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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 GL_UPPER_LEFT .

Version Support[edit]

gl_FrontFacing 1.1+

See Also[edit]

 gl_FragCoord , glClipControl 


Copyright© 2011-2014 Khronos Group. This material may be distributed subject to the terms and conditions set forth in the Open Publication License, v 1.0, 8 June 1999.