Method GL.glStencilOp()

Method glStencilOp

void glStencilOp(int sfail, int dpfail, int dppass)

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 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 (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.

Parameter 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.

Parameter 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.

Parameter 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.

Throws

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

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

Note

GL_DECR_WRAP and GL_INCR_WRAP are available only if the GL version is 1.4 or greater.

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.