Name

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 xwi ywi+1 - xwi+1 ywi

where xwi and ywi are the x and y window coordinates of the ith vertex of the n-vertex polygon.

Version Support

Variable Version 1.00 Version 3.00
gl_FrontFacing Y Y

See Also

gl_FragCoord

Copyright

Copyright © 2011-2012 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/.