OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Knowing Where You Are

Once your compute shader is executing, it likely has the responsibility to set the value of one or more elements of some output array (such as an image or an array of atomic counters), or to read data from a specific location in an input array. To do this, you will need to know where in the local workgroup you are and where that workgroup is within the larger global workgroup. For these purposes, OpenGL provides several built-in variables to compute shaders. These built-in variables are implicitly declared as shown in Example 12.4.

Example 12.4. Declaration of Compute Shader Built-in Variables

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const uvec3 gl_WorkGroupSize;in    uvec3 gl_NumWorkGroups;in    uvec3 gl_LocalInvocationID;in    uvec3 ...