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
|OpenGL Shading Language Version|
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