gl_Position - contains the position of the current vertex

       Copyright © 2011-2014 Khronos Group

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

gl_Position is a member of the gl_PerVertex named block:

           out gl_PerVertex {
               vec4 gl_Position;
               float gl_PointSize;
               float gl_ClipDistance[];
           };

       In the vertex, tessellation evaluation and geometry languages, a single global instance of the
       gl_PerVertex named block is available and its gl_Position member is an output that receives the
       homogeneous vertex position. It may be written at any time during shader execution. The value written to
       gl_Position will be used by primitive assembly, clipping, culling and other fixed functionality
       operations, if present, that operate on primitives after vertex processing has occurred.

       In the tessellation control language, the gl_PerVertex named block is used to construct an array,
       gl_out[], whose gl_Position members hold the homogeneous control point position, which become available
       as inputs to the subsequent tessellation evaluation shader.

       The value of gl_Position (or the gl_Position member of the gl_out[] array, in the case of the
       tessellation control shader) is undefined after the vertex, tessellation control, and tessellation
       evaluation shading stages if the corresponding shader executable does not write to gl_Position. It is
       also undefined after the geometry processing stage if the geometry shader executable calls EmitVertex()
       without having written gl_Position since the last call to EmitVertex() (or hasn't written it at all).

       In the tessellation control, tessellation evaluation and geometry languages, the gl_PerVertex named block
       is used to construct an array, gl_in[] of per-vertex or per-control point inputs whose content represents
       the corresponding outputs written by the previous stage.

       gl_Position - contains the position of the current vertex

gl_PointSize(), gl_ClipDistance()

       ┌─────────────┬───────────────────────────────────────────────────────────────────────────────────┐
       │             │        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_Position │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │
       └─────────────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┘

       Versions 1.10 to 1.40 - vertex shader only.

       Versions 1.50 to 3.30 - vertex and geometry shaders only.