GLSTENCILOP - Online Linux Manual PageSection : 3G
NAME"glStencilOp − set stencil test actions
C SPECIFICATIONvoid glStencilOp( GLenum fail, GLenum zfail, GLenum zpass )
PARAMETERSfail  Specifies the action to take when the stencil test fails. Six symbolic constants are accepted: ­GL_KEEP, ­GL_ZERO, ­GL_REPLACE, ­GL_INCR, ­GL_DECR, and ­GL_INVERT. The initial value is ­GL_KEEP. zfail  Specifies the stencil action when the stencil test passes, but the depth test fails. zfail accepts the same symbolic constants as fail. The initial value is ­GL_KEEP. zpass  Specifies the stencil action when both the stencil test and the depth test pass, or when the stencil test passes and either there is no depth buffer or depth testing is not enabled. zpass accepts the same symbolic constants as fail. The initial value is ­GL_KEEP.
DESCRIPTIONStenciling, 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 value in the stencil buffer and a reference value. To enable and disable the test, call ­glEnable and ­glDisable with argument ­GL_STENCIL_TEST; to control it, call ­glStencilFunc. ­glStencilOp takes three arguments that indicate what happens to the stored stencil value while stenciling is enabled. If the stencil test fails, no change is made to the pixel's color or depth buffers, and fail specifies what happens to the stencil buffer contents. The following six actions are possible. ­GL_KEEP  Keeps the current value. ­GL_ZERO  Sets the stencil buffer value to 0. ­GL_REPLACE  Sets the stencil buffer value to ref, as specified by ­glStencilFunc. ­GL_INCR  Increments the current stencil buffer value. Clamps to the maximum representable unsigned value. ­GL_DECR  Decrements the current stencil buffer value. Clamps to 0. ­GL_INVERT  Bitwise inverts the current stencil buffer value. Stencil buffer values are treated as unsigned integers. When incremented and decremented, values are clamped to 0 and 2n-1, where n is the value returned by querying ­GL_STENCIL_BITS. The other two arguments to ­glStencilOp specify stencil buffer actions that depend on whether subsequent depth buffer tests succeed (zpass) or fail (zfail) (see
­glDepthFunc). The actions are specified using the same six symbolic constants as fail. Note that zfail is ignored when there is no depth buffer, or when the depth buffer is not enabled. In these cases, fail and zpass specify stencil action when the stencil test fails and passes, respectively.
NOTESInitially the stencil test is disabled. If there is no stencil buffer, no stencil modification can occur and it is as if the stencil tests always pass, regardless of any call to ­glStencilOp.
ERRORS­GL_INVALID_ENUM is generated if fail, zfail, or zpass is any value other than the six defined constant values. ­GL_INVALID_OPERATION is generated if ­glStencilOp is executed between the execution of ­glBegin and the corresponding execution of ­glEnd.
ASSOCIATED GETS­glGet with argument ­GL_STENCIL_FAIL
­glGet with argument ­GL_STENCIL_PASS_DEPTH_PASS
­glGet with argument ­GL_STENCIL_PASS_DEPTH_FAIL
­glGet with argument ­GL_STENCIL_BITS
­glIsEnabled with argument ­GL_STENCIL_TEST
SEE ALSO­glAlphaFunc(3G), ­glBlendFunc(3G), ­glDepthFunc(3G), ­glEnable(3G), ­glLogicOp(3G), ­glStencilFunc(3G) 0
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