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, 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.

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 value in the stencil buffer and a reference value. To enable and disable stencil test, call glEnable and glDisable with argument GL_STENCIL_TEST. To control it, call glStencilFunc. Stenciling is initially disabled.

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

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.

Associated Gets

glGet with argument GL_STENCIL_BITS

See Also

glAlphaFunc, glBlendFunc, glDepthFunc, glEnable, glGet, glLogicOp, glStencilFunc

Copyright

Copyright © 2003-2004 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.