1991-2006
Silicon Graphics, Inc.
glStencilFunc
3G

# glStencilFunc

set front and back function and reference value for stencil testing

## C Specification

                    void glStencilFunc
                    GLenum func
                    GLint ref
                    GLuint mask

## Parameters

`func`

  Specifies the test function. Eight symbolic constants are valid: `GL_NEVER`, `GL_LESS`, `GL_LEQUAL`, `GL_GREATER`, `GL_GEQUAL`, `GL_EQUAL`, `GL_NOTEQUAL`, and `GL_ALWAYS`. The initial value is `GL_ALWAYS`.

`ref`

  Specifies the reference value for the stencil test. `ref` is clamped to the range 0 2 n - 1 , where n is the number of bitplanes in the stencil buffer. The initial value is 0.

`mask`

  Specifies a mask that is ANDed with both the reference value and the stored stencil value when the test is done. The initial value is all 1's.

## Description

Stenciling, like depth-buffering, enables and disables drawing on a per-pixel basis. Stencil planes are first drawn into using GL drawing primitives, then geometry and images are rendered 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 reference value and the value in the stencil buffer. To enable and disable the test, call glEnable and glDisable with argument `GL_STENCIL_TEST`. To specify actions based on the outcome of the stencil test, call glStencilOp or glStencilOpSeparate.

There can be two separate sets of `func`, `ref`, and `mask` parameters; one affects back-facing polygons, and the other affects front-facing polygons as well as other non-polygon primitives. glStencilFunc sets both front and back stencil state to the same values. Use glStencilFuncSeparate to set front and back stencil state to different values.

`func` is a symbolic constant that determines the stencil comparison function. It accepts one of eight values, shown in the following list. `ref` is an integer reference value that is used in the stencil comparison. It is clamped to the range 0 2 n - 1 , where n is the number of bitplanes in the stencil buffer. `mask` is bitwise ANDed with both the reference value and the stored stencil value, with the ANDed values participating in the comparison.

If stencil represents the value stored in the corresponding stencil buffer location, the following list shows the effect of each comparison function that can be specified by `func`. Only if the comparison succeeds is the pixel passed through to the next stage in the rasterization process (see glStencilOp). All tests treat stencil values as unsigned integers in the range 0 2 n - 1 , where n is the number of bitplanes in the stencil buffer.

The following values are accepted by `func`:

`GL_NEVER`

  Always fails.

`GL_LESS`

  Passes if ( `ref` & `mask` ) < ( stencil & `mask` ).

`GL_LEQUAL`

  Passes if ( `ref` & `mask` ) <= ( stencil & `mask` ).

`GL_GREATER`

  Passes if ( `ref` & `mask` ) > ( stencil & `mask` ).

`GL_GEQUAL`

  Passes if ( `ref` & `mask` ) >= ( stencil & `mask` ).

`GL_EQUAL`

  Passes if ( `ref` & `mask` ) = ( stencil & `mask` ).

`GL_NOTEQUAL`

  Passes if ( `ref` & `mask` ) != ( stencil & `mask` ).

`GL_ALWAYS`

  Always passes.

## 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 test always passes.

glStencilFunc is the same as calling glStencilFuncSeparate with `face` set to `GL_FRONT_AND_BACK`.

## Errors

`GL_INVALID_ENUM` is generated if `func` is not one of the eight accepted values.

`GL_INVALID_OPERATION` is generated if `glStencilFunc` is executed between the execution of glBegin and the corresponding execution of glEnd.

## Associated Gets

glGet with argument `GL_STENCIL_FUNC`, `GL_STENCIL_VALUE_MASK`, `GL_STENCIL_REF`, `GL_STENCIL_BACK_FUNC`, `GL_STENCIL_BACK_VALUE_MASK`, `GL_STENCIL_BACK_REF`, or `GL_STENCIL_BITS`

glIsEnabled with argument `GL_STENCIL_TEST`

## See Also

glAlphaFunc, glBlendFunc, glDepthFunc, glEnable, glLogicOp, glStencilFuncSeparate, glStencilMask, glStencilMaskSeparate, glStencilOp, 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/.