glStencilFunc man page on RedHat
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GLSTENCILFUNC(3G) OpenGL Manual GLSTENCILFUNC(3G)
NAME
glStencilFunc - set front and back function and reference value for
stencil testing
C SPECIFICATION
void glStencilFunc(GLenum func, GLint ref, GLuint mask);
PARAMETERS
func
Specifies the test function. Eight symbolic constants 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 0 2 n - 1, where 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.
DESCRIPTION
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 the test, call glEnable() and
glDisable() with argument GL_STENCIL_TEST. To specify actions based on
the outcome of the stencil test, call glStencilOp() or
glStencilOpSeparate().
There can be two separate sets of func, ref, and mask parameters; one
affects back-facing polygons, and the other affects front-facing
polygons as well as other non-polygon primitives. glStencilFunc() sets
both front and back stencil state to the same values. Use
glStencilFuncSeparate() to set front and back stencil state to
different values.
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 0 2 n - 1, where 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 0 2 n - 1, where 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 ( ref & mask ) < ( stencil & mask ).
GL_LEQUAL
Passes if ( ref & mask ) <= ( stencil & mask ).
GL_GREATER
Passes if ( ref & mask ) > ( stencil & mask ).
GL_GEQUAL
Passes if ( ref & mask ) >= ( stencil & mask ).
GL_EQUAL
Passes if ( ref & mask ) = ( stencil & mask ).
GL_NOTEQUAL
Passes if ( ref & mask ) != ( stencil & mask ).
GL_ALWAYS
Always passes.
NOTES
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.
glStencilFunc() is the same as calling glStencilFuncSeparate() with
face set to GL_FRONT_AND_BACK.
ERRORS
GL_INVALID_ENUM is generated if func is not one of the eight accepted
values.
ASSOCIATED GETS
glGet() with argument GL_STENCIL_FUNC, GL_STENCIL_VALUE_MASK,
GL_STENCIL_REF, GL_STENCIL_BACK_FUNC, GL_STENCIL_BACK_VALUE_MASK,
GL_STENCIL_BACK_REF, or GL_STENCIL_BITS
glIsEnabled() with argument GL_STENCIL_TEST
SEE ALSO
glBlendFunc(), glDepthFunc(), glEnable(), glLogicOp(),
glStencilFuncSeparate(), glStencilMask(), glStencilMaskSeparate(),
glStencilOp(), glStencilOpSeparate()
COPYRIGHT
Copyright © 1991-2006 Silicon Graphics, Inc. This document is licensed
under the SGI Free Software B License. For details, see
http://oss.sgi.com/projects/FreeB/.
AUTHORS
opengl.org
opengl.org 03/16/2013 GLSTENCILFUNC(3G)
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