Indicates whether a primitive is front or back facing.
Available only in the fragment language,gl_FrontFacingis 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 byglFrontFace. One way to compute this area is:
where [math][/math] and [math][/math] are the x and y window coordinates of theith 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
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