gl_FrontFacing - indicates whether a primitive is front or back facing

       Copyright © 2011-2014 Khronos Group

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

       in bool gl_FrontFacing ;.SH "DESCRIPTION"

       Available only in the fragment language, gl_FrontFacing is an input variable whose value is true if the
       fragment belongs to a front-facing primitive and false otherwise. The determination of whether a triangle
       primitive is front-facing is made by examining the sign of the area of the triangle, including a possible
       reversal of this sign as controlled by glFrontFace. One way to compute this area is:

       a=12 ∑ j=0 n-1 x w i y w i+1 - x w i+1 y w i

       where x w i and y w i are the x and y window coordinates of the ith vertex of the n-vertex polygon.

       The sign of this computation is negated when the value of GL_CLIP_ORIGIN (the clip volume origin, set
       with glClipControl()) is GL_UPPER_LEFT.

       gl_FrontFacing - indicates whether a primitive is front or back facing

gl_FragCoord(), glClipControl()

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       │                │        OpenGLShadingLanguageVersion                                            │
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       ├────────────────┼──────┼──────┼──────┼──────┼──────┼──────┼──────┼──────┼──────┼──────┼──────┼──────┤
       │ gl_FrontFacing │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │  ✔   │
       └────────────────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┴──────┘