Contains the invocation index of the current shader.
In the tessellation control language,gl_InvocationIDcontains the index of the output patch vertex assigned to the shader invocation. It is assigned an integer value in the range [0, N-1] where N is the number of output patch vertices.
In the geometry language,gl_InvocationIDidentifies the invocation number assigned to the geometry shader invocation. It is assigned an integer value in the range [0, N-1] where N is the number of geometry shader invocations per primitive.
 Versions 1.50 to 3.30 - geometry shaders only.
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