2011 Khronos Group
gl_FrontFacing 3G
gl_FrontFacing indicates whether a primitive is front or back facing

## Declaration

`in bool gl_FrontFacing;`

## 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=1 2 ∑ 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.

## Version Support

Variable | Version 1.10 | Version 1.20 | Version 1.30 | Version 1.40 | Version 1.50 | Version 3.30 | Version 4.00 | Version 4.10
--- | --- | --- | --- | --- | --- | --- | --- | ---
gl_FrontFacing | Y | Y | Y | Y | Y | Y | Y | Y

## See Also

gl_FragCoord

## Copyright

Copyright 2011 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. http://opencontent.org/openpub/.