OpenGL man pages
glStencilFunc - set function and reference value for stencil testing
void glStencilFunc( GLenum func,
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
ref Specifies the reference value for the stencil test. ref is clamped
to the range [0,2 -1], where n is the number of bitplanes in the
mask Specifies a mask that is ANDed with both the reference value and the
stored stencil value when the test is done.
Stenciling, like z-buffering, enables and disables drawing on a per-pixel
basis. You draw into the stencil planes using GL drawing primitives, then
render geometry and images, 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.
The test is enabled by glEnable and glDisable with argument
GL_STENCIL_TEST. Actions taken based on the outcome of the stencil test
are specified with glStencilOp.
func is a symbolic constant that determines the stencil comparison
function. It accepts one of eight values, shown below. ref is an integer
reference value that is used in the stencil comparison. It is clamped to
the range [0,2n-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 [0,2n-1], where n is the number of bitplanes in the stencil buffer.
Here are the values 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.
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
GL_INVALID_ENUM is generated if func is not one of the eight accepted
GL_INVALID_OPERATION is generated if glStencilFunc is executed between the
execution of glBegin and the corresponding execution of glEnd.
glGet with argument GL_STENCIL_FUNC
glGet with argument GL_STENCIL_VALUE_MASK
glGet with argument GL_STENCIL_REF
glGet with argument GL_STENCIL_BITS
glIsEnabled with argument GL_STENCIL_TEST
glAlphaFunc, glBlendFunc, glDepthFunc, glEnable, glIsEnabled, glLogicOp,
Fri Dec 6 11:18:03 EST 1996
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