gl_FrontFacing — indicates whether a primitive is front or back facing
in bool gl_FrontFacing ; 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:
where and 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.
| OpenGL Shading Language Version | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Variable Name | 1.10 | 1.20 | 1.30 | 1.40 | 1.50 | 3.30 | 4.00 | 4.10 | 4.20 | 4.30 | 4.40 | 4.50 |
| gl_FrontFacing | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ |
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