Compute Shader/Defined Inputs

Compute Shaders have the following built-in input variables.

in uvec3 gl_NumWorkGroups;
in uvec3 gl_WorkGroupID;
in uvec3 gl_LocalInvocationID;
in uvec3 gl_GlobalInvocationID;
in uint  gl_LocalInvocationIndex;

gl_NumWorkGroups

This variable contains the number of work groups passed to the dispatch function.

gl_WorkGroupID

This is the current work group for this shader invocation. Each of the XYZ components will be on the half-open range [0, gl_NumWorkGroups.XYZ).

gl_LocalInvocationID

This is the current invocation of the shader within the work group. Each of the XYZ components will be on the half-open range [0, gl_WorkGroupSize.XYZ).

gl_GlobalInvocationID

This value uniquely identifies this particular invocation of the compute shader among all invocations of this compute dispatch call. It's a short-hand for the math computation:

gl_WorkGroupID * gl_WorkGroupSize + gl_LocalInvocationID;

gl_LocalInvocationIndex

This is a 1D version of gl_LocalInvocationID. It identifies this invocation's index within the work group. It is short-hand for this math computation:

  gl_LocalInvocationIndex =
          gl_LocalInvocationID.z * gl_WorkGroupSize.x * gl_WorkGroupSize.y +
          gl_LocalInvocationID.y * gl_WorkGroupSize.x + 
          gl_LocalInvocationID.x;