glStencilOp

NAME

glStencilOp - set stencil test actions

C SPECIFICATION

void glStencilOp(GLenum fail,
                 GLenum zfail,
                 GLenum zpass)

PARAMETERS

fail 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, GL_INVERT, GL_INCR_WRAP_EXT, and GL_DECR_WRAP_EXT. 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.

DESCRIPTION

Stenciling, like depth-buffering, enables and disables drawing on a per-pixel basis. GL drawing primitives are used to draw into the stencil planes which can then be used to mask out portions of the screen when rendering geometry and images. 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 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 zero.

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 zero.

GL_INVERT
Bitwise inverts the current stencil buffer value.

GL_INCR_WRAP_EXT
Increments the current stencil buffer value such that incrementing the maximum representable value results in a value of 0.

GL_DECR_WRAP_EXT
Decrements the current stencil buffer value such that decrementing the a value of 0 results in the maximum representable 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.

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 tests always pass, regardless of any call to glStencilOp.

EXTENSIONS

Some routines and constants for this function are part of an extension, not part of the core GL command set. The following extension names must be present in the string returned by glGetString when called with argument GL_EXTENSIONS to use these routines and constants.
GL_EXT_stencil_wrap
GL_INCR_WRAP_EXT and GL_DECR_WRAP_EXT are part of the EXT_stencil_wrap extension.

ERRORS

GL_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 GETS

glGet 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 ALSO

glAlphaFunc, glBlendFunc, glDepthFunc, glEnable, glLogicOp, glStencilFunc