Name
gl.StencilOp -- set stencil test actions
Synopsis
gl.StencilOp(fail, zfail, zpass)
Function
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 gl.Enable() and gl.Disable() with argument #GL_STENCIL_TEST; to control it, call gl.StencilFunc().

gl.StencilOp() 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 eight 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 gl.StencilFunc().

#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 2^n - 1, where n is the value returned by querying #GL_STENCIL_BITS.

The other two arguments to gl.StencilOp() specify stencil buffer actions that depend on whether subsequent depth buffer tests succeed (zpass) or fail (zfail) (See gl.DepthFunc for details.). The actions are specified using the same eight 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.

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 gl.StencilOp().

Please consult an OpenGL reference manual for more information.

Inputs
fail
specifies the action to take when the stencil test fails; the initial value is #GL_KEEP (see above)
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
Errors
#GL_INVALID_ENUM is generated if fail, zfail, or zpass is any value other than the eight defined constant values.

#GL_INVALID_OPERATION is generated if gl.StencilOp() is executed between the execution of gl.Begin() and the corresponding execution of gl.End().

Associated gets
gl.Get() with one of the following arguments: #GL_STENCIL_FAIL, #GL_STENCIL_PASS_DEPTH_PASS, #GL_STENCIL_PASS_DEPTH_FAIL, #GL_STENCIL_BACK_FAIL, or #GL_STENCIL_BITS

gl.IsEnabled() with argument #GL_STENCIL_TEST

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