glStencilFunc

NAME

glStencilFunc -- set function and reference value for stencil testing

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.
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.
mask: Specifies a mask that is ANDed with both the reference value and the stored stencil value when the test is done.

DESCRIPTION

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, 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.

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.

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 called between a call to glBegin and the corresponding call to 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