glStencilOp

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, 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.

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 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.

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.

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