Name
gl.StencilFunc -- set function and reference value for stencil testing
Synopsis
gl.StencilFunc(func, ref, mask)
Function
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 gl.Enable() and gl.Disable() with argument #GL_STENCIL_TEST. To specify actions based on the outcome of the stencil test, call gl.StencilOp().

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 gl.StencilOp for details.). 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.

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.

Please consult an OpenGL reference manual for more information.

Inputs
func
specifies the test function; the initial value is #GL_ALWAYS (see above)
ref
specifies the reference value for the stencil test; 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
Errors
#GL_INVALID_ENUM is generated if func is not one of the eight accepted values.

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

Associated gets
gl.Get() with argument #GL_STENCIL_FUNC, #GL_STENCIL_VALUE_MASK, #GL_STENCIL_REF, or #GL_STENCIL_BITS

gl.IsEnabled() with argument #GL_STENCIL_TEST


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