2011-2014
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=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.

## Version Support

gl_FrontFacing

## See Also

gl_FragCoord

## Copyright

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. http://opencontent.org/openpub/.