glStencilFunc — set function and reference value for stencil testing
void fsfuncglStencilFunc( | GLenum func, |
| GLint ref, | |
GLuint mask); |
func
Specifies the test function. Eight tokens are valid:
GL_NEVER,
GL_LESS,
GL_LEQUAL,
GL_GREATER,
GL_GEQUAL,
GL_EQUAL,
GL_NOTEQUAL, and
GL_ALWAYS. The initial value is
GL_ALWAYS.
ref
Specifies the reference value for the stencil test.
ref
is clamped to the range
n is
the number of bitplanes in the stencil buffer. The
initial value is 0.
mask
Specifies a mask that is ANDed with both the reference value and the stored stencil value when the test is done. The initial value is all 1's.
Stenciling, like depth-buffering, enables and disables drawing on a per-pixel basis. Stencil planes are first drawn into using GL drawing primitives, then geometry and images are rendered using the stencil planes to mask out portions of the screen. Stenciling is typically used in multipass rendering algorithms to achieve special effects, such as decals, outlining, and constructive solid geometry rendering.
The stencil test conditionally eliminates a pixel based
on the outcome of a comparison between the reference value and
the value in the stencil buffer. To enable and disable stencil
test, call
glEnable and
glDisable
with argument
GL_STENCIL_TEST.
Stencil test is initially disabled.
To specify actions based on the outcome of the stencil test, call
glStencilOp.
func
is a symbolic constant that determines the stencil comparison
function. It accepts one of eight values, shown in the
following list. ref
is an integer reference value that is used in the stencil
comparison. It is clamped to the range
n
is the number of bitplanes in the stencil buffer.
mask
is bitwise ANDed with both the reference value and the stored
stencil value, with the ANDed values participating in the
comparison.
If stencil
represents the value stored in the corresponding stencil buffer
location, the following list shows the effect of each
comparison function that can be specified by
func.
Only if the comparison succeeds is the pixel passed through
to the next stage in the rasterization process (see
glStencilOp).
All tests treat stencil
values as unsigned integers in the range
n
is the number of bitplanes in the stencil buffer.
The following values are accepted by
func:
GL_NEVER
Always fails.
GL_LESS
Passes if
GL_LEQUAL
Passes if
GL_GREATER
Passes if
GL_GEQUAL
Passes if
GL_EQUAL
Passes if
GL_NOTEQUAL
Passes if
GL_ALWAYS
Always passes.
Initially, the stencil test is disabled. If there is no stencil buffer, no stencil modification can occur and it is as if the stencil test always passes.
Copyright © 2003-2004 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.