HP Workstation zx2000 OpenGL 1.1 Reference for HP-UX 11.x - Page 406

S glStencilOp 406 glStencilOp glStencilOp: set stencil test actions. C Specification void glStencilOp( GLenum fail, GLenum zfail, GLenum zpass) Parameters fail zfail zpass Specifies the action to take when the stencil test fails. Six symbolic constants are accepted: GL_KEEP, GL_ZERO, GL_REPLACE, GL_INCR, GL_DECR, and GL_INVERT. The initial value is GL_KEEP. Specifies the stencil action when the stencil test passes, but the depth test fails. zfail accepts the same symbolic constants as fail. The initial value is GL_KEEP. 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. zpass accepts the same symbolic constants as fail. The initial value is GL_KEEP. 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. 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 fail specifies what happens to the stencil buffer contents. The following six 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. Chapter 17