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
GL_ALWAYS. The initial value is GL_ALWAYS.
ref Specifies the reference value for the stencil test. ref is clamped to the range [0,2n-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
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 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.
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 [0,2n-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.
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.
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
GL_INVALID_ENUM is generated if func is not one of the eight accepted values.
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(3G), glBlendFunc(3G), glDepthFunc(3G), glEnable(3G), glIsEnabled(3G), glLogicOp(3G), glStencilOp(3G)