glStencilFunc
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## Name

glStencilFunc — set function and reference value for stencil testing

## C Specification

`void `glStencilFunc`` | `(` | `GLenum `func`, GLint `ref`, GLuint `mask`);`

## Python Specification

``glStencilFunc`` | `(` | ``func`, `ref`, `mask`) → `None``

## Parameters

`func`
  Specifies the test function. Eight tokens 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. 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 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`.

`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.

## 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`

`glGet` with argument `GL_STENCIL_VALUE_MASK`

`glGet` with argument `GL_STENCIL_REF`

`glGet` with argument `GL_STENCIL_BITS`

`glIsEnabled` with argument `GL_STENCIL_TEST`

## See Also

glAlphaFunc, glBlendFunc, glDepthFunc, glEnable, glIsEnabled, glLogicOp, glStencilOp

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