1991-2006 Silicon Graphics, Inc.
glStencilOp 3G
glStencilOp set front and back stencil test actions

## C Specification

void `glStencilOp` GLenum sfail GLenum dpfail GLenum dppass

## Parameters

sfail

  Specifies the action to take when the stencil test fails. Eight symbolic constants are accepted: `GL_KEEP`, `GL_ZERO`, `GL_REPLACE`, `GL_INCR`, `GL_INCR_WRAP`, `GL_DECR`, `GL_DECR_WRAP`, and `GL_INVERT`. The initial value is `GL_KEEP`.

dpfail

  Specifies the stencil action when the stencil test passes, but the depth test fails. dpfail accepts the same symbolic constants as sfail. The initial value is `GL_KEEP`.

dppass

  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. dppass accepts the same symbolic constants as sfail. 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 the test, call glEnable and glDisable with argument `GL_STENCIL_TEST`; to control it, call glStencilFunc or glStencilFuncSeparate.

There can be two separate sets of sfail, dpfail, and dppass parameters; one affects back-facing polygons, and the other affects front-facing polygons as well as other non-polygon primitives. glStencilOp sets both front and back stencil state to the same values. Use glStencilOpSeparate to set front and back stencil state to different values.

`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 sfail 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 glStencilFunc.

`GL_INCR`

  Increments the current stencil buffer value. Clamps to the maximum representable unsigned value.

`GL_INCR_WRAP`

  Increments the current stencil buffer value. Wraps stencil buffer value to zero when incrementing the maximum representable unsigned value.

`GL_DECR`

  Decrements the current stencil buffer value. Clamps to 0.

`GL_DECR_WRAP`

  Decrements the current stencil buffer value. Wraps stencil buffer value to the maximum representable unsigned value when decrementing a stencil buffer value of zero.

`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 `glStencilOp` specify stencil buffer actions that depend on whether subsequent depth buffer tests succeed (dppass) or fail (dpfail) (see glDepthFunc). The actions are specified using the same eight symbolic constants as sfail. Note that dpfail is ignored when there is no depth buffer, or when the depth buffer is not enabled. In these cases, sfail and dppass 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`.

glStencilOp is the same as calling glStencilOpSeparate with face set to `GL_FRONT_AND_BACK`.

## Errors

`GL_INVALID_ENUM` is generated if sfail, dpfail, or dppass is any value other than the eight defined symbolic constant values.

## Associated Gets

glGet with argument `GL_STENCIL_FAIL`, `GL_STENCIL_PASS_DEPTH_PASS`, `GL_STENCIL_PASS_DEPTH_FAIL`, `GL_STENCIL_BACK_FAIL`, `GL_STENCIL_BACK_PASS_DEPTH_PASS`, `GL_STENCIL_BACK_PASS_DEPTH_FAIL`, or `GL_STENCIL_BITS`

glIsEnabled with argument `GL_STENCIL_TEST`

## See Also

glBlendFunc, glDepthFunc, glEnable, glStencilFunc, glStencilFuncSeparate, glStencilMask, glStencilMaskSeparate, glStencilOpSeparate

## Copyright

Copyright 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.