// In tessellation control, tessellation evaluation, and // fragment shaders
in int gl_PrimitiveID ;
// In geometry shaders
out int gl_PrimitiveID ;
gl_PrimitiveID is a tessellation control, tessellation evaluation and fragment language input variable. For the tessellation control and tessellation evaluation languages, it holds the number of primitives processed by the shader since the current set of rendering primitives was started. The first primitive processed by the drawing command is numbered zero and the primitive ID counter is incremented after every individual point, line or triangle primitive is processed. For triangles drawn in point or line mode, the primitive ID counter is incremented only once, even through multiple points or lines may actually be drawn. Restarting a primitive topology using the primitive restart index has no effect on the primitive ID counter.
In the geometry language, gl_PrimitiveID is an output variable that is passed to the corresponding gl_PrimitiveID input variable in the fragment shader. If no geomery shader is present then gl_PrimitiveID in the fragment language behaves identically as it would in the tessellation control and evaluation languages. If a geometry shader is present but does not write to gl_PrimitiveID, the value of gl_PrimitiveID in the fragment shader is undefined.
OpenGL Shading Language Version
|gl_PrimitiveID (Fragment and Geometry Languages)||-||-||-||-||✔||✔||✔||✔||✔||✔||✔||
|gl_PrimitiveID (Tessellation Control and Evaluation Languages)||-||-||-||-||-||-||✔||✔||✔||✔||✔||
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Copyright © 2011-2014 Khronos Group