glStencilFunc
[New
- Windows 95, OEM Service Release 2]
The glStencilFunc
function sets the function and reference value for stencil testing.
void glStencilFunc(
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GLenum func, |
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GLint ref, |
|
|
GLuint mask |
|
|
); |
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Parameters
func
The test
function. The following eight tokens are valid:
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.
ref
The reference
value for the stencil test. The ref parameter is clamped to the range [0,2^n
- 1], where n is
the number of bitplanes in the stencil buffer.
mask
A mask that
is ANDed with both the reference value and the stored stencil value when
the test is done.
Remarks
Stenciling,
like z-buffering, enables and disables drawing on a per-pixel basis. You
draw into the stencil planes using OpenGL 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. Actions taken based on the
outcome of the stencil test are specified with glStencilOp.
The func
parameter is a symbolic constant that determines the stencil comparison
function. It accepts one of the eight values shown above. The ref
parameter 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.
The mask parameter 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
preceding 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.
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.
The following
functions retrieve information related to glStencilFunc:
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
Error Codes
The following
are the error codes generated and their conditions.
|
Error
Code |
Condition |
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GL_INVALID_ENUM
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func was not one of the eight accepted values. |
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GL_INVALID_OPERATION
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glStencilFunc was called between a call to glBegin and the
corresponding call to glEnd. |
See Also