gl_FrontFacing

Available only in the fragment language, 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 . One way to compute this area is:

$a=\frac{1}{2}{\sum }_{j=0}^{n-1}{x}_w^i{y}_w^{i+1}-x_w^{i+1}{y}_w^i$

where $x_w^i$ and $y_w^i$ are the x and y window coordinates of the i th 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_FragCoord , glClipControl

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