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Shady is a nifty CLI tool for rendering GLSL fragment shaders for easy development and hacking.



go install


The basic setup is a single fragment shader, like a regular fragments shader, calculates the color for each pixel. But instead of receiving vertex and normal and transformation information from the vertex shader, it defines it's own algorithm for shapes.

The best supported format/environment for shaders is that of Example:

void mainImage(out vec4 fragColor, in vec2 fragCoord) {
  vec2 uv = fragCoord.xy / iResolution.xy;
  fragColor = vec4(uv.x, uv.y, 0, 1);

Currently, the iTime, iTimeDelta, iFrame, iDate, iMouse, and iResolution, iChannelResolution uniforms are supported. Other uniforms are defined but not initialized.

See also for info on how to write shaders for Shadertoy.

Including other source files

To include another GLSL file, you may use the directive below:

#pragma use "path/to/file.glsl"

This allows you to use functions and such from other GLSL files so it becomes possible to create libraries. There is no namespacing or generation of forward function declarations, it just takes a source file and dumps it in the place of this directive much like C does. However, it does prevent including the same file more than once in recursive inclusion.

File paths are resolved relative to the source file that declared the include directive.


It is possible use resources like images, videos and audio from shaders in this environment by using the iChannelX samplers. On the website, one can select this resource input mapping using dialogs. This implementation requires these mappings to be specified in the shader source. The respective uniform is declared automatically.

Mappings can refer to other files such as images, videos and audio. Relative paths are resolved relative to the GLSL file that declared them.

Mappings are declared in a special directive that is parsed by shady. These are typically inserted at the top of the file. Its format is:

#pragma map <uniform name>=<loader>:<value>

Mappings can also be set on the command line when invoking shady by using the -map flag. The format is the same as described earlier. Mappings set on the command line take precedence over mappings declared in source files. There are no hard requirements for the namespace being the same as in the shader, so it is possible to e.g. override an image with a video. Example:

shady -i thing.glsl -ofmt x11 -g 1366x768 -f 60 -map myTexture=video:myVideo.mkv

uniform name is the name of the sampler uniform that is inserted into the source of the fragment shader. Unlike, which names all samples as iChannelX, the name can be of any value as long as it is a valid GLSL variable name. loader specifies how value should be interpreted. There are a couple of loaders that you can choose from:

The "builtin" loader

The builtin loader gives access to some of the presets that can be found on Shadertoy. Accepted values for builtin are:

  • Back Buffer: creates a sampler2D containing the previously rendered image.
  • RGBA Noise Small: creates a sampler2D texture with pseudo-random noise. The randomness is deterministic.
  • RGBA Noise Medium: the same as above, but bigger.

Example: Enable the sampler named iChannel0 as a noise texture:

#pragma map iChannel0=builtin:RGBA Noise Medium

The "image" loader

Setting the loader to image interprets the value as a path to an image file and creates a sampler2D containing a static texture containing the RGBA data of the image file. Supported formats are JPEG, PNG and GIF, the latter using the first frame of the animation for the texture.

For each mapped image, an additional vec3 uniform is created with the original size of the image named ${uniform name}Size. The Z component of this vector is reserved.


#pragma map myTexture=image:yoloswag.png

The "audio" loader

Audio files can be loaded as a texture with a size of 512x2. Row 0 contains the FFT of the current window and row 1 contains the actual sound wave. For regular audio files, The playback rate is determined by the duration and framerate flags. For realtime audio, the window is the most recently produced audio, skipping information if rendering can not keep up.

If the value is just a file, this file is used as audio. FFmpeg is invoked to decode the file, so any format supported by FFmpeg can be played.

For realtime audio, only raw PCM pipes are supported. The filename must be followed by the PCM format settings as ;<rate>:<channels>:<encoding>. encoding is the sign as s or u followed by the number of bits per sample and then the endianness as le or be, e.g. s16le.

Example: Map audio to the audio of an MP3 file:

#pragma map music=audio:whatever.mp3

The "video" loader

Using videos as textures is very similar to images, there is a sampler2D uniform containing the current video frame and a ${uniform name}Size vector for the resolution. There is an additional {uniform name}CurTime float which is the current time in seconds in the video. This value is the same as iTime, but wraps when the video is restarted from the beginning.

The sound of the video is not available, although this may be implemented in the future.


#pragma map video=video:party.mkv

The "buffer" loader

It is possible to map another shader as a texture by using the buffer loader. This is equivalent of just calling the mainImage function of this other shader and using the calculated color as texel. However, buffers have a separate resolution and Back Buffer. Because the render output of a buffer in raster format, the size of the texture should be specified in the mapping by appending ;WxH to the shader filename.

Like videos, the buffer is declared as a sampler2D along with a ${uniform name}Size vector.


#pragma map thing=buffer:other-shader.glsl;512x512

NOTE: Buffer support is not very well tested, your mileage may vary.

The "kinect" loader

If Shady was compiled using the kinect build tag, it is possible to use a Kinect's RGB and depth image in shaders. Just pass -tags kinect to go build when building and use #pragma map kinect=kinect:on to create a sampler2D of the Kinect's video stream. The alpha channel holds the depth image.

Internally, libfreenect is used which only supports the earlier Kinect versions for the XBox 360.

Combining with other tools


Ledcat is a program that can be used to control lots of LEDs over lots of protocols. Shady can be combined with Ledcat to bring the fireworks to your LED-displays!

It can be installed like this when you have the Rust Language:

cargo install ledcat

To aid development, Ledcat can be used to simulate a display in a terminal like this:

# LEDCAT_GEOMETRY is a special env var that Ledcat and Shady use to set the
# display size. It is also possible to use the -g flag on both programs.
export LEDCAT_GEOMETRY=128x128

shady -i example.glsl -ofmt rgb24 -f 20 | ledcat -f 20 show


FFmpeg may be used to render to video files:

# Render at 1024x768 at 20 fps and show it, the same as using `-ofmt x11`:
shady -i example.glsl -ofmt rgb24 -g 1024x768 -f 20 \
  | ffplay -f rawvideo -pixel_format rgb24 -video_size 1024x768 -f 20 -

# The same, but render 12 seconds to an MP4 file
shady -i example.glsl -ofmt rgb24 -g 1024x768 -f 10 \
  | ffmpeg -f rawvideo -pixel_format rgb24 -video_size 1024x768 \
    -framerate 10 -t 12 -i - example.mp4


Visualising the output of MPD is possible by adding the following to your MPD config:

audio_output {
  type   "fifo"
  name   "FIFO"
  path   "~/.mpd/mpd.fifo"
  format "22000:16:1"

And then creating a PCM audio mapping like this:

#pragma map music=audio:~/.mpd/mpd.fifo;22000:1:s16le


My performance is really bad

Some shaders can really ask a lot from a system, in these cases it may not be possible to animate real time. If it is acceptable to have the animation be of finite length and restart after a while, write a series of frame to a file, and load them in a loop.

# Render a 20 second loop to a file:
shady -i example.glsl -g 64x64 -f 60 -n $((20*60)) -ofmt rgb24 -o ./my-animation.bin

# Play the animation repeatedly:
while true; do
    cat ./my-animation.bin | ledcat -g 64x64 -f 60 show

Optionally, you could use something like gzip to reduce the file size.

EGL is not initialized, or could not be initialized

Headless rendering is possible. If $DISPLAY is unset because X11 is not running, try running shady with the EGL_PLATFORM env var set to surfaceless or drm.

If you still are not able to get shady to run headless, animate to a file and play from that file in real time. See above.


Error compiling fragment shader:
0:2(1): error: syntax error, unexpected NEW_IDENTIFIER

The above error could be caused by a precision mediump float; being present. Because this is an OpenGL ES directive, it is not supported. Try removing it or wrapping with a preprocessor macro:

#ifdef GL_ES
precision mediump float;


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