GLSTENCILFUNCSEPARAT(3G)

NAME
glStencilFuncSeparate - set front and/or back function and reference value for stencil testing

C SPECIFICATION

void glStencilFuncSeparate(GLenum face, GLenum func, GLint ref, GLuint mask);

PARAMETERS

face
    Specifies whether front and/or back stencil state is updated. Three symbolic constants are valid: GL_FRONT, GL_BACK, and GL_FRONT_AND_BACK.

func
    Specifies the test function. Eight symbolic constants 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 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 reference 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 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() or glStencilOpSeparate().

There can be two separate sets of func, ref, and mask parameters; one affects back-facing polygons, and the other affects front-facing polygons as well as other non-polygon primitives. glStencilFunc() sets both front and back stencil state to the same values, as if glStencilFuncSeparate() were called with face set to GL_FRONT_AND_BACK.

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 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.

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

glStencilFuncSeparate is available only if the GL version is 2.0 or greater.

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 glStencilFuncSeparate is executed between the execution of glBegin() and the corresponding execution of glEnd().

ASSOCIATED GETS

glGet() with argument GL_STENCIL_FUNC, GL_STENCIL_VALUE_MASK, GL_STENCIL_REF, GL_STENCIL_BACK_FUNC, GL_STENCIL_BACK_VALUE_MASK, GL_STENCIL_BACK_REF, or GL_STENCIL_BITS

glIsEnabled() with argument GL_STENCIL_TEST

SEE ALSO

glAlphaFunc(), glBlendFunc(), glDepthFunc(), glEnable(), glLogicOp(), glStencilFunc(), glStencilMask(), glStencilMaskSeparate(), glStencilOp(), glStencilOpSeparate()

COPYRIGHT

Copyright © 2006 Khronos Group. This material may be distributed subject to the terms and conditions set forth in the Open Publication License, v 1.0, 8 June 1999. http://opencontent.org/openpub/.