Computational geometry is the study of efficient algorithms for solving geometric problems. The nearest neighbor problem involves identifying one point, out of a set of points, that is nearest to the query point according to some measure of distance. The nearest neighborhood problem involves identifying the locus of points lying nearer to the query point than to any other point in the set. This package provides functions for solving these and related problems in the case of planar points and the Euclidean distance metric.
ConvexHull [{{x1,y1},{x2,y2},…}] compute the convex hull of a set of points in the plane
compute the Delaunay triangulation of a set of points in the plane
compute the Voronoi diagram of a set of points in the plane
Computational geometry functions.
The convex hull of a set S is the boundary of the smallest set containing S. The Voronoi diagram of S is the collection of nearest neighborhoods for each of the points in S. For points in the plane, these neighborhoods are polygons. The Delaunay triangulation of S is a triangulation of the points in S such that no triangle contains a point of S in its circumcircle. This is equivalent to connecting the points in S according to whether their neighborhood polygons share a common side.
This loads the package.
Here is a list of points in the plane.
This gives the indices of the points lying on the convex hull in counterclockwise order.
Duplicate points are ignored.
This gives the counterclockwise vertex adjacency list for each point in the Delaunay triangulation. For example, the entry {1,{4,3,2}} indicates that the first point in data2D is connected in counterclockwise order to the fourth, third, and second points.
While DelaunayTriangulation need only specify the connections between points, VoronoiDiagram must specify both a set of diagram vertices and the connections between those vertices. Another difference between the two functions is that while a triangulation consists of segments, a diagram consists of both segments and rays. For example, in the case of a Voronoi diagram, points in the interior of the convex hull will have nearest neighborhoods that are closed polygons, but the nearest neighborhoods of points on the convex hull will be open polygons.
These considerations make the output of VoronoiDiagram more complex than that of DelaunayTriangulation . The diagram is given as a list of diagram vertices followed by a diagram vertex adjacency list. The finite vertices of the diagram are listed first in the vertex list. The vertices lying at infinity have head Ray and are listed last.
This assigns the list of Voronoi diagram vertices to vorvert and the Voronoi diagram vertex adjacency list to vorval.
The first vertex in
vorvert is a finite diagram vertex having coordinates
{-0.0158537,8.44146}. The last vertex in
vorvert is a point at infinity. This point is represented by a
Ray object having origin
{10.5172,3.46115} and containing
{13.95,0.2}.
Each entry in vorval gives the index of a point in data2D followed by a counterclockwise list of the Voronoi diagram vertices that comprise the point's nearest neighborhood polygon.
Here is the Voronoi polygon vertex adjacency list for the first point in data2D.
This selects the coordinates of the polygon vertices from
vorvert. The first two vertices have head
List , while the last two have head
Ray . Thus, the Voronoi polygon associated with the first point in
data2D is open and is defined by a segment and two rays.
compute the Voronoi diagram using the Delaunay triangulation vertex adjacency list delval
compute the Voronoi diagram using the Delaunay triangulation vertex adjacency list delval and the convex hull index list convexhull
Computing the Voronoi diagram using the Delaunay triangulation and the convex hull.
This computes the Voronoi diagram of data2D more efficiently by making use of the Delaunay triangulation vertex adjacency list.
Here the Voronoi diagram is computed using both the Delaunay triangulation and the convex hull.
plot the Delaunay triangulation of the points
plot the graph depicted by the counterclockwise list of indices in indexlist
plot the graph depicted by the vertex adjacency list val
DiagramPlot [{{x1,y1},{x2,y2},…}] plot the Voronoi diagram of the points
plot the diagram depicted by the vertex list diagvert and the vertex adjacency list diagval
plot the surface according to the Delaunay triangulation established by projecting the points onto the x‐y plane
plot the surface according to the triangulation depicted by the vertex adjacency list trival
Computational geometry plotting functions.
The default of
PlanarGraphPlot is a plot of the Delaunay triangulation of the points.
This plots the convex hull of the points.
Here is an alternative triangulation.
This plots the triangulation of data2D given by trival.
The default of
DiagramPlot is a plot of the Voronoi diagram of the points.
Here is an alternative set of diagram vertices.
Here is an alternative diagram vertex adjacency list.
This plots the diagram of data2D given by diagvert and diagval.
Here is a set of three‐dimensional points having the same x-y coordinates as data2D.
The default of
TriangularSurfacePlot is a plot of the
z coordinates according to the connectivity established by the Delaunay triangulation of the
x-
y coordinates.
This plots the z coordinates according to the connectivity established by the triangulation trival.
give the minimum set of points needed to define the convex hull
give both the Delaunay triangulation vertex adjacency list and the convex hull
plot the Delaunay triangulation without labels
plot the Voronoi diagram without labels
plot the Voronoi diagram with the outermost ray plus (n-1) of the outermost diagram vertices trimmed
Options for computational geometry functions.
This gives the minimum set of points needed to define the convex hull.
This returns both the Delaunay triangulation and the convex hull.
Here is a set of random numbers uniformly distributed on ×.
This computes the Voronoi diagram of random.
The diagram plot is dominated by outlier vertices.
TrimPoints->2 means that the diagram is plotted so that both the outermost ray and the outermost vertex are eliminated.
The
TrimPoints option can be used to magnify the diagram until the points in
random fill the plot.
give True if the vertex adjacency list trival represents a Delaunay triangulation of the points, and False otherwise
Testing for a Delaunay triangulation.
delval is a Delaunay triangulation, so this returns
True .
This returns
False because
trival is not a Delaunay triangulation.
compute the bounded Voronoi diagram of a set of points {x,y}, where the bound is the convex polygon described by the points {a,b}
BoundedDiagram [{{a1,b1},{a2,b2},…},{{x1,y1},{x2,y2},…},delval]
compute the bounded Voronoi diagram using the Delaunay triangulation vertex adjacency list delval
BoundedDiagram [{{a1,b1},{a2,b2},…},{{x1,y1},{x2,y2},…},delval,convexhull]
compute the bounded Voronoi diagram using the Delaunay triangulation vertex adjacency list delval and the convex hull index list convexhull
Computing the bounded Voronoi diagram.
When spatial data is collected within a finite region of the plane, the unbounded Voronoi diagram of the points may not offer an accurate picture of the region of influence of each point. A tile on the periphery of the diagram will be open, indicating an infinite region of influence, when in fact an open tile is simply due to the limited extent of the spatial sampling. It is sometimes useful to intersect the unbounded Voronoi diagram of the data with the boundary of the convex region from which the data was collected. Then each point in the data can be associated with a closed tile or finite region of influence.
BoundedDiagram begins by finding the unbounded Voronoi diagram. It then works counterclockwise around the boundary, integrating bounding polygon vertices into the diagram, and deleting Voronoi diagram vertices falling outside of the boundary. Bounding an open tile of the Voronoi diagram allows you to approximate the true underlying closed tile you would have if the data collection had not been limited to a portion of the plane.
The bounded diagram is represented as two lists: (1) a vertex coordinate list, and (2) a vertex adjacency list, one entry for each point in the original unbounded diagram indicating the associated bounded polygon vertices in counterclockwise order.
Since BoundedDiagram requires the unbounded Voronoi diagram, the computation of the bounded diagram can be made more efficient by providing additional arguments, such as the Delaunay triangulation vertex adjacency list and the convex hull.
These are the coordinates of the rectangular region from which the data2D sample was drawn.
This assigns the list of bounded diagram vertices to diagvert1 and the bounded diagram vertex adjacency list to diagval1.
Here is a plot of the bounded diagram of data2D given by diagvert1 and diagval1.
TileAreas [{{x1,y1},{x2,y2},…},{{q1, q2,…},val}]
find the areas of the tiles centered on {xi,yi} and having vertices qj, as stipulated by the vertex adjacency list val
Computing tile areas.
You can make use of Voronoi diagrams to build spatial interaction models, or simply calculate the area of influence of individual tiles.
In an unbounded Voronoi diagram, some of the tiles have infinite area.
In a bounded diagram, each of the tiles has finite area.
This gives the tile areas scaled by the area of the rectangle from which the sample was drawn.