gl_InstanceID — contains the index of the current primitive in an instanced draw command
in int gl_InstanceID ;
gl_InstanceID is a vertex language input variable that holds the integer index of the current primitive in an instanced draw command such as
glDrawArraysInstanced. If the current primitive does not originate from an instanced draw command, the value of
gl_InstanceID is zero.
|OpenGL Shading Language Version|
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