GLSTENCILFUNC() MachTen Programmer’s Manual GLSTENCILFUNC()

NAME
glStencilFunc - set function and reference value for sten-
cil testing

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C SPECIFICATION
void glStencilFunc( GLenum func,
GLint ref,
GLuint mask )

PARAMETERS
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 [0,$2 sup n - 1$], 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 refer-
ence 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. 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 render-
ing.

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 dis-
able the test, call glEnable and glDisable with argument
GL_STENCIL_TEST. 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 [0,$2 sup n - 1$], where $n$ is the
number of bitplanes in the stencil buffer. mask is bit-
wise 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 correspond-
ing 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 pro-
cess (see glStencilOp). All tests treat stencil values as
unsigned integers in the range [0,$2 sup n - 1$], 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.

ERRORS
GL_INVALID_ENUM is generated if func is not one of the
eight accepted values.

GL_INVALID_OPERATION is generated if glStencilFunc is exe-
cuted between the execution of glBegin and the correspond-
ing execution of glEnd.

ASSOCIATED GETS
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

SEE ALSO
glAlphaFunc, glBlendFunc, glDepthFunc, glEnable, glIsEn-
abled, glLogicOp, glStencilOp

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