gl_PrimitiveIDIn - contains the index of the current primitive

       Copyright © 2011-2014 Khronos Group

[FIXME: source]                                    11/18/2024                               GL_PRIMITIVEIDIN(3G)

       in int gl_PrimitiveIDIn ;.SH "DESCRIPTION"

       gl_PrimitiveIDIn is a geometry language input variable that 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.

       gl_PrimitiveIDIn - contains the index of the current primitive

gl_InstanceID()

       ┌──────────────────┬───────────────────────────────────────────────────────────────────────────────────┐
       │                  │        OpenGLShadingLanguageVersion                                            │
       ├──────────────────┼──────┬──────┬──────┬──────┬──────┬──────┬──────┬──────┬──────┬──────┬──────┬──────┤
       │ Variable         │ 1.10 │ 1.20 │ 1.30 │ 1.40 │ 1.50 │ 3.30 │ 4.00 │ 4.10 │ 4.20 │ 4.30 │ 4.40 │ 4.50 │
       │ Name             │      │      │      │      │      │      │      │      │      │      │      │      │
       ├──────────────────┼──────┼──────┼──────┼──────┼──────┼──────┼──────┼──────┼──────┼──────┼──────┼──────┤
       │ gl_PrimitiveIDIn │  -   │  -   │  -   │  -   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │
       └──────────────────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┘