gl_FrontFacing.3G man page

gl_FrontFacing — indicates whether a primitive is front or back facing


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.

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