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, as if glStencilOpSeparate() were called with face set to GL_FRONT_AND_BACK.
glStencilOpSeparate 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.
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 glStencilOpSeparate 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.
GL_INVALID_ENUM is generated if face is any value other than GL_FRONT, GL_BACK, or GL_FRONT_AND_BACK.
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
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