interfaces are provided: one is feature-based and the other is example-based. Although both interfaces are effective in providing personalized preferences, our user study showed that example-based was preferred by twice as many people as feature-based. [less]
Example-Based Wrinkle Synthesis for Clothing Animation
Huamin Wang, Florian Hecht, Ravi Ramamoorthi, James F. O'Brien
SIGGRAPH 2010
This paper describes a method for animating the appearance of clothing, such as pants or a shirt, that fits closely to a figure's body. Compared to flowing cloth, such as loose ... [more] This paper describes a method for animating the appearance of clothing, such as pants or a shirt, that fits closely to a figure's body. Compared to flowing cloth, such as loose dresses or capes, these types of garments involve nearly continuous collision contact and small wrinkles, that can be troublesome for traditional cloth simulation methods. Based on the observation that the wrinkles in close-fitting clothing behave in a predominantly kinematic fashion, we have developed an example-based wrinkle synthesis technique. Our method drives wrinkle generation from the pose of the figure's kinematic skeleton. This approach allows high quality clothing wrinkles to be combined with a coarse cloth simulation that computes the global and dynamic aspects of the clothing motion. While the combined results do not exactly match a high-resolution reference simulation, they do capture many of the characteristic fine-scale features and wrinkles. Further, the combined system runs at interactive rates, making it suitable for applications where high-resolution offline simulations would not be a viable option. The wrinkle synthesis method uses a precomputed database built by simulating the high-resolution clothing as the articulated figure is moved over a range of poses. In principle, the space of poses is exponential in the total number of degrees of freedom; however clothing wrinkles are primarily affected by the nearest joints, allowing each joint to be processed independently. During synthesis, mesh interpolation is used to consider the influence of multiple joints, and combined with a coarse simulation to produce the final results at interactive rates. [less]
Dynamic Local Remeshing for Elastoplastic Simulation
Martin Wicke, Daniel Ritchie, Bryan Klingner, Sebastian Burke, Jonathan Shewchuk, James F. O'Brien
SIGGRAPH 2010
We propose a finite element simulation method that addresses the full range of material behavior, from purely elastic to highly plastic, for physical domains that are substantially ... [more] We propose a finite element simulation method that addresses the full range of material behavior, from purely elastic to highly plastic, for physical domains that are substantially reshaped by plastic flow, fracture, or large elastic deformations. To mitigate artificial plasticity, we maintain a simulation mesh in both the current state and the rest shape, and store plastic offsets only to represent the non-embeddable portion of the plastic deformation. To maintain high element quality in a tetrahedral mesh undergoing gross changes, we use a dynamic meshing algorithm that attempts to replace as few tetrahedra as possible, and thereby limits the visual artifacts and artificial diffusion that would otherwise be introduced by repeatedly remeshing the domain from scratch. Our dynamic mesher also locally refines and coarsens a mesh, and even creates anisotropic tetrahedra, wherever a simulation requests it. We illustrate these features with animations of elastic and plastic behavior, extreme deformations, and fracture. [less]
My Search for Symmetrical Embeddings of Regular Maps
Carlo H. Séquin
Bridges 2010
Various approaches are discussed for obtaining highly symmetrical and aesthetically pleasing space models of regular maps embedded in surfaces of genus 2 to 5. For many ... [more] Various approaches are discussed for obtaining highly symmetrical and aesthetically pleasing space models of regular maps embedded in surfaces of genus 2 to 5. For many cases, geometrical intuition and preliminary visualization models made from paper strips or plastic pipes are quite competitive with exhaustive computer searches. A couple of particularly challenging problems are presented as detailed case studies. The symmetrical patterns discovered could be further modified to create Escher-like tilings on low-genus handle bodies. [less]
Two New Approaches to Depth of Field Post-Processing: Pyramid Spreading and Tensor Filtering
Todd J. Kosloff, Brian A. Barsky
VISIGRAPP 2010
Depth of field refers to the swath that is imaged in sharp focus through an optics system, such as a camera lens. Control over depth of field is an important artistic tool, which ... [more] Depth of field refers to the swath that is imaged in sharp focus through an optics system, such as a camera lens. Control over depth of field is an important artistic tool, which can be used, for example, to emphasize the subject of a photograph. The most efficient algorithms for simulating depth of field are post-processing methods. Post-processing can be made more efficient by making various approximations. We start with the assumption that the point spread function (PSF) is Gaussian. This assumption introduces structure into the problem which we exploit to achieve speed. Two methods will be presented. In our first approach, which we call pyramid spreading, PSFs are spread into a pyramid. By writing larger PSFs to coarser levels of the pyramid, the performance remains constant, independent of the size of the PSFs. After spreading all the PSFs, the pyramid is then collapsed to yield the final blurred image. Our second approach, called the tensor method, exploits the fact that blurring is a linear operator. The operator is treated as a large tensor which is compressed by finding structure in it. The compressed representation is then used to directly blur the image. Both methods present new perspectives on the problem of efficiently blurring an image. [less]
Simulation of Needle Insertion and Tissue Deformation for Modeling Prostate Brachytherapy
Nuttapong Chentanez, Ron Alterovitz, Daniel Ritchie, Lita Cho, Kris Hauser, Ken Goldberg, Jonathan Shewchuk, James F. O'Brien
ABS 2010
Realistic modeling of needle insertion during brachytherapy can be used for training and in automated planning to reduce errors between intended and actual placement ... [more] Realistic modeling of needle insertion during brachytherapy can be used for training and in automated planning to reduce errors between intended and actual placement of the needle tip. We have developed a three-dimensional tetrahedral finite element simulation that models tissue deformation, needle flexation, and their coupled interaction. [less]
Using Blur to Affect Perceived Distance and Size
Robert (Robin) Held, Emily Cooper, James F. O'Brien, Marty Banks
TOG 2010
We present a probabilistic model of how viewers may use defocus blur in conjunction with other pictorial cues to estimate the absolute distances to objects in a scene. Our ... [more] We present a probabilistic model of how viewers may use defocus blur in conjunction with other pictorial cues to estimate the absolute distances to objects in a scene. Our model explains how the pattern of blur in an image together with relative depth cues indicates the apparent scale of the image’s contents. From the model, we develop a semi-automated algorithm that applies blur to a sharply rendered image and thereby changes the apparent distance and scale of the scene’s contents. To examine the correspondence between the model/algorithm and actual viewer experience, we conducted an experiment with human viewers and compared their estimates of absolute distance to the model’s predictions. We did this for images with geometrically correct blur due to defocus and for images with commonly used approximations to the correct blur. The agreement between the experimental data and model predictions was excellent. The model predicts that some approximations should work well and that others should not. Human viewers responded to the various types of blur in much the way the model predicts. The model and algorithm allow one to manipulate blur precisely and to achieve the desired perceived scale efficiently. [less]
An intuitive explanation of third-order surface behavior
Pushkar P. Joshi, Carlo H. Séquin
CAGD
We present a novel parameterization-independent exposition of the third-order geometric behavior of a surface point. Unlike existing algebraic expositions, our work ... [more] We present a novel parameterization-independent exposition of the third-order geometric behavior of a surface point. Unlike existing algebraic expositions, our work produces an intuitive explanation of third-order shape, analogous to the principal curvatures and directions that describe second-order shape. We extract four parameters that provide a quick and concise understanding of the third-order surface behavior at any given point. Our shape parameters are useful for easily characterizing different third-order surface shapes without having to use tensor algebra. Our approach generalizes to higher orders, allowing us to extract similarly intuitive parameters that fully describe fourth- and higher-order surface behavior. [less]
2009
Generating Surface Crack Patterns
Hayley Iben, James F. O'Brien
Graphical Models
We present a method for generating surface crack patterns that appear in materials such as mud, ceramic glaze, and glass. To model these phenomena, we build upon existing physically ... [more] We present a method for generating surface crack patterns that appear in materials such as mud, ceramic glaze, and glass. To model these phenomena, we build upon existing physically based methods. Our algorithm generates cracks from a stress field defined heuristically over a triangle discretization of the surface. The simulation produces cracks by evolving this field over time. The user can control the characteristics and appearance of the cracks using a set of simple parameters. By changing these parameters, we have generated examples similar to a variety of crack patterns found in the real world. We assess the realism of our results by comparison with photographs of real-world examples. Using a physically based approach also enables us to generate animations similar to time-lapse photography. [less]
Simulating Gaseous Fluids with Low and High Speeds
Yue Gao, Chen-Feng Li, Shi-Min Hu, Brian A. Barsky
Pacific Graphics 09
Gaseous fluids may move slowly, as smoke does, or at high speed, such as occurs with explosions. High-speed gas flow is always accompanied by low-speed gas flow, which produces ... [more] Gaseous fluids may move slowly, as smoke does, or at high speed, such as occurs with explosions. High-speed gas flow is always accompanied by low-speed gas flow, which produces rich visual details in the fluid motion. Realistic visualization involves a complex dynamic flow field with both low and high speed fluid behavior. In computer graphics, algorithms to simulate gaseous fluids address either the low speed case or the high speed case, but no algorithm handles both efficiently. With the aim of providing visually pleasing results, we present a hybrid algorithm that efficiently captures the essential physics of both low- and high-speed gaseous fluids. We model the low speed gaseous fluids by a grid approach and use a particle approach for the high speed gaseous fluids. In addition, we propose a physically sound method to connect the particle model to the grid model. By exploiting complementary strengths and avoiding weaknesses of the grid and particle approaches, we produce some animation examples and analyze their computational performance to demonstrate the effectiveness of the new hybrid method. [less]
Three Techniques for Rendering Generalized Depth of Field Effects
Todd J. Kosloff, Brian A. Barsky
MI 09
Depth of field refers to the swath that is imaged in sufficient focus through an optics system, such as a camera lens. Control over depth of field is an important artistic tool ... [more] Depth of field refers to the swath that is imaged in sufficient focus through an optics system, such as a camera lens. Control over depth of field is an important artistic tool that can be used to emphasize the subject of a photograph. In a real camera, the control over depth of field is limited by the laws of physics and by physical constraints. Depth of field has been rendered in computer graphics, but usually with the same limited control as found in real camera lenses. In this paper, we generalize depth of field in computer graphics by allowing the user to specify the distribution of blur throughout a scene in a more flexible manner. Generalized depth of field provides a novel tool to emphasize an area of interest within a 3D scene, to select objects from a crowd, and to render a busy, complex picture more understandable by focusing only on relevant details that may be scattered throughout the scene. We present three approaches for rendering generalized depth of field based on nonlinear distributed ray tracing, compositing, and simulated heat diffusion. Each of these methods has a different set of strengths and weaknesses, so it is useful to have all three available. The ray tracing approach allows the amount of blur to vary with depth in an arbitrary way. The compositing method creates a synthetic image with focus and aperture settings that vary per-pixel. The diffusion approach provides full generality by allowing each point in 3D space to have an arbitrary amount of blur. [less]
Interactive Simulation of Surgical Needle Insertion and Steering
Nuttapong Chentanez, Ron Alterovitz, Daniel Ritchie, Lita Cho, Kris Hauser, Ken Goldberg, Jonathan Shewchuk, James F. O'Brien
SIGGRAPH 2009
We present algorithms for simulating and visualizing the insertion and steering of needles through deformable tissues for surgical training and planning. Needle insertion ... [more] We present algorithms for simulating and visualizing the insertion and steering of needles through deformable tissues for surgical training and planning. Needle insertion is an essential component of many clinical procedures such as biopsies, injections, neurosurgery, and brachytherapy cancer treatment. The success of these procedures depends on accurate guidance of the needle tip to a clinical target while avoiding vital tissues. Needle insertion deforms body tissues, making accurate placement difficult. Our interactive needle insertion simulator models the coupling between a steerable needle and deformable tissue. We introduce (1) a novel algorithm for local remeshing that quickly enforces the conformity of a tetrahedral mesh to a curvilinear needle path, enabling accurate computation of contact forces, (2) an efficient method for coupling a 3D finite element simulation with a 1D inextensible rod with stick-slip friction, and (3) optimizations that reduce the computation time for physically based simulations.We can realistically and interactively simulate needle insertion into a prostate mesh of 13,375 tetrahedra and 2,763 vertices at a 25 Hz frame rate on an 8-core 3.0 GHz Intel Xeon PC. The simulation models prostate brachytherapy with needles of varying stiffness, steering needles around obstacles, and supports motion planning for robotic needle insertion. We evaluate the accuracy of the simulation by comparing against real-world experiments in which flexible, steerable needles were inserted into gel tissue phantoms. [less]
Real-Time Deformation and Fracture in a Game Environment
Eric G. Parker, James F. O'Brien
SCA 2009
This paper describes a simulation system that has been developed to model
the deformation and fracture of solid objects in a real-time gaming context.
Based around a corotational ...
[more]
This paper describes a simulation system that has been developed to model
the deformation and fracture of solid objects in a real-time gaming context.
Based around a corotational tetrahedral finite element method, this system
has been constructed from components published in the graphics and
computational physics literatures. The goal of this paper is to describe how
these components can be combined to produce an engine that is robust to
unpredictable user interactions, fast enough to model reasonable scenarios
at real-time speeds, suitable for use in the design of a game level, and
with appropriate controls allowing content creators to match artistic
direction. Details concerning parallel implementation, solver design,
rendering method, and other aspects of the simulation are elucidated with
the intent of providing a guide to others wishing to implement similar
systems. Examples from in-game scenes captured on the Xbox 360, PS3, and PC
platforms are included.
This paper recieved the award for best paper at SCA 2009.
[less]
3D Clothing Fitting Based on the Geometric Feature Matching
Zhong Li, Xiaogang Jin, Brian Barsky, Jun Liu
CAD/Graphics 2009
The 3D clothing fitting on a body model is an important research topic in the garment computer aided design (GCAD). During the fitting process, the match between the clothing ... [more] The 3D clothing fitting on a body model is an important research topic in the garment computer aided design (GCAD). During the fitting process, the match between the clothing and body models is still a problem for researchers. In this paper, we provide a 3D clothing fitting method based on the feature point match. We firstly use a new cubic-order weighted fitting patch to estimate the geometric properties of each vertex on two mesh models. Feature points are then extracted from two models and a new matching function is constructed to match them according to curvature and torsion. We interactively select several key feature points from two limited feature point sets to compute the transformation matrix of the clothing model. Finally the second match is performed to achieve the precise match between the clothing and body models. The experimental results show that our 3D clothing fitting method is simple and effective. [less]
Tubular Sculptures
Carlo H. Séquin
Bridges 2009
This paper reviews ways in which many artists have constructed large sculptures from tubular elements, ranging from single cylinders to toroidal or knotted structures ... [more] This paper reviews ways in which many artists have constructed large sculptures from tubular elements, ranging from single cylinders to toroidal or knotted structures, to assemblies of a large number of bent tubes. A few parameterized generators are introduced that facilitate design and evaluation of a variety of such sculptural forms. [less]
Visualizing High-Order Surface Geometry
Pushkar P. Joshi, Carlo H. Séquin
CAD&A 2009
We have derived parameters that describe the higher-order geometric behavior of smooth surfaces. Our parameters are similar in spirit to the principal directions and ... [more] We have derived parameters that describe the higher-order geometric behavior of smooth surfaces. Our parameters are similar in spirit to the principal directions and principal curvatures that succinctly capture second-order shape behavior. We derive our parameters from a cylindrical Fourier decomposition around the surface normal. We present a visualization program for studying the influence of the various terms of different degrees on the shape of the local neighborhood of a surface point. We display a small surface patch that is controlled by two sets of parameters: One set is a simple polynomial description of the surface geometry in Cartesian coordinates. The other one is a set of Fourier components grouped by angular frequency and by their phase shifts. Manipulating the values in one parameter set changes the geometry of the patch and also updates the parameter values of the other set. [less]
An Effective Third-order Local Fitting Patch and Its Application
Zhong Li, Brian Barsky, Xiaogang Jin
SMI 2009
In this paper, we extend Razdan and Bae’s second-order local fitting method [11] to construct an effective third-order fitting patch. Compared to other estimation algorithms ... [more] In this paper, we extend Razdan and Bae’s second-order local fitting method [11] to construct an effective third-order fitting patch. Compared to other estimation algorithms, this weighted bicubic B é zier patch more accurately obtains the normal vector and curvature estimation of a triangular mesh model. Furthermore, we define the principal geodesic torsion of each vertex on the mesh model and estimate it through this local fitting patch. In the end of this paper, we apply the third-order fitting patch for the mesh smoothing and hole-filling which can get the satisfactory results. [less]
Ribbed Surfaces for Art, Architecture, and Visualization
James Hamlin, Carlo H. Séquin
CAD&A 2009
Sequences of parameterized Hermite curves following with their endpoints along two guide rails are used to create "transparent" surfaces and tubular sculptures. This parameterized ... [more] Sequences of parameterized Hermite curves following with their endpoints along two guide rails are used to create "transparent" surfaces and tubular sculptures. This parameterized set-up allows modeling a wide variety of shapes in a natural way by just changing a few parameters. Potential applications range from mathematical visualization models to architecture. [less]
CAD Tools for Creating Space-filling 3D Escher Tiles
Mark Howison, Carlo H. Séquin
CAD&A 2009
We discuss the design and implementation of CAD tools for creating decorative solids that tile 3-space in a regular, isohedral manner. Starting with the simplest case of ... [more] We discuss the design and implementation of CAD tools for creating decorative solids that tile 3-space in a regular, isohedral manner. Starting with the simplest case of extruded 2D tilings, we describe geometric algorithms used for maintaining boundary representations of 3D tiles, including a Java implementation of an interactive constrained Delaunay triangulation library and a mesh-cutting algorithm used in layering extruded tiles to create more intricate designs. Finally, we demonstrate a CAD tool for creating 3D tilings that are derived from cubic lattices. The design process for these 3D tiles is more constrained, and hence more difficult, than in the 2D case, and it raises additional user interface issues. [less]
Interpolating Splines: Which is the fairest of them all?
Raph Levien, Carlo H. Séquin
CAD&A 2009
Interpolating splines are a basic primitive for designing planar curves. There is a wide diversity in the literature but no consensus on a "best" spline, or even criteria for ... [more] Interpolating splines are a basic primitive for designing planar curves. There is a wide diversity in the literature but no consensus on a "best" spline, or even criteria for preferring one spline over another. For the case of G2-continuous splines, we emphasize two properties that can arguably be expected in any definition of "best" and show that any such spline is made from segments cut from a single generator curve, such as the Euler spiral. [less]
Depth of Field Postprocessing For Layered Scenes Using Constant-Time Rectangle Spreading
Todd Kosloff, Michael W. Tao, Brian Barsky
GI 2009
Control over what is in focus and what is not in focus in an image is an important artistic tool. The range of depth in a 3D scene that is imaged in sufficient focus through an optics ... [more] Control over what is in focus and what is not in focus in an image is an important artistic tool. The range of depth in a 3D scene that is imaged in sufficient focus through an optics system, such as a camera lens, is called depth of field. Without depth of field, the entire scene appears completely in sharp focus, leading to an un- natural, overly crisp appearance. Current techniques for rendering depth of field in computer graphics are either slow or suffer from artifacts, or restrict the choice of point spread function (PSF). In this paper, we present a new image filter based on rectangle spread- ing which is constant time per pixel. When used in a layered depth of field framework, our filter eliminates the intensity leakage and depth discontinuity artifacts that occur in previous methods. We also present several extensions to our rectangle spreading method to allow flexibility in the appearance of the blur through control over the PSF. [less]
Refolding Planar Polygons
Hayley Iben, James F. O'Brien, Erik Demaine
DCG
This paper describes an algorithm for generating a guaranteed intersection-free interpolation sequence between any pair of compatible polygons. Our algorithm builds ... [more] This paper describes an algorithm for generating a guaranteed intersection-free interpolation sequence between any pair of compatible polygons. Our algorithm builds on prior results from linkage unfolding, and if desired it can ensure that every edge length changes monotonically over the course of the interpolation sequence. The computational machinery that ensures against self-intersection is independent from a distance metric that determines the overall character of the interpolation sequence. This decoupled approach provides a powerful control mechanism for determining how the interpolation should appear, while still assuring against intersection and guaranteeing termination of the algorithm. Our algorithm also allows additional control by accommodating a set of algebraic constraints that can be weakly enforced throughout the interpolation sequence. [less]
2008
Intricate Isohedral Tilings of 3D Euclidean Space
Carlo H. Séquin
Bridges 2008
Various methods to create intricate tilings of 3D space are presented. They include modulated extrusions of 2D Escher tilings, free-form deformations of the fundamental ... [more] Various methods to create intricate tilings of 3D space are presented. They include modulated extrusions of 2D Escher tilings, free-form deformations of the fundamental domain of various 3D symmetry groups, highly symmetrical polyhedral toroids of genus 1, higher-genus cage structures derived from the cubic lattice as well as from the diamond and triamond lattices, and finally interlinked tiles with the connectivity of simple knots. [less]
Making Big Things Look Small: Blur combined with other depth cues affects perceived size and distance
Robert (Robin) Held, Emily Cooper, James F. O'Brien, Marty Banks
VSS 2008
Blur is commonly considered a weak distance cue, but photographic techniques that manipulate blur cause significant and compelling changes in the perceived distance ... [more] Blur is commonly considered a weak distance cue, but photographic techniques that manipulate blur cause significant and compelling changes in the perceived distance and size of objects. One such technique is "tilt-shift miniaturization," in which a camera's lens is translated and slanted relative to the film plane. The result is an exaggerated vertical blur gradient that makes scenes with a vertical distance gradient (e.g., bird's-eye view of landscape) appear significantly nearer and therefore smaller. We will begin by demonstrating this compelling effect, and then describe how we used it to examine the visual system's use of blur as a cue to distance and size. In a psychophysical experiment, we presented computer-generated, bird's-eye images of a highly realistic model of a city. Blur was manipulated in four ways: 1) sharp images with no blur; 2) horizontal blur gradients were applied to those images; 3) vertical gradients were applied; 4) a large aperture (diameter up to 60m) was used to create an image with an accurate correlation between blur and depth for realizable, small-scale scenes. Observers indicated the perceived distance to objects in the images. Technique 1 produced a convincing impression of a full-sized scene. Technique 2 produced no systematic miniaturization. Techniques 3 and 4 produced significant and similar miniaturization. Thus, the correlation between blur and the depth indicated by other cues affects perceived distance and size. The correlation must be only reasonably accurate to produce a significant and systematic effect. We developed a probabilistic model of the relationship between blur and distance. An interesting prediction of the model is that blur only affects perceived distance when coupled with other distance cues, which is manifested in the tilt-shift effect we observed in humans. Thus, blur is a useful cue to absolute distance when coupled with other depth information. [less]
A Precomputed Polynomial Representation for Interactive BRDF Editing with Global Illumination
Aner Ben-Artzi, Kevin Egan, Kevin Egan, Frédo Durand, Ravi Ramamoorthi
TOG
The ability to interactively edit BRDFs in their final placement within a computer graphics scene is vital to making informed choices for material properties. We significantly ... [more] The ability to interactively edit BRDFs in their final placement within a computer graphics scene is vital to making informed choices for material properties. We significantly extend previous work on BRDF editing for static scenes (with fixed lighting and view) by developing a precomputed polynomial representation that enables interactive BRDF editing with global illumination. Unlike previous precomputation-based rendering techniques, the image is not linear in the BRDF when considering interreflections. We introduce a framework for precomputing a multibounce tensor of polynomial coefficients that encapsulates the nonlinear nature of the task. Significant reductions in complexity are achieved by leveraging the low-frequency nature of indirect light. We use a high-quality representation for the BRDFs at the first bounce from the eye and lower-frequency (often diffuse) versions for further bounces. This approximation correctly captures the general global illumination in a scene, including color-bleeding, near-field object reflections, and even caustics. We adapt Monte Carlo path tracing for precomputing the tensor of coefficients for BRDF basis functions. At runtime, the high-dimensional tensors can be reduced to a simple dot product at each pixel for rendering. We present a number of examples of editing BRDFs in complex scenes with interactive feedback rendered with global illumination. [less]
Geometrically exact dynamic splines
Adrien Theetten, Laurent Grisoni, Claude Andriot, Brian Barsky
Computer-Aided Design 2008
We propose a complete model handling the physical simulation of deformable 1D objects. We formulate continuous expressions for stretching, bending and twisting energies ... [more] We propose a complete model handling the physical simulation of deformable 1D objects. We formulate continuous expressions for stretching, bending and twisting energies. These expressions are mechanically rigorous and geometrically exact. Both elastic and plastic deformations are handled to simulate a wide range of materials.We validate the proposed model in several classical test configurations. The use of geometrical exact energies with dynamic splines provides very accurate results as well as interactive simulation times, which shows the suitability of the proposed model for constrained CAD applications. We illustrate the application potential of the proposed model by describing a virtual system for cable positioning, which can be used to test compatibility between planned fixing clip positions, and mechanical cable properties. [less]
2007
Liquid Simulation on Lattice-Based Tetrahedral Meshes
Nuttapong Chentanez, Bryan Feldman, François Labelle, James F. O'Brien, Jonathan Shewchuk
SCA 2007
This paper describes a simulation method for animating the behavior of incompressible liquids with complex free surfaces. The region occupied by the liquid is discretized ... [more] This paper describes a simulation method for animating the behavior of incompressible liquids with complex free surfaces. The region occupied by the liquid is discretized with a boundary-conforming tetrahedral mesh that grades from fine resolution near the surface to coarser resolution on the interior. At each time-step, semi-Lagrangian techniques are used to advect the fluid and its boundary forward, and a new conforming mesh is then constructed over the fluid-occupied region. The tetrahedral meshes are built using a variation of the body-centered cubic lattice structure that allows octree grading and deviation from the lattice-structure at boundaries. The semi-regular mesh structure can be generated rapidly and allows efficient computation and storage while still conforming well to boundaries and providing a mesh-quality guarantee. Pressure projection is performed using an algebraic multigrid method, and a thickening scheme is used to reduce volume loss when fluid features shrink below mesh resolution. Examples are provided to demonstrate that the resulting method can capture complex liquid motions that include fine detail on the free surfaces without suffering from excessive amounts volume loss or artificial damping. [less]
An Algorithm for Rendering Generalized Depth of Field Effects Based on Simulated Heat Diffusion
Todd Kosloff, Brian Barsky
ICCSA 2007
Depth of field refers to the swath through a 3Dscene that is imaged in acceptable focus through an optics system, such as a camera lens. Control over depth of field is an important ... [more] Depth of field refers to the swath through a 3Dscene that is imaged in acceptable focus through an optics system, such as a camera lens. Control over depth of field is an important artistic tool that can be used to emphasize the sub- ject of a photograph. In a real camera, the control over depth of field is limited by the nature of the image formation process and by physical constraints. The depth of field effect has been simulated in computer graphics, but with the same limited control as found in real camera lenses. In this paper, we use diffusion in a non-homogeneous medium to generalize depth of field in computer graphics by enabling the user to independently specify the degree of blur at each point in three-dimensional space. Generalized depth of field provides a novel tool to emphasize an area of interest within a 3D scene, to pick objects out of a crowd, and to render a busy, complex picture more understandable by focusing only on relevant details that may be scattered throughout the scene. Our algorithm oper- ates by blurring a sequence of nonplanar layers that form the scene. Choosing a suitable blur algorithm for the layers is critical; thus, we develop appropriate blur semantics such that the blur algorithm will properly generalize depth of field. We found that diffusion in a non-homogeneous medium is the process that best suits these semantics. [less]
Symmetric Embedding of Locally Regular Hyperbolic Tilings
Carlo H. Séquin
Bridges 2007
Various methods to create intricate tilings of 3D space are presented. They include modulated extrusions of 2D Escher tilings, free-form deformations of the fundamental ... [more] Various methods to create intricate tilings of 3D space are presented. They include modulated extrusions of 2D Escher tilings, free-form deformations of the fundamental domain of various 3D symmetry groups, highly symmetrical polyhedral toroids of genus 1, higher-genus cage structures derived from the cubic lattice as well as from the diamond and triamond lattices, and finally interlinked tiles with the connectivity of simple knots. [less]
Computer-Aided Design and Realization of Geometrical Sculptures
Carlo H. Séquin
The use of computer-aided design tools in the conception and realization of large-scale geometrical bronze sculptures is described. An inspirational piece of sculpture ... [more] The use of computer-aided design tools in the conception and realization of large-scale geometrical bronze sculptures is described. An inspirational piece of sculpture is analyzed and then captured in procedural form including several design parameters. These parameters not only allow the sculpture to be scaled to different sizes and individually optimized for each scale, but also facilitate the design of new sculptures that lie in the same conceptual family. The parameterized representation takes care of constraints and limitations in several of the implementation steps and provides additional aids for the assembly of a large sculpture from many smaller and more easily manufacturable pieces. [less]
A Method for Cartoon-Style Rendering of Liquid Animations
Ashley M. Eden, Adam Bargteil, Tolga Goktekin, Sara Beth Eisinger, James F. O'Brien
GI 2007
In this paper we present a visually compelling and informative cartoon rendering style for liquid animations. Our style is inspired by animations such as Futurama, The Little ... [more] In this paper we present a visually compelling and informative cartoon rendering style for liquid animations. Our style is inspired by animations such as Futurama, The Little Mermaid, and Bambi. We take as input a liquid surface obtained from a three-dimensional physically based liquid simulation system and output animations that evoke a cartoon style and convey liquid movement. Our method is based on four cues that emphasize properties of the liquid's shape and motion. We use bold outlines to emphasize depth discontinuities, patches of constant color to highlight near-silhouettes and areas of thinness, and, optionally place temporally coherent oriented textures on the liquid surface to help convey motion. [less]
Hyper-Seeing the Regular Hendeca-choron
Carlo H. Séquin
ISAMA 2007
The hendecachoron is an abstract 4-dimensional polytope composed of eleven cells in the form of hemi-icosahedra. This paper tries to foster an understanding of this intriguing ... [more] The hendecachoron is an abstract 4-dimensional polytope composed of eleven cells in the form of hemi-icosahedra. This paper tries to foster an understanding of this intriguing object of high symmetry by discussing its construction in bottom-up and top down ways and providing visualization by computer graphics models. [less]
Design and Implementation of Pax Mundi II
Carlo H. Séquin
On January 18, 2007 a ten foot tall bronze sculpture Pax Mundi II was installed in the courtyard of the H&R Block headquarters in Kansas City. This paper describes the computer-aided ... [more] On January 18, 2007 a ten foot tall bronze sculpture Pax Mundi II was installed in the courtyard of the H&R Block headquarters in Kansas City. This paper describes the computer-aided re-design process that started from the original Pax Mundi wood sculpture, as well as the fabrication and installation of the final sculpture. [less]
4D compression and relighting with high-resolution light transport matrices
Ewen Cheslack-Postava, Nolan Goodnight, Ren Ng, Ravi Ramamoorthi, Greg Humphreys
I3D 2007
This paper presents a method for efficient compression and relighting with high-resolution, precomputed light transport matrices. We accomplish this using a 4D wavelet ... [more] This paper presents a method for efficient compression and relighting with high-resolution, precomputed light transport matrices. We accomplish this using a 4D wavelet transform, transforming the columns of the transport matrix, in addition to the 2D row transform used in previous work. We show that a standard 4D wavelet transform can actually inflate portions of the matrix, because high-frequency lights lead to high-frequency images that cannot easily be compressed. Therefore, we present an adaptive 4D wavelet transform that terminates at a level that avoids inflation and maximizes sparsity in the matrix data. Finally, we present an algorithm for fast relighting from adaptively compressed transport matrices. Combined with a GPU-based precomputation pipeline, this results in an image and geometry relighting system that performs significantly better than 2D compression techniques, on average 2x-3x better in terms of storage cost and rendering speed for equal quality matrices. [less]
2006
A semi-Lagrangian contouring method for fluid simulation
Adam Bargteil, Tolga Goktekin, James F. O'Brien, John A. Strain
ACM Trans. Graphics
In this paper we present a semi-Lagrangian surface tracking method for use with fluid simulations. Our method maintains an explicit polygonal mesh that defines the surface ... [more] In this paper we present a semi-Lagrangian surface tracking method for use with fluid simulations. Our method maintains an explicit polygonal mesh that defines the surface, and an octree data structure that provides both a spatial index for the mesh and a means for efficiently approximating the signed distance to the surface. At each timestep a new surface is constructed by extracting the zero set of an advected signed-distance function. Semi-Lagrangian backward path tracing is used to advect the signed-distance function. One of the primary advantages of this formulation is that it enables tracking of surface characteristics, such as color or texture coordinates, at negligible additional cost. We include several examples demonstrating that the method can be effectively used as part of a fluid simulation to animate complex and interesting fluid behaviors. [less]
Optimization of HDR brachytherapy dose distributions using linear programming with penalty costs
Ron Alterovitz, Etienne Lessard, Jean Pouliot, I-Chow Joe Hsu, James F. O'Brien, Ken Goldberg
J. Medical Physics
Prostate cancer is increasingly treated with high-dose-rate (HDR) brachytherapy, a type of radiotherapy in which a radioactive source is guided through catheters temporarily ... [more] Prostate cancer is increasingly treated with high-dose-rate (HDR) brachytherapy, a type of radiotherapy in which a radioactive source is guided through catheters temporarily implanted in the prostate. Clinicians must set dwell times for the source inside the catheters so the resulting dose distribution minimizes deviation from dose prescriptions that conform to patient-specific anatomy. The primary contribution of this paper is to take the well-established dwell times optimization problem defined by Inverse Planning by Simulated Annealing (IPSA) developed at UCSF and exactly formulate it as a linear programming (LP) problem. Because LP problems can be solved exactly and deterministically, this formulation provides strong performance guarantees: one can rapidly find the dwell times solution that globally minimizes IPSA's objective function for any patient case and clinical criteria parameters. For a sample of 20 prostates with volume ranging from 23 to 103 cc, the new LP method optimized dwell times in less than 15 s per case on a standard PC. The dwell times solutions currently being obtained clinically using simulated annealing (SA), a probabilistic method, were quantitatively compared to the mathematically optimal solutions obtained using the LP method. The LP method resulted in significantly improved objective function values compared to SA (P = 1.54 * 10-7), but none of the dosimetric indices indicated a statistically significant difference (P ≤ 0.01). The results indicate that solutions generated by the current version of IPSA are clinically equivalent to the mathematically optimal solutions. [less]
A Texture Synthesis Method for Liquid Animations
Adam Bargteil, Funshing Sin, Jonathan E. Michaels, Tolga Goktekin, James F. O'Brien
SCA 2006
In this paper we present a method for synthesizing textures on animated liquid surfaces generated by a physically based fluid simulation system. Rather than advecting ... [more] In this paper we present a method for synthesizing textures on animated liquid surfaces generated by a physically based fluid simulation system. Rather than advecting texture coordinates on the surface, we synthesize a new texture for every frame. We synthesize the texture with an optimization procedure which attempts to match the surface texture to an input sample texture. By synthesizing a new texture for every frame, our method is able to overcome the discontinuities and distortions of an advected parameterization. We achieve temporal coherence by initializing the surface texture with color values advected from the surface at the previous frame and including these colors in the energy function used during optimization. [less]
Generating Surface Crack Patterns
Hayley Iben, James F. O'Brien
SCA 2006
We present a method for generating surface crack patterns that
appear in materials such as mud, ceramic glaze, and glass. To
model these phenomena, we build upon existing physically ...
[more]
We present a method for generating surface crack patterns that
appear in materials such as mud, ceramic glaze, and glass. To
model these phenomena, we build upon existing physically based
methods. Our algorithm generates cracks from a stress field
defined heuristically over a triangle discretization of the
surface. The simulation produces cracks by evolving this field
over time. The user can control the characteristics and
appearance of the cracks using a set of simple parameters. By
changing these parameters, we have generated examples similar to a
variety of crack patterns found in the real world. We assess the
realism of our results by a comparison with photographs of
real-world examples. Using a physically based approach also
enables us to generate animations similar to time-lapse
photography.
Awarded best paper at SCA 2006.
[less]
Simultaneous Coupling of Fluids and Deformable Bodies
Nuttapong Chentanez, Tolga Goktekin, Bryan Feldman, James F. O'Brien
SCA 2006
This paper presents a method for simulating the two-way interaction between fluids and deformable solids. The fluids are simulated using an incompressible Eulerian formulation ... [more] This paper presents a method for simulating the two-way interaction between fluids and deformable solids. The fluids are simulated using an incompressible Eulerian formulation where a linear pressure projection on the fluid velocities enforces mass conservation. Similarly, elastic solids are simulated using a semi-implicit integrator implemented as a linear operator applied to the forces acting on the nodes in Lagrangian formulation.The proposed method enforces coupling constraints between the fluid and the elastic systems by combining both the pressure projection and implicit integration steps into one set of simultaneous equations. Because these equations are solved simultaneously the resulting combined system treats closed regions in physically correct fashion, and has good stability characteristics allowing relatively large time steps. This general approach is not tied to any particular volume discretization of fluid or solid, and we present results implemented using both grid based and tetrahedral simulations. [less]
Fluid Animation with Dynamic Meshes
Bryan Klingner, Bryan Feldman, Nuttapong Chentanez, James F. O'Brien
SIGGRAPH 2006
This paper presents a method for animating fluid with unstructured tetrahedral meshes that change at each time step. Meshes that conform well to changing boundaries and ... [more] This paper presents a method for animating fluid with unstructured tetrahedral meshes that change at each time step. Meshes that conform well to changing boundaries and that focus computation in the visually important parts of the domain can be generated quickly and reliably using existing techniques. We also describe a new approach to two-way coupling of fluid and rigid bodies that, while general, benefits from remeshing. Overall, the method provides a flexible environment for creating complex scenes involving fluid animation. [less]
A Texture Synthesis Method for Liquid Animations
Adam Bargteil, Funshing Sin, Jonathan Michaels, James F. O'Brien
SIGGRAPH 2006 Tech Sketch
In this sketch we present a method for synthesizing textures on animated liquid surfaces generated by a physically based fluid simulation system. Rather than advecting ... [more] In this sketch we present a method for synthesizing textures on animated liquid surfaces generated by a physically based fluid simulation system. Rather than advecting texture coordinates on the surface, we synthesize a new texture for every frame. We synthesize the texture with an optimization procedure which attempts to match the surface texture to an input sample texture. By synthesizing a new texture for every frame, our method is able to overcome the discontinuities and distortions of an advected parameterization. We achieve temporal coherence by initializing the surface texture with color values advected from the surface at the previous frame and including these colors in the energy function used during optimization. [less]
Acquiring Scattering Properties of Participating Media by Dilution
Srinivasa G. Narasimhan, Mohit Gupta, Craig Donner , Ravi Ramamoorthi, Shree Nayar, Henrik Wann Jensen
SIGGRAPH 2006
The visual world around us displays a rich set of volumetric effects due to participating media. The appearance of these media is governed by several physical properties ... [more] The visual world around us displays a rich set of volumetric effects due to participating media. The appearance of these media is governed by several physical properties such as particle densities, shapes and sizes, which must be input (directly or indirectly) to a rendering algorithm to generate realistic images. While there has been significant progress in developing rendering techniques (for instance, volumetric Monte Carlo methods and analytic approximations), there are very few methods that measure or estimate these properties for media that are of relevance to computer graphics. In this paper, we present a simple device and technique for robustly estimating the properties of a broad class of participating media that can be either (a) diluted in water such as juices, beverages, paints and cleaning supplies, or (b) dissolved in water such as powders and sugar/salt crystals, or (c) suspended in water such as impurities. The key idea is to dilute the concentrations of the media so that single scattering effects dominate and multiple scattering becomes negligible, leading to a simple and robust estimation algorithm. Furthermore, unlike previous approaches that require complicated or separate measurement setups for different types or properties of media, our method and setup can be used to measure media with a complete range of absorption and scattering properties from a single HDR photograph. Once the parameters of the diluted medium are estimated, a volumetric Monte Carlo technique may be used to create renderings of any medium concentration and with multiple scattering. We have measured the scattering parameters of forty commonly found materials, that can be immediately used by the computer graphics community. We can also create realistic images of combinations or mixtures of the original measured materials, thus giving the user a wide flexibility in making realistic images of participating media. [less]
Simultaneous Coupling of Fluids and Deformable Bodies
Nuttapong Chentanez, Tolga Goktekin, Bryan Feldman, James F. O'Brien
SIGGRAPH 2006 Tech Sketch
We describe a method for simultaneous two-way coupling of fluid and deformable bodies. The interaction between a fluid and deformable body can create complex and interesting ... [more] We describe a method for simultaneous two-way coupling of fluid and deformable bodies. The interaction between a fluid and deformable body can create complex and interesting motion that would be difficult to convincingly animate by hand. [less]
Patterns on the Genus-3 Klein Quartic
Carlo H. Séquin
Bridges 2006
Projections of Klein's quartic surface of genus 3 into 3D space are used as canvases on which we present regular tessellations, Escher tilings, knot- and graph-embedding ... [more] Projections of Klein's quartic surface of genus 3 into 3D space are used as canvases on which we present regular tessellations, Escher tilings, knot- and graph-embedding problems, Hamiltonian cycles, Petrie polygons and equatorial weaves derived from them. Many of the solutions found have also been realized as small physical models made on rapid-prototyping machines. [less]
Refolding Planar Polygons
Hayley Iben, James F. O'Brien, Erik Demaine
SoCG 2006
This paper describes an algorithm for generating a
guaranteed-intersection-free interpolation sequence between any pair
of compatible polygons. Our algorithm builds ...
[more]
This paper describes an algorithm for generating a
guaranteed-intersection-free interpolation sequence between any pair
of compatible polygons. Our algorithm builds on prior results from
linkage unfolding, and if desired it can ensure that every edge
length changes monotonically over the course of the interpolation
sequence. The computational machinery that ensures against
self-intersection is independent from the distance metric that
determines the overall character of the interpolation sequence.
This approach provides a powerful control mechanism for determining
how the interpolation should appear, while still assuring against
intersection and guaranteeing termination of the algorithm. Our
algorithm also allows additional control by accommodating set of
algebraic constraints that can be weakly enforced throughout the
interpolation sequence.
Awarded best paper at SoCG 2006.
[less]
Extensions for 3D Graphics Rendering Engine used for Direct Tessellation of Spline Surfaces
Adrien Sfarti, Brian Barsky, Todd Kosloff, Egon Pasztor, Alex Kozlowski, Eric Roman, Alex Perelman
ICCS 2006
In current 3D graphics architectures, the bus between the triangle server and the rendering engine GPU is clogged with triangle vertices and their many attributes (normal ... [more] In current 3D graphics architectures, the bus between the triangle server and the rendering engine GPU is clogged with triangle vertices and their many attributes (normal vectors, colors, texture coordinates). We have developed a new 3D graphics architecture using data compression to unclog the bus between the triangle server and the rendering engine. This new architecture has been described in [1]. In the present paper we describe further developments of the newly proposed architecture. The current paper shows several interesting extensions of our architecture such as backsurface rejection, NURBS real time tesselation and a description of a surface based API. We also show how the implementation of our architecture operates on top of the pixel shaders. [less]
New 3D Graphics Rendering Engine Architecture for Direct Tessellation of Spline Surfaces
Adrien Sfarti, Brian Barsky, Todd Kosloff, Egon Pasztor, Alex Kozlowski, Eric Roman, Alex Perelman
3IA 2006
In current 3D graphics architectures, the bus between the triangle server and the rendering engine GPU is clogged with triangle vertices and their many attributes (normal ... [more] In current 3D graphics architectures, the bus between the triangle server and the rendering engine GPU is clogged with triangle vertices and their many attributes (normal vectors, colors, texture coordinates). We develop a new 3D graphics architecture using data compression to unclog the bus between the triangle server and the rendering engine. The data compression is achieved by replacing the conventional idea of a GPU that renders triangles with a GPU that tessellates surface patches into triangles. [less]
Human Vision Based Detection of Non-Uniform Brightness on LCD Panels
Jee Hong Kim, Brian Barsky
MVAII 2004
We propose a method to detect defects due to spatially non-uniform brightness on LCD panels by using a machine vision technique. The detection method is based on human vision ... [more] We propose a method to detect defects due to spatially non-uniform brightness on LCD panels by using a machine vision technique. The detection method is based on human vision so that proper subjective assessment experiments were conducted to investigate the correlation between the parameters related to non-uniformity and the degree how easily observable it is. The visibility of the defects reveals to depend mainly on the spatial gradient of brightness variation. Thus, in the proposed method, the spatial gradient that is calculated by using extracted contours will be utilized to detect the defects due to non-uniform brightness. The detection method comprises four parts: contour extraction, spatial gradient calculation, decision of defects, and display of defects. We applied the method to the images captured from practical LCD panels with non-uniformity defects and the results were consistent with detection by a human inspector. [less]
Computational Studies of Human Motion: Tracking and Motion Synthesis
David Forsyth, Okan Arikan, Leslie Ikemoto, James F. O'Brien, Deva Ramanan
Foundations and Trends
We review methods for kinematic tracking of the human body in video. The review is part of a projected book that is intended to cross-fertilize ideas about motion representation ...
[more]
We review methods for kinematic tracking of the human body in video. The review is part of a projected book that is intended to cross-fertilize ideas about motion representation between the animation and computer vision communities. The review confines itself to the earlier stages of motion, focusing on tracking and motion synthesis; future material will cover activity representation and motion generation.
In general, we take the position that tracking does not necessarily involve (as is usually thought) complex multimodal inference problems. Instead, there are two key problems, both easy to state.
The first is lifting, where one must infer the configuration of the body in three dimensions from image data. Ambiguities in lifting can result in multimodal inference problem, and we review what little is known about the extent to which a lift is ambiguous. The second is data association, where one must determine which pixels in an image come from the body. We see a tracking by detection approach as the most productive, and review various human detection methods.
Lifting, and a variety of other problems, can be simplified by observing temporal structure in motion, and we review the literature on data- driven human animation to expose what is known about this structure. Accurate generative models of human motion would be extremely useful in both animation and tracking, and we discuss the profound difficulties encountered in building such models. Discriminative methods – which should be able to tell whether an observed motion is human or not – do not work well yet, and we discuss why.
There is an extensive discussion of open issues. In particular, we discuss the nature and extent of lifting ambiguities, which appear to be significant at short timescales and insignificant at longer timescales. This discussion suggests that the best tracking strategy is to track a 2D representation, and then lift it. We point out some puzzling phenomena associated with the choice of human motion representation – joint angles vs. joint positions. Finally, we give a quick guide to resources.
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2005
Interactive Procedural Computer-Aided Design
Carlo H. Séquin
CAD/Graphics 2005
The typical engineering design process can be decomposed into several phases: creative exploration of ideas, testing soundness of proposed concepts, refining concepts ... [more] The typical engineering design process can be decomposed into several phases: creative exploration of ideas, testing soundness of proposed concepts, refining concepts to realizable solutions, optimizing viable solutions with respect to performance/cost. Powerful computer algorithms have been developed for many of these tasks. Often these modules are rigid, allowing for little intervention by the designer, and the management of the interactions of these tasks mostly relies on human intelligence. Better user interfaces are required to integrate more fully human ingenuity and the assistance of the computer into the overall design process. The most powerful CAD systems should combine the power of programming, graphical visualization, and interactive adjustment of crucial design parameters. [less]
Semi-Automated Ultrasound Interpretation System Using Anatomical Knowledge Representation
Michael S. Downes, Brian Barsky
VC 2005
Interpreting ultrasound data presents a significant challenge to medical personnel, which limits the clinical applications of the technology. We have addressed this ... [more] Interpreting ultrasound data presents a significant challenge to medical personnel, which limits the clinical applications of the technology. We have addressed this issue by developing a prototype computer-based system designed to aid non-expert medical practitioners in using ultrasound devices in a variety of different diagnostic situations. Essentially, the system treats the collection of images generated during an ultrasound examination as an ordered sequence of views of the anatomical environment and picks out key views in which the contents of the scan image changes. It stores descriptions of expected key views and matches incoming images to this key view sequence during an orientation phase of an examination. The prototype can guide a novice user through an examination of a patient’s abdomen and automatically identify anatomical structures within the region. Overall, the design represents a novel approach to processing and augmenting ultrasound data and to representing spatial knowledge. [less]
Elimination of Artifacts Due to Occlusion and Discretization Problems in Image Space Blurring Techniques
Brian Barsky, Michael Tobias, Derrick P. Chu, Daniel R. Horn
GM 2005
Traditional computer graphics methods render images that appear sharp at all depths. Adding blur can add realism to a scene, provide a sense of scale, and draw a viewerÕs attention ... [more] Traditional computer graphics methods render images that appear sharp at all depths. Adding blur can add realism to a scene, provide a sense of scale, and draw a viewerÕs attention to a particular region of a scene. Our image-based blur algorithm needs to distinguish whether a portion of an image is either from a single object or is part of more than one object. This motivates two approaches to identify objects after an image has been rendered. We illustrate how these techniques can be used in conjunction with our image space method to add blur to a scene. [less]
Animating Gases with Hybrid Meshes
Bryan Feldman, James F. O'Brien, Bryan Klingner
SIGGRAPH 2005
This paper presents a method for animating gases on unstructured tetrahedral meshes to efficiently model the interaction of the fluids with irregularly shaped obstacles ... [more] This paper presents a method for animating gases on unstructured tetrahedral meshes to efficiently model the interaction of the fluids with irregularly shaped obstacles. Because our discretization scheme parallels that of the standard staggered grid mesh we are able to combine tetrahedral cells with regular hexahedral cells in a single mesh. This hybrid mesh offers both accuracy near obstacles and efficiency in open regions. [less]
Fast and Detailed Approximate Global Illumination by Irradiance Decomposition
Okan Arikan, David A. Forsyth, James F. O'Brien
SIGGRAPH 2005
In this paper we present an approximate method for accelerated computation of the final gathering step in a global illumination algorithm. Our method operates by decomposing ... [more] In this paper we present an approximate method for accelerated computation of the final gathering step in a global illumination algorithm. Our method operates by decomposing the radiance field close to surfaces into separate far- and near-field components that can be approximated individually. By computing surface shading using these approximations, instead of directly querying the global illumination solution, we have been able to obtain rendering time speed ups on the order of 10x compared to previous acceleration methods. Our approximation schemes rely mainly on the assumptions that radiance due to distant objects will exhibit low spatial and angular variation, and that the visibility between a surface and nearby surfaces can be reasonably predicted by simple location- and orientation-based heuristics. Motivated by these assumptions, our far-field scheme uses scattered-data interpolation with spherical harmonics to represent spatial and angular variation, and our near-field scheme employs and aggressively simple visibility heuristic. For our test scenes, the errors introduced when our assumptions fail do not result in visually objectionable artifacts or easily noticeable deviation from a ground-truth solution. We also discuss how our near-field approximation can be used with standard local illumination algorithms to produce significantly improved images at only negligible additional costs. [less]
Fluids in Deforming Meshes
Bryan Feldman, James F. O'Brien, Bryan Klingner, Tolga Goktekin
SCA 2005
This paper describes a simple modification to an Eulerian fluid simulation that permits the underlying mesh to deform independent of the simulated fluid's motion. The modification ... [more] This paper describes a simple modification to an Eulerian fluid simulation that permits the underlying mesh to deform independent of the simulated fluid's motion. The modification consists of a straightforward adaptation of the commonly used semi-Lagrangian advection method to account for the mesh's motion. Because the method does not require more interpolation steps than standard semi-Lagrangian integration, it does not suffer from additional smoothing and requires only the added cost of updating the mesh. By specifying appropriate boundary conditions, mesh boundaries can behave like moving obstacles that act on the fluid resulting in a number of interesting effects. The paper includes several examples that have been computed on moving tetrahedral meshes. [less]
Pushing People Around
Okan Arikan, David Forsyth, James F. O'Brien
SCA 2005
We present an algorithm for animating characters being pushed by an external source such as a user or a game environment. We start with a collection of motions of a real person ... [more] We present an algorithm for animating characters being pushed by an external source such as a user or a game environment. We start with a collection of motions of a real person responding to being pushed. When a character is pushed, we synthesize new motions by picking a motion from the recorded collection and modifying it so that the character responds to the push from the desired direction and location on its body. Determining the deformation parameters that realistically modify a recorded response motion is difficult. Choosing the response motion that will look best when modified is also non-trivial, especially in real-time. To estimate the envelope of deformation parameters that yield visually plausible modifications of a given motion, and to find the best motion to modify, we introduce an oracle. The oracle is trained using a set of synthesized response motions that are identified by a user as good and bad. Once trained, the oracle can, in real-time, estimate the visual quality of all motions in the collection and required deformation parameters to serve a desired push. Our method performs better than a baseline algorithm of picking the closest response motion in configuration space, because our method can find visually plausible transitions that do not necessarily correspond to similar motions in terms of configuration. Our method can also start with a limited set of recorded motions and modify them so that they can be used to serve different pushes on the upper body. [less]
A semi-Lagrangian contouring method for fluid simulation
Adam Bargteil, Tolga Goktekin, James F. O'Brien, John A. Strain
SIGGRAPH 2005 Tech Sketch
In this sketch we present a semi-Lagrangian surface tracking method for use with fluid simulations. Our method main- tains an explicit polygonal mesh that defines the surface ... [more] In this sketch we present a semi-Lagrangian surface tracking method for use with fluid simulations. Our method main- tains an explicit polygonal mesh that defines the surface, and an octree data structure that provides both a spatial index for the mesh and an efficient means for evaluating the signed- distance function away from the surface. At each time step the surface is reconstructed from an implicit function defined by the composition of backward advection and the previous signed-distance function. One of the primary advantages of this formulation is that it enables tracking of surface charac- teristics, such as color or texture coordinates, at negligible additional cost. We include several examples demonstrating that the method can be used as part of a fluid simulation to effectively animate complex and interesting fluid behaviors. [less]
Symmetrical Hamiltonian Manifolds on Regular 3D and 4d Polytopes
Carlo H. Séquin
Coxeter Day 2005
Hamiltonian cycles on the edge graphs of the regular polytopes in three and four dimensions are investigated with the primary goal of finding complete multi-colored coverages ... [more] Hamiltonian cycles on the edge graphs of the regular polytopes in three and four dimensions are investigated with the primary goal of finding complete multi-colored coverages of all the edges in the graph. The concept of a Hamiltonian path is then extended to the notion of Hamiltonian twomanifolds that visit all the given edges exactly once. For instance, the 4D simplex can be covered by a strip of 5 triangular facets that form a Moebius band! The use of Hamiltonian cycles to create physical dissection puzzles as well as geometrical sculptures is also investigated. The concepts are illustrated with computer graphics imagery and with small maquettes made with rapid prototyping techniques. [less]
Splitting Tori, Knots, and Moebius Bands
Carlo H. Séquin
Bridges 2005
A study of sculptures and puzzles resulting from splitting lengthwise, tori, Moebius bands, various knots and graphs, illustrated with many models made on rapid prototyping ... [more] A study of sculptures and puzzles resulting from splitting lengthwise, tori, Moebius bands, various knots and graphs, illustrated with many models made on rapid prototyping machines. [less]
Skeletal Parameter Estimation from Optical Motion Capture Data
Adam Kirk, James F. O'Brien, David Forsyth
CVPR 2005
In this paper we present an algorithm for automatically estimating a subject’s skeletal structure from optical mo- tion capture data. Our algorithm consists of a series ... [more] In this paper we present an algorithm for automatically estimating a subject’s skeletal structure from optical mo- tion capture data. Our algorithm consists of a series of steps that cluster markers into segment groups, determine the topological connectivity between these groups, and lo- cate the positions of their connecting joints. Our problem formulation makes use of fundamental distance constraints that must hold for markers attached to an articulated struc- ture, and we solve the resulting systems using a combination of spectral clustering and nonlinear optimization. We have tested our algorithms using data from both passive and ac- tive optical motion capture devices. Our results show that the system works reliably even with as few as one or two markers on each segment. For data recorded from human subjects, the system determines the correct topology and qualitatively accurate structure. Tests with a mechanical calibration linkage demonstrate errors for inferred segment lengths on average of only two percent. We discuss appli- cations of our methods for commercial human figure ani- mation, and for identifying human or animal subjects based on their motion independent of marker placement or feature selection. [less]
New 3D Graphics Rendering Engine Architecture for Direct Tessellation of Spline Surfaces
Adrien Sfarti, Brian Barsky, Todd Kosloff, Egon Pasztor, Alex Kozlowski, Eric Roman, Alex Perelman
ICCS 2005
In current 3D graphics architectures, the bus between the triangle server and the rendering engine GPU is clogged with triangle vertices and their many attributes (normal ... [more] In current 3D graphics architectures, the bus between the triangle server and the rendering engine GPU is clogged with triangle vertices and their many attributes (normal vectors, colors, texture coordinates). We develop a new 3D graphics architecture using data compression to unclog the bus between the triangle server and the rendering engine. The data compression is achieved by replacing the conventional idea of a GPU that renders triangles with a GPU that tessellates surface patches into triangles. [less]
2004
A Method for Animating Viscoelastic Fluids
Tolga Goktekin, Adam Bargteil, James F. O'Brien
SIGGRAPH 2004
This paper describes a technique for animating the behavior of viscoelastic fluids, such as mucus, liquid soap, pudding, toothpaste, or clay, that exhibit a combination of both ... [more] This paper describes a technique for animating the behavior of viscoelastic fluids, such as mucus, liquid soap, pudding, toothpaste, or clay, that exhibit a combination of both fluid and solid characteristics. The technique builds upon prior Eulerian methods for animating incompressible fluids with free surfaces by including additional elastic terms in the basic Navier-Stokes equations. The elastic terms are computed by integrating and advecting strain-rate throughout the fluid. Transition from elastic resistance to viscous flow is controlled by von Mises’s yield condition, and subsequent behavior is then governed by a quasi-linear plasticity model. [less]
Skeletal Parameter Estimation from Optical Motion Capture Data
Adam Kirk, James F. O'Brien, David Forsyth
SIGGRAPH 2004 Tech Sketch
In this sketch we present an algorithm for automatically estimating a subject's skeletal structure from optical motion capture data without using any a priori skeletal ... [more] In this sketch we present an algorithm for automatically estimating a subject's skeletal structure from optical motion capture data without using any a priori skeletal model. Our algorithm consists of a series of four steps that cluster markers into groups approximating rigid bodies, determine the topological connectivity between those groups, locate the positions of the connecting joints, and project those joint positions onto a rigid skeleton. These steps make use of a combination of spectral clustering and nonlinear optimization. Because it does not depend on prior rotation estimates, our algorithm can work reliably even when only one or two markers are attached to each body part, and our results do not suffer from error introduced by inaccurate rotation estimates. Furthermore, for applications where skeletal rotations are required, the skeleton computed by our algorithm actually provides an accurate and reliable means for computing them. We have tested an implementation of this algorithm with both passive and active motion capture data and found it to work well. Its computed skeletal estimates closely match measured values, and the algorithm behaves robustly even in the presence of noise, marker occlusion, and other errors typical of motion capture data. [less]
Refolding Planar Polygons
Hayley Iben, James F. O'Brien, Erik Demaine
SIGGRAPH 2004 Tech Sketch
This sketch describes a guaranteed technique for generating intersection-free interpolation sequences between arbitrary, non-intersecting, planar polygons. The computational ... [more] This sketch describes a guaranteed technique for generating intersection-free interpolation sequences between arbitrary, non-intersecting, planar polygons. The computational machinery that ensures against self intersection guides a user-supplied distance heuristic that determines the overall character of the interpolation sequence. Additional control is provided to the user through specifying algebraic constraints that can be enforced throughout the sequence. [less]
Interpolating and Approximating Implicit Surfaces from Polygon Soup
Chen Shen, James F. O'Brien, Jonathan Shewchuk
SIGGRAPH 2004
This paper describes a method for building interpolating or approximating implicit surfaces from polygonal data. The user can choose to generate a surface that exactly ... [more] This paper describes a method for building interpolating or approximating implicit surfaces from polygonal data. The user can choose to generate a surface that exactly interpolates the polygons, or a surface that approximates the input by smoothing away features smaller than some user-specified size. The implicit functions are represented using a moving least-squares formulation with constraints integrated over the polygons. The paper also presents an improved method for enforcing normal constraints and an iterative procedure for ensuring that the implicit surface tightly encloses the input vertices. [less]
Radiance Caching and Local Geometry Correction
Okan Arikan, David A. Forsyth, James F. O'Brien
SIGGRAPH 2004 Tech Sketch
We present a final gather algorithm which splits the irradiance integral into two components. One component captures the incident radiance due to distant surfaces. This ... [more] We present a final gather algorithm which splits the irradiance integral into two components. One component captures the incident radiance due to distant surfaces. This incident radiance due to far field illumination is represented as a spatially varying field of spherical harmonic coefficients. Since distant surfaces do not cause rapid changes in incident radiance, this field is smooth and slowly varying and can be computed quickly and represented efficiently.In contrast, nearby surfaces may create drastic changes in irradiance, because their positions on the visible hemisphere can change quickly. We can find such nearby surfaces (scene triangles) by a local search. By assuming nearby surfaces are always visible, we can correct the far field irradiance estimate we obtain using the spherical harmonics, and restore the high frequency detail in indirect lighting. This correction can be performed efficiently because finding nearby surfaces is a local operation. [less]
An Opponent Process Approach to Modeling the Blue Shift of the Human Color Vision System
Brian Barsky, Todd Kosloff, Steven D. Upstill
APGV 2004
Low light level affects human visual perception in various ways. Visual acuity is reduced and scenes appear bluer, darker, less saturated, and with reduced contrast. We confine ... [more] Low light level affects human visual perception in various ways. Visual acuity is reduced and scenes appear bluer, darker, less saturated, and with reduced contrast. We confine our attention to an approach to modeling the appearance of the bluish cast in dim light, which is known as blue shift. Both photographs and computer-generated images of night scenes can be made to appear more realistic by understanding these phenomena as well as how they are produced by the retina. The retina comprises two kinds of photoreceptors, called rods and cones. The rods are more sensitive in dim light than are the cones. Although there are three different kinds of cones with different spectral sensitivity curves, all rods have the same spectral response curve. Consequently, rods provide luminance information but no color discrimination. Thus, when the light is too dim to fully excite the cones, scenes appear desaturated. The opponent process theory of color vision [Hurvich and Jameson 1957] states that the outputs of the rods and cones are encoded as red-green, yellow-blue, and white-black opponent channels. We model loss of saturation and blue shift in this opponent color space. [less]
Rendering Skewed Plane of Sharp Focus and Associated Depth of Field
Brian Barsky, Egon Pasztor
SIGGRAPH 2004
Depth of field is the region of a scene that is in focus in an image. This is measured relative to a plane-of-sharp focus. When using a physical camera, this plane is perpendicular ... [more] Depth of field is the region of a scene that is in focus in an image. This is measured relative to a plane-of-sharp focus. When using a physical camera, this plane is perpendicular to the optical axis of the camera lens, unless the camera is a view camera. This special camera enables many effects, including skewing the plane-of- sharp focus and associated depth of field. Using a view camera, the photographer can position and orient the lens plane and film plane independently; in fact, the film plane need not be perpendicular to the optical axis of the lens. This enables the photographer to control two unique types of effects: perspective correction, and arbitrary orientation of the plane-of- sharp-focus anywhere in the viewing volume. Perspective correction is vital for architecture photography, where it is desirable to maintain parallel vertical lines even when the view direction is angled up from the horizontal, as is the case, for example, in photographing a tall building from ground level. Vertical lines converge when they are not parallel to the film plane. This effect is not discussed in this sketch. The ability to orient the plane-of-sharp-focus seems to be unknown in computer graphics. Whenever depth of field has been rendered, it is always aligned with the viewing direction. Previous algorithms for rendering images with depth of field did not recognize that it can be possible for the volume of space that is "in focus" to be at any orientation with respect to the viewing direction (see Fig. 1). The effect is possible with a physical camera in the case of a view camera. [less]
Vision-Realistic Rendering: Simulation of the Scanned Foveal Image from Wavefront Data of Human Subjects
Brian Barsky
APGV 2004
We introduce the concept of vision-realistic rendering – the com- puter generation of synthetic images that incorporate the charac- teristics of a particular individual ... [more] We introduce the concept of vision-realistic rendering – the com- puter generation of synthetic images that incorporate the charac- teristics of a particular individual’s entire optical system. Specif- ically, this paper develops a method for simulating the scanned foveal image from wavefront data of actual human subjects, and demonstrates those methods on sample images. First, a subject’s optical system is measured by a Shack- Hartmann wavefront aberrometry device. This device outputs a measured wavefront which is sampled to calculate an object space point spread function (OSPSF). The OSPSF is then used to blur in- put images. This blurring is accomplished by creating a set of depth images, convolving them with the OSPSF, and finally compositing to form a vision-realistic rendered image. Applications of vision-realistic rendering in computer graphics as well as in optometry and ophthalmology are discussed. [less]
Triple Product Wavelet Integrals for All-Frequency Relighting
Ren Ng, Ravi Ramamoorthi, Pat Hanrahan
SIGGRAPH 2004
This paper focuses on efficient rendering based on pre-computed light transport, with realistic materials and shadows under all-frequency direct lighting such as environment ... [more] This paper focuses on efficient rendering based on pre-computed light transport, with realistic materials and shadows under all-frequency direct lighting such as environment maps. The basic difficulty is representation and computation in the 6D space of light direction, view direction, and surface position. While image-based and synthetic methods for real-time rendering have been proposed, they do not scale to high sampling rates with variation of both lighting and viewpoint. Current approaches are therefore limited to lower dimensionality (only lighting or viewpoint variation, not both) or lower sampling rates (low frequency lighting and materials). We propose a new mathematical and computational analysis of pre-computed light transport. We use factored forms, separately pre-computing and representing visibility and material properties. Rendering then requires computing triple product integrals at each vertex, involving the lighting, visibility and BRDF. Our main contribution is a general analysis of these triple product integrals, which are likely to have broad applicability in computer graphics and numerical analysis. We first determine the computational complexity in a number of bases like point samples, spherical harmonics and wavelets. We then give efficient linear and sublinear-time algorithms for Haar wavelets, incorporating non-linear wavelet approximation of lighting and BRDFs. Practically, we demonstrate rendering of images under new lighting and viewing conditions in a few seconds, significantly faster than previous techniques. [less]
Spectral Surface Reconstruction from Noisy Point Clouds
Ravi Kolluri, Jonathan Shewchuk, James F. O'Brien
SGP 2004
We introduce a noise-resistant algorithm for reconstructing a watertight surface from point cloud data. It forms a Delaunay tetrahedralization, then uses a variant of ... [more] We introduce a noise-resistant algorithm for reconstructing a watertight surface from point cloud data. It forms a Delaunay tetrahedralization, then uses a variant of spectral graph partitioning to decide whether each tetrahedron is inside or outside the original object. The reconstructed surface triangulation is the set of triangular faces where inside and outside tetrahedra meet. Because the spectral partitioner makes local decisions based on a global view of the model, it can ignore outliers, patch holes and undersampled regions, and surmount ambiguity due to measurement errors. Our algorithm can optionally produce a manifold surface. We present empirical evidence that our implementation is substantially more robust than several closely related surface reconstruction programs. [less]
An Energy-Driven Approach to Linkage Unfolding
Jason Cantarella, Erik Demaine, Hayley Iben, James F. O'Brien
SoCG 2004
We present a new algorithm for unfolding planar polygonal linkages without self-intersection based on following the gradient flow of a "repulsive" energy function. This ...
[more]
We present a new algorithm for unfolding planar polygonal linkages without self-intersection based on following the gradient flow of a "repulsive" energy function. This algorithm has several advantages over previous methods. (1) The output motion is represented explicitly and exactly as a piecewise-linear curve in angle space. As a consequence, an exact snapshot of the linkage at any time can be extracted from the output in strongly polynomial time (on a real RAM supporting arithmetic, sin and arcsin). (2) Each linear step of the motion can be computed exactly in O(n2) time on a real RAM where n is the number of vertices. (3) We explicitly bound the number of linear steps (and hence running time) as a polynomial in n and the ratio between the maximum edge length and the initial minimum distance between a vertex and an edge. (4) Our method is practical and easy to implement. We provide a publicly accessible Java applet that implements the algorithm.
Best paper award at SoCG 2004.
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2003
Motion Synthesis from Anotations
Okan Arikan, David Forsyth, James F. O'Brien
SIGGRAPH 2003
This paper describes a framework that allows a user to synthesize human motion while retaining control of its qualitative properties. The user paints a timeline with annotations ... [more] This paper describes a framework that allows a user to synthesize human motion while retaining control of its qualitative properties. The user paints a timeline with annotations --- like walk, run or jump --- from a vocabulary which is freely chosen by the user. The system then assembles frames from a motion database so that the final motion performs the specified actions at specified times. The motion can also be forced to pass through particular configurations at particular times, and to go to a particular position and orientation. Annotations can be painted positively (for example, must run), negatively (for example, may not run backwards) or as a don't-care. The system uses a novel search method, based around dynamic programming at several scales, to obtain a solution efficiently so that authoring is interactive. Our results demonstrate that the method can generate smooth, natural-looking motion. The annotation vocabulary can be chosen to fit the application, and allows specification of composite motions (run and jump} simultaneously, for example). The process requires a collection of motion data that has been annotated with the chosen vocabulary. This paper also describes an effective tool, based around repeated use of support vector machines, that allows a user to annotate a large collection of motions quickly and easily so that they may be used with the synthesis algorithm. [less]
Animating Suspended Particle Explosions
Bryan Feldman, James F. O'Brien, Okan Arikan
SIGGRAPH 2003
This paper describes a method for animating suspended particle explosions. Rather than modeling the numerically troublesome, and largely invisible blast wave, the method ... [more] This paper describes a method for animating suspended particle explosions. Rather than modeling the numerically troublesome, and largely invisible blast wave, the method uses a relatively stable incompressible fluid model to account for the motion of air and hot gases. The fluid's divergence field is adjusted directly to account for detonations and the generation and expansion of gaseous combustion products. Particles immersed in the fluid track the motion of particulate fuel and soot as they are advected by the fluid. Combustion is modeled using a simple but effective process governed by the particle and fluid systems. The method has enough flexibility to also approximate sprays of burning liquids. This paper includes several demonstrative examples showing air bursts, explosions near obstacles, confined explosions, and burning sprays. Because the method is based on components that allow large time integration steps, it only requires a few seconds of computation per frame for the examples shown. [less]
All-Frequncy Shadows Using Non-linear Wavelet Lighting Approximation
Ren Ng, Ravi Ramamoorthi, Pat Hanrahan
SIGGRAPH 2003
We present a method, based on pre-computed light transport, for real-time rendering of objects under all-frequency, time-varying illumination represented as a high-resolution ... [more] We present a method, based on pre-computed light transport, for real-time rendering of objects under all-frequency, time-varying illumination represented as a high-resolution environment map. Current techniques are limited to small area lights, with sharp shadows, or large low-frequency lights, with very soft shadows. Our main contribution is to approximate the environment map in a wavelet basis, keeping only the largest terms (this is known as a non-linear approximation). We obtain further compression by encoding the light transport matrix sparsely but accurately in the same basis. Rendering is performed by multiplying a sparse light vector by a sparse transport matrix, which is very fast. For accurate rendering, using non-linear wavelets is an order of magnitude faster than using linear spherical harmonics, the current best technique. [less]
Spectral Watertight Surface Reconstruction
Ravi Kolluri, Jonathan Shewchuk, James F. O'Brien
SIGGRAPH 2003 Tech Sketch
We use spectral partitioning to reconstruct a watertight surface from point cloud data. This method is particularly effective for noisy and undersampled point sets with ... [more] We use spectral partitioning to reconstruct a watertight surface from point cloud data. This method is particularly effective for noisy and undersampled point sets with outliers, because decisions about the reconstructed surface are based on a global view of the model. [less]
Investigating Occlusion and Discretization Problems in Image Space Blurring Techniques
Brian Barsky, Michael J. Tobias, Daniel R. Horn, Derrick P. Chu
VVG 2003
Traditional computer graphics methods render images that appear sharp at all depths. Adding blur can add realism to a scene, provide a sense of scale, and draw a viewer’s attention ... [more] Traditional computer graphics methods render images that appear sharp at all depths. Adding blur can add realism to a scene, provide a sense of scale, and draw a viewer’s attention to a particular region of a scene. Our image based blur algorithm needs to distinguish whether a portion of an image is either from a single object or is part of more than one object. This motivates two approaches to identify objects after an image has been rendered. We illustrate how these techniques can be used in conjunction with our image space method to add blur to a scene. [less]
Interactive Deformation Using Modal Analysis with Constraints
Kris Hauser, Chen Shen, James F. O'Brien
Graphics Interface 2003
Modal analysis provides a powerful tool for efficiently simulating the behavior of deformable objects. This paper shows how manipulation, collision, and other constraints ... [more] Modal analysis provides a powerful tool for efficiently simulating the behavior of deformable objects. This paper shows how manipulation, collision, and other constraints may be implemented easily within a modal framework. Results are presented for several example simulations. These results demonstrate that for many applications the errors introduced by linearization are acceptable, and that the resulting simulations are fast and stable even for complex objects and stiff materials. [less]
Camera Models and Optical Systems Used in Computer Graphics: Part I, Object Based Techniques
Brian Barsky, Daniel R. Horn, Stanley A. Klein, Jeffrey A. Pang, Meng Yu
ICCSA 03
Images rendered with traditional computer graphics techniques, such as scanline rendering and ray tracing, appear focused at all depths. However, there are advantages to ... [more] Images rendered with traditional computer graphics techniques, such as scanline rendering and ray tracing, appear focused at all depths. However, there are advantages to having blur, such as adding realism to a scene or drawing attention to a particular place in a scene. In this paper we describe the optics underlying camera models that have been used in computer graphics, and present object space techniques for rendering with those models. In our companion paper [3], we survey im- age space techniques to simulate these models. These techniques vary in both speed and accuracy. [less]
Camera Models and Optical Systems Used in Computer Graphics: Part II, Image Based Techniques
Brian Barsky, Daniel R. Horn, Stanley A. Klein, Jeffrey A. Pang, Meng Yu
ICCSA 03
In our companion paper [5], we described the optics underlying camera models that have been used in computer graphics, and presented object space techniques for rendering ... [more] In our companion paper [5], we described the optics underlying camera models that have been used in computer graphics, and presented object space techniques for rendering with those models. In this paper, we survey image space techniques to simulate these models, and address topics including linear filtering, ray distribution buffers, light fields, and simulation techniques for interactive applications. [less]
2002
An Energy-Driven Approach to Linkage Unfolding
Jason Cantarella, Erik Demaine, Hayley Iben, James F. O'Brien
DIMACS 2002
In this paper, we introduce a new energy-driven approach for straightening polygonal arcs and convexifying polygonal cycles without self-intersection based on following ... [more] In this paper, we introduce a new energy-driven approach for straightening polygonal arcs and convexifying polygonal cycles without self-intersection based on following the gradient flow of a "repulsive" energy function. [less]
Modelling with Implicit Surfaces that Interpolate
Greg Turk, James F. O'Brien
TOG
We introduce new techniques for modelling with interpolating implicit surfaces. This form of implicit surface was first used for problems of surface reconstruction and ... [more] We introduce new techniques for modelling with interpolating implicit surfaces. This form of implicit surface was first used for problems of surface reconstruction and shape transformation, but the emphasis of our work is on model creation. These implicit surfaces are described by specifying locations in 3D through which the surface should pass, and also identifying locations that are interior or exterior to the surface. A 3D implicit function is created from these constraints using a variational scattered data interpolation approach, and the iso-surface of this function describes a surface. Like other implicit surface descriptions, these surfaces can be used for CSG and interference detection, may be interactively manipulated, are readily approximated by polygonal tilings, and are easy to ray trace. A key strength for model creation is that interpolating implicit surfaces allow the direct specification of both the location of points on the surface and the surface normals. These are two important manipulation techniques that are difficult to achieve using other implicit surface representations such as sums of spherical or ellipsoidal Gaussian functions ("blobbies"). We show that these properties make this form of implicit surface particularly attractive for interactive sculpting using the particle sampling technique introduced by Witkin and Heckbert. Our formulation also yields a simple method for converting a polygonal model to a smooth implicit model, as well as a new way to form blends between objects. [less]
Graphical Modeling and Animation of Ductile Fracture
James F. O'Brien, Adam Bargteil, Jessica Hodgins
SIGGRAPH 2002
In this paper, we describe a method for realistically animating ductile fracture in common solid materials such as plastics and metals. The effects that characterize ductile ... [more] In this paper, we describe a method for realistically animating ductile fracture in common solid materials such as plastics and metals. The effects that characterize ductile fracture occur due to interaction between plastic yielding and the fracture process. By modeling this interaction, our ductile fracture method can generate realistic motion for a much wider range of materials than could be realized with a purely brittle model. This method directly extends our prior work on brittle fracture [O'Brien and Hodgins, SIGGRAPH 99]. We show that adapting that method to ductile as well as brittle materials requires only a simple to implement modification that is computationally inexpensive. This paper describes this modification and presents results demonstrating some of the effects that may be realized with it. [less]
Synthesizing sounds from rigid-body simulations
James F. O'Brien, Chen Shen, Christine Gatchalian
SCA 2002
This paper describes a real-time technique for generating realistic and compelling sounds that correspond to the motions of rigid objects. By numerically precomputing ... [more] This paper describes a real-time technique for generating realistic and compelling sounds that correspond to the motions of rigid objects. By numerically precomputing the shape and frequencies of an object's deformation modes, audio can be synthesized interactively directly from the force data generated by a standard rigid-body simulation. Using sparse-matrix eigen-decomposition methods, the deformation modes can be computed efficiently even for large meshes. This approach allows us to accurately model the sounds generated by arbitrarily shaped objects based only on a geometric description of the objects and a handful of material parameters. [less]
Modal Analysis for Real-Time Viscoelastic Deformation
Chen Shen, Kris Hauser, Christine Gatchalian, James F. O'Brien
SIGGRAPH 2002 Tech Sketch
This technical sketch describes how a standard analysis technique known as modal decomposition can be used for real-time model- ing of viscoelastic deformation. While ... [more] This technical sketch describes how a standard analysis technique known as modal decomposition can be used for real-time model- ing of viscoelastic deformation. While most prior work on inter- active deformation has relied on geometrically simple models and advantageously selected material parameters to achieve interactive speeds, the approach described here has two qualities that we believe should be required of a real-time deformation method: the simulation cost is decoupled from both the model’s geometric complexity and from stiffness of the material’s parameters. Additionally, the simulation may be advanced at arbitrarily large time-steps without introducing objectionable errors such as artificial damping. [less]
Modeling the Accumulation of Wind-Driven Snow
Bryan Feldman, James F. O'Brien
SIGGRAPH 2002 Tech Sketch
This technical sketch presents a method for modeling the appearance of snow drifts formed by the accumulation of wind-blown snow near buildings and other obstacles. Our ... [more] This technical sketch presents a method for modeling the appearance of snow drifts formed by the accumulation of wind-blown snow near buildings and other obstacles. Our method combines previous work on snow accumulation with techniques for incompressible fluid flows. By computing the three-dimensional flow of air in the volume around the obstacles our method is able to model how the snow is convected, deposited, and lifted by the wind. The results demonstrate realistic snow accumulation patterns with deep windward and leeward drifts, furrows, and low accumulation in wind shadowed areas. [less]
2001
Synthesizing Sounds from Physically Based Motion
James F. O'Brien, Perry R. Cook, Georg Essl
SIGGRAPH 2001
The goal of this work is to develop techniques for approximating sounds that are generated by the motions of solid objects. Our methods builds on previous work in the field ... [more] The goal of this work is to develop techniques for approximating sounds that are generated by the motions of solid objects. Our methods builds on previous work in the field of physically based animation that use deformable models to simulate the behavior of the solid objects. As the motions of the objects are computed, their surfaces are analyzed to determine how the motion will induce acoustic pressure waves in the surrounding medium. The waves are propogated to the listener where the results are used to generate sounds corresponding to the behavior of the simulated objects. [less]
Image Based Rendering and Illumination Using Spherical Mosaics
Chen Shen, Heung-Yeung Shum, James F. O'Brien
SIGGRAPH 2001 Tech Sketch
The work described here extends the concentric mosaic representation developed by Shum and He to spherical mosaics that allow the viewer greater freedom of movement. Additionally ... [more] The work described here extends the concentric mosaic representation developed by Shum and He to spherical mosaics that allow the viewer greater freedom of movement. Additionally, by precomputing maps for diffuse and specular lighting terms, we use high dynamic range image data to compute realistic illumination for objects that can be interactively manipulated within the scene. [less]
Implicit Surfaces that Interpolate
Greg Turk, GHuong Quynh Dinh, James F. O'Brien, Gary Yngve
Shape Modeling International 2001
Implicit surfaces are often created by summing a collection of radial basis functions. Recently, researchers have begun to create implicit surfaces that exactly interpolate ... [more] Implicit surfaces are often created by summing a collection of radial basis functions. Recently, researchers have begun to create implicit surfaces that exactly interpolate a given set of points by solving a simple linear system to assign weights to each basis function. Due to their ability to interpolate, these implicit surfaces are more easily controllable than traditional "blobby" implicits. There are several additional forms of control over these surfaces that make them attractive for a variety of applications. Surface normals may be directly specified at any location over the surface, and this allows the modeller to pivot the normal while still having the surface pass through the constraints. The degree of smoothness of the surface can be controlled by changing the shape of the basis functions, allowing the surface to be pinched or smooth. On a point-by-point basis the modeller may decide whether a constraint point should be exactly interpolated or approximated. Applications of these implicits include shape transformation, creating surfaces from computer vision data, creation of an implicit surface from a polygonal model, and medical surface reconstruction. [less]
2000
Animating Fracture
James F. O'Brien, Jessica Hodgins
CACM
We have developed a simulation technique that uses non-linear finite element analysis and elastic fracture mechanics to compute physically plausible motion for three-dimensional ... [more] We have developed a simulation technique that uses non-linear finite element analysis and elastic fracture mechanics to compute physically plausible motion for three-dimensional, solid objects as they break, crack, or tear. When these objects deform beyond their mechanical limits, the system automatically determines where fractures should begin and in what directions they should propagate. The system allows fractures to propagate in arbitrary directions by dynamically restructuring the elements of a tetrahedral mesh. Because cracks are not limited to the original element boundaries, the objects can form irregularly shaped shards and edges as they shatter. The result is realistic fracture patterns such as the ones shown in our examples. This paper presents an overview of the fracture algorithm, the details are presented in our ACM SIGGRAPH 1999 and 2002 papers. [less]
Combining Active and Passive Simulations for Secondary Motion
James F. O'Brien, Victor Zordan, Jessica Hodgins
CG&A
Objects that move in response to the actions of a main character often make an important contribution to the visual richness of an animated scene. We use the term "secondary ... [more] Objects that move in response to the actions of a main character often make an important contribution to the visual richness of an animated scene. We use the term "secondary motion" to refer to passive motions generated in response to the movements of characters and other objects or environmental forces. Secondary motions aren't normally the mail focus of an animated scene, yet their absence can distract or disturb the viewer, destroying the illusion of reality created by the scene. We describe how to generate secondary motion by coupling physically based simulations of passive objects to actively controlled characters. [less]
Animating Explosions
Gary D. Yngve, James F. O'Brien, Jessica K. Hodgins
SIGGRAPH 2000
In this paper, we introduce techniques for animating explosions and their effects. The primary effect of an explosion is a disturbance that causes a shock wave to propagate ... [more] In this paper, we introduce techniques for animating explosions and their effects. The primary effect of an explosion is a disturbance that causes a shock wave to propagate through the surrounding medium. This disturbance determines the behavior of nearly all other secondary effects seen in explosions. We simulate the propagation of an explosion through the surrounding air using a computational fluid dynamics model based on the equations for compressible, viscous flow. To model the numerically stable formulation of shocks along blast wave fronts, we employ an integration method that can handle steep gradients without introducing inappropriate damping. The system includes two-way coupling between solid objects and surrounding fluid. Using this technique, we can generate a variety of effects including shaped explosive charges, a projectile propelled from a chamber by an explosion, and objects damaged by a blast. With appropriate rendering techniques, our explosion model can be used to create such visual effects such as fireballs, dust clouds, and the refraction of light caused by a blast wave. [less]
Automatic Joint Parameter Estimation from Magnetic Motion Capture Data
James F. O'Brien, Robert Bodenheimer, Gabriel Brostow, Jessica Hodgins
GI 2000
This paper describes a technique for using magnetic motion capture data to determine the joint parameters of an articulated hierarchy. This technique makes it possible ... [more] This paper describes a technique for using magnetic motion capture data to determine the joint parameters of an articulated hierarchy. This technique makes it possible to determine limb lengths, joint locations, and sensor placement for a human subject without external measurements. Instead, the joint parameters are inferred with high accuracy from the motion data acquired during the capture session. The parameters are computed by performing a linear least squares fit of a rotary joint model to the input data. A hierarchical structure for the articulated model can also be determined in situations where the topology of the model is not known. Once the system topology and joint parameters have been recovered, the resulting model can be used to perform forward and inverse kinematic procedures. We present the results of using the algorithm on human motion capture data, as well as validation results obtained with data from a simulation and a wooden linkage of known dimensions. [less]
1999
Graphical Modeling and Animation of Brittle Fracture
James F. O'Brien, Jessica Hodgins
SIGRAPH 1999
In this paper, we augment existing techniques for simulating flexible objects to include models for crack initiation and propagation in three-dimensional volumes. By ...
[more]
In this paper, we augment existing techniques for simulating flexible objects to include models for crack initiation and propagation in three-dimensional volumes. By analyzing the stress tensors computed over a finite element model, the simulation determines where cracks should initiate and in what directions they should propagate. We demonstrate our results with animations of breaking bowls, cracking walls, and objects that fracture when they collide. By varying the shape of the objects, the material properties, and the initial conditions of the simulations, we can create strikingly different effects ranging from a wall that shatters when it is hit by a wrecking ball to a bowl that breaks in two when it is dropped on edge.
This paper received the SIGGRAPH 99 Impact Award.
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Shape Transformation Using Variational Implicit Functions
James F. O'Brien, Greg Turk
SIGGRAPH 1999
Traditionally, shape transformation using implicit functions is performed in two distinct steps: 1) creating two implicit functions, and 2) interpolating between these ... [more] Traditionally, shape transformation using implicit functions is performed in two distinct steps: 1) creating two implicit functions, and 2) interpolating between these two functions. We present a new shape transformation method that combines these two tasks into a single step. We create a transformation between two N-dimensional objects by casting this as a scattered data interpolation problem in N + 1 dimensions. For the case of 2D shapes, we place all of our data constraints within two planes, one for each shape. These planes are placed parallel to one another in 3D. Zero-valued constraints specify the locations of shape boundaries and positive-valued constraints are placed along the normal direction in towards the center of the shape. We then invoke a variational interpolation technique (the 3D generalization of thin-plate interpolation), and this yields a single implicit function in 3D. Intermediate shapes are simply the zero-valued contours of 2D slices through this 3D function. Shape transformation between 3D shapes can be performed similarly by solving a 4D interpolation problem. To our knowledge, ours is the first shape transformation method to unify the tasks of implicit function creation and interpolation. The transformations produced by this method appear smooth and natural, even between objects of differing topologies. If desired, one or more additional shapes may be introduced that influence the intermediate shapes in a sequence. Our method can also reconstruct surfaces from multiple slices that are not restricted to being parallel to one another. [less]
Animating Sand, Mud, and Snow
Robert Sumner, James F. O'Brien, Jessica Hodgins
CGF
Computer animations often lack the subtle environmental changes that should occur due to the actions of the characters. Squealing car tires usually leave no skid marks ... [more] Computer animations often lack the subtle environmental changes that should occur due to the actions of the characters. Squealing car tires usually leave no skid marks, airplanes rarely leave jet trails in the sky, and most runners leave no footprints. In this paper, we describe a simulation model of ground surfaces that can be deformed by the impact of rigid body models of animated characters. To demonstrate the algorithms, we show footprints made by a runner in sand, mud, and snow as well as bicycle tire tracks, a bicycle crash, and a falling runner. The shapes of the footprints in the three surfaces are quite different, but the effects were controlled through only five essentially independent parameters. To assess the realism of the resulting motion, we compare the simulated footprints to human footprints in sand. [less]
1998
Perception of Human Motion with Different Geometric Models
Jessica Hodgins, James F. O'Brien, Jack Tumblin
TVCG 1998
Human figures have been animated using a variety of geometric models including stick figures, polygonal models, and NURBS-based models with muscles, flexible skin, or clothing ... [more] Human figures have been animated using a variety of geometric models including stick figures, polygonal models, and NURBS-based models with muscles, flexible skin, or clothing. This paper reports on experimental results indicating that a viewer’s perception of motion characteristics is affected by the geometric model used for rendering. Subjects were shown a series of paired motion sequences and asked if the two motions in each pair were the same or different. The motion sequences in each pair were rendered using the same geometric model. For the three types of motion variation tested, sensitivity scores indicate that subjects were better able to observe changes with the polygonal model than they were with the stick figure model. [less]
Animating Sand, Mud, and Snow
Robert Sumner, James F. O'Brien, Jessica Hodgins
GI 98
Computer animations often lack the subtle environmental changes that should occur due to the actions of the char- acters. Squealing car tires usually leave no skid marks ...
[more]
Computer animations often lack the subtle environmental changes that should occur due to the actions of the char- acters. Squealing car tires usually leave no skid marks, airplanes rarely leave jet trails in the sky, and most run- ners leave no footprints. In this paper, we describe a sim- ulation model of ground surfaces that can be deformed by the impact of rigid body models of animated characters. To demonstrate the algorithms, we show footprints made by a runner in sand, mud, and snow as well as bicycle tire tracks, a bicycle crash, and a falling runner. The shapes of the footprints in the three surfaces are quite different, but the effects were controlled through only five essentially independent parameters. To assess the realism of the re- sulting motion, we compare the simulated footprints to video footage of human footprints in sand.
Received the Michael A. J. Sweeney award for best student paper.
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1997
Combining Active and Passive Simulations for Secondary Motion
James F. O'Brien, Victor Zordan, Jessica Hodgins
SIGGRAPH 1997 Tech Sketch
The secondary motion of passive objects in the scene is essential for appealing and natural-looking animated characters, but because of the difficulty of controlling ... [more] The secondary motion of passive objects in the scene is essential for appealing and natural-looking animated characters, but because of the difficulty of controlling the motion of the primary character, most research in computer animation has largely ignored secondary motion. We use dynamic simulation to generate secondary motion. Simulation is an appropriate technique because secondary motion is passive, dictated only by forces from the environment or the primary actor and not from an internal source of energy in the object itself. Secondary motion does not lend itself easily to keyframing, procedural approaches, or motion capture because of the many degrees of freedom that must move in synchrony with the primary motion of the animated figure. [less]
Do Geometric Models Affect Judgments of Human Motion?
Jessica Hodgins, James F. O'Brien, Jack Tumblin
GI 97
Human figures have been animated using a wide variety of geometric models including stick figures, polygonal models, and NURBS-based models with muscles, flexible skin ... [more] Human figures have been animated using a wide variety of geometric models including stick figures, polygonal models, and NURBS-based models with muscles, flexible skin, or clothing. This paper re- ports on experiments designed to ascertain whether a viewer’s perception of motion characteristics is affected by the geometric model used for rendering. Subjects were shown a series of paired motion sequences and asked if the two motions in each pair were "the same" or "different." The two motion sequences in each pair used the same geometric model. For each trial, the pairs of motion sequences were grouped into two sets where one set was rendered with a stick figure model and the other set was rendered with a polygonal model. Sensitivity measures for each trial indicate that for these sequences subjects were better able to discriminate motion variations with the polygonal model than with the stick figure model. [less]
1995
Animating Human Athletics
Jessica Hodgins, Wayne Wooten, David Brogan, James F. O'Brien
SIGGRAPH 1995
This paper describes algorithms for the animation of men and women performing three dynamic athletic behaviors: running, bicycling, and vaulting. We animate these behaviors ... [more] This paper describes algorithms for the animation of men and women performing three dynamic athletic behaviors: running, bicycling, and vaulting. We animate these behaviors using control algorithms that cause a physically realistic model to perform the desired maneuver. For example, control algorithms allow the simulated humans to maintain balance while moving their arms, to run or bicycle at a variety of speeds, and to perform a handspring vault. Algorithms for group behaviors allow a number of simulated bicyclists to ride as a group while avoiding simple patterns of obstacles. We add secondary motion to the animations with spring- mass simulations of clothing driven by the rigid-body motion of the simulated human. For each simulation, we compare the computed motion to that of humans performing similar maneuvers both qualitatively through the comparison of real and simulated video images and quantitatively through the comparison of simulated and biomechanical data. [less]
Dynamic Simulation of Splashing Fluids
James F. O'Brien, Jessica Hodgins
Computer Animation 95
In this paper we describe a method for modeling the dynamic behavior of splashing fluids. The model simulates the behavior of a fluid when objects impact or float on its surface ... [more] In this paper we describe a method for modeling the dynamic behavior of splashing fluids. The model simulates the behavior of a fluid when objects impact or float on its surface. The forces generated by the objects create waves and splashes on the surface of the fluid. To demonstrate the realism and limitations of the model, images from a computer- generated animation are presented and compared with video frames of actual splashes occurring under similar Abstract: initial conditions. [less]