F
glFrontFace
Chapter 6
158
glFrontFace
glFrontFace: define front- and back-facing polygons.
C Specification
void glFrontFace(
GLenum mode)
Parameters
mode Specifies the orientation of front-facing polygons. GL_CW and
GL_CCW are accepted. The initial value is GL_CCW.
Description
In a scene composed entirely of opaque closed surfaces, back-facing polygons are never
visible. Eliminating these invisible polygons has the obvious benefit of speeding up the
rendering of the image. To enable and disable elimination of back-facing polygons, call
glEnable and glDisable with argument GL_CULL_FACE.
The projection of a polygon to window coordinates is said to have clockwise winding if an
imaginary object following the path from its first vertex, its second vertex, and so on, to
its last vertex, and finally back to its first vertex, moves in a clockwise direction about
the interior of the polygon. The polygon’s winding is said to be counterclockwise if the
imaginary object following the same path moves in a counterclockwise direction about
the interior of the polygon. glFrontFace specifies whether polygons with clockwise
winding in window coordinates, or counterclockwise winding in window coordinates, are
taken to be front-facing. Passing GL_CCW to mode selects counterclockwise polygons as
front-facing; GL_CW selects clockwise polygons as front-facing. By default,
counterclockwise polygons are taken to be front-facing.
Errors
• GL_INVALID_ENUM is generated if mode is not an accepted value.
• GL_INVALID_OPERATION is generated if glFrontFace is executed between the
execution of glBegin and the corresponding execution of glEnd.
Associated Gets
glGet with argument GL_FRONT_FACE
See Also
glCullFace,
glLightModel
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