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.
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.
|
|
OpenGL Shading Language Version
| |||||||||||
| Variable Name | 1.10 | 1.20 | 1.30 | 1.40 | 1.50 | 3.30 | 4.00 | 4.10 | 4.20 | 4.30 | 4.40 |
4.50
|
| gl_FrontFacing | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ |
✔
|
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. m[blue]http://opencontent.org/openpub/m[].