OpenGL Shading Language (abbreviated: GLSL or GLslang), is a high-level shading language with a syntax based on the C programming language. It was created by the OpenGL ARB (OpenGL Architecture Review Board) to give developers more direct control of the graphics pipeline.

Some benefits of using GLSL are:

  • Cross-platform compatibility on multiple operating systems, including GNU/Linux, macOS and Windows.
  • The ability to write shaders that can be used on any hardware vendor's graphics card that supports the OpenGL Shading Language.
  • Each hardware vendor includes the GLSL compiler in their driver, thus allowing each vendor to create code optimized for their particular graphics card’s architecture.


Fragment usage of the graphics pipeline is restricted to a single fragment program which will be executed by the GPU for each pixels of the graphical score/canvas.

The fragment program is written in GLSL (OpenGL Shading Language) which has a syntax quite similar to the C language but is much simpler to learn.

You can do anything from ray-traced 3D to drawing simple lines and define their behaviors with Fragment, the only requirement is high school mathematics.

Here is a simple example of GLSL code that Fragment accept which just set all pixels to black and all oscillators to off:

void main () {
  gl_FragColor = vec4(0., 0., 0., 0.);
  synthOutput = vec4(0., 0., 0., 0.); // WebGL 2.0 only

Pre-defined uniforms

Fragment has many pre-defined uniforms which can be used to access different informations.

Here is a list of Fragment pre-defined uniforms

  • vec2 resolution viewport resolution (in pixels)
  • float globalTime playback time (in seconds)
  • float baseFrequencyscore base frequency
  • float octavescore octave range
  • vec4 mousenormalized mouse pixel coords.
  • vec4 dateyear, month, day, time in seconds
  • sampler2D iInputNImported data access, typical usage : texture2D(iInput0, uv);
  • sampler2D pFrame The previous frame available as a texture
  • sampler2D pFrameSynthThe previous synth frame available as a texture (WebGL 2)
  • int frame The current frame
  • float htoy take a frequency as argument and return its vertical position on the canvas (in pixels units)
  • float fline take a frequency as argument and return either 1 or 0 (shortcut to draw a horizontal line)
  • vec4[N] keyboardMIDI note-on/note-off events : frequency, velocity, elapsed time since the key was pressed, MIDI channel
  • vec4[N+1] keyboardEnhanced MIDI (MPE) : pitch bend, timbre (CC74), pressure (aftertouch), release velocity
  • vec3[N] pKeystore the previous note-off for each channels, frequency, velocity, elapsed time since the key was pressed

Reference cards

The Khronos Group (authors of the language specification) released several reference cards of the GLSL specification, the reference cards are compact reference of the full language specification which can be used in conjunctions with code samples to learn the OpenGL Shading Language quickly.

Since there is plenty of resources to learn the OpenGL Shading Language, the documentation provided in this section will only provide the official reference cards, many GLSL tutorials are available on the internet and are valid for Fragment.

As Fragment can use WebGL 2.0 if your browser has support for it, the reference cards for both WebGL 1.0 GLSL and WebGL 2.0 GLSL (more functionalities) is provided, keep in mind that if Fragment run fine with your browser, you can safely use the WebGL 1.0 GLSL reference card as a starting point:


Fragment support GLSL 3.0 version automatically if it detect support for the WebGL 2.0 API (which is recommended), GLSL 3.0 allow to use dynamical indexes with arrays among many other things, it also allow to use shortcuts.

If WebGL 2.0 is supported, there is actually two output in the fragment shader, gl_FragColor or fragColor for the visuals and synthOutput for the pixels data which will be used by the audio synthesis engine, this allow to do visuals alongside stereo audio synthesis, when WebGL 2.0 is enabled and the EXT_color_buffer_float extension is available, the pixels data output will be encoded as 32-bit float, this give higher quality sounds and possibilities.

There is also many applications that let you create stunning visuals in your browser by the same method, one of the most popular one and compatible with Fragment (by using the convert ShaderToy button of the toolbar) is ShaderToy, this program let you build visuals and audio at the same time, just like Fragment but with a more conventional approach non-pixels based audio synthesis.