gl_VertexID - contains the index of the current vertex

       Copyright © 2011-2014 Khronos Group

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

       in int gl_VertexID ;.SH "DESCRIPTION"

       gl_VertexID is a vertex language input variable that holds an integer index for the vertex. The index is
       impliclty generated by glDrawArrays and other commands that do not reference the content of the
       GL_ELEMENT_ARRAY_BUFFER, or explicitly generated from the content of the GL_ELEMENT_ARRAY_BUFFER by
       commands such as glDrawElements.

       gl_VertexID - contains the index of the current vertex

gl_InstanceID()

       ┌─────────────┬───────────────────────────────────────────────────────────────────────────────────┐
       │             │        OpenGLShadingLanguageVersion                                            │
       ├─────────────┼──────┬──────┬──────┬──────┬──────┬──────┬──────┬──────┬──────┬──────┬──────┬──────┤
       │ Function    │ 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_VertexID │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │
       └─────────────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┘