Shanat

Simple, zero-dependency shader sketch for the Raspberry Pi 3B's composite video output

Building and running

To build, run build.sh from the root. The executable is bin/shanat.

Code structure

In the C++ code, all the low-level code related to EGL, GBM and DRM (don't even ask me what all that shit is) is tucked away in horrors.h and horrors.cpp. The code in main.cpp is a render loop where only the OpenGL calls remain. That's not pretty either, but it's relatively little, and it's the same old ugliness you find in WebGL.

The specific sketch with the balls dancing around the Lissajous curve is under /lissaj. I'm calling shaders_incldes.sh as a "pre-build step" so the shaders can live in their own files with a GLSL extension. The step puts them into string literals inside shaders.h.

geo and fps contain utilities for vector math and frame rate management. They do reinvent the wheel, but this way the sketch has no dependencies.

webgl-proto-lissaj is not strictly part of this sketch. It is its own thing where I prototyped the animation in WebGL+Javascript.

Environment

I've built this specifically for the Raspberry Pi 3B. I'm running the latest stable OS as of September 2025, which is based on Debian Bookworm. They've changed around a lot of things realated to display management over the last few versions, so YMMV.

I installed the OS's headless, "light" version, so there is no graphical UI. I'm connecting over SSH.

• In /boot/firmware/config.txt, you need to use KMS (not the earlier "fake" thing called FKMS) with the following line:
  dtoverlay=vc4-kms-v3d,composite
• In /boot/firmware/cmdline.txt add vc4.tv_norm=PAL video=HDMI-A-1:720x576@50M

I have nothing plugged into HDMI. If you want composite out and HDMI, you'll probably need to play around with hdmi_force_hotplug and hdmi_ignore_hotplug in config.txt.

Prerequisites

The code is dependency-free in that it uses no frameworks or graphics libraries. However, it does need the system modules installed in order to access the GPU (Broadcom VideoCore IV). In my headless setup that meant manually installing these packages:

sudo apt install -y \
 build-essential git cmake pkg-config \
 libegl1-mesa-dev libsdl2-dev libgles2-mesa-dev \
 libdrm-dev libgbm-dev

Not needed to build, but these are also useful to have around:

sudo apt install -y \
 xvfb mesa-utils libdrm-tests

When looking around for the available displays, these two commands were my best friend:

ls -l /dev/dri/
sudo modetest -D /dev/dri/card0

In modetest's output, you want to look for connectors like those below. They are buried in a lot of noise. What I needed for the composite output was Composite-1 to show up. This info might help you figure out your own setup.

id	encoder	status		name		size (mm)	modes	encoders
33	0	disconnected	HDMI-A-1 	0x0		0	32
51	50	unknown Composite-1 	0x0		4	50