OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Stencil Examples

Probably the most typical use of the stencil test is to mask out an irregularly shaped region of the screen to prevent drawing from occurring within it. To do this, fill the stencil mask with zeros, and then draw the desired shape in the stencil buffer with ones. You can’t draw geometry directly into the stencil buffer, but you can achieve the same result by drawing into the color buffer and choosing a suitable value for the zpass function (such as GL_REPLACE). Whenever drawing occurs, a value is also written into the stencil buffer (in this case, the reference value). To prevent the stencil-buffer drawing from affecting the contents of the color buffer, set the color mask to zero (or GL_FALSE). You might also want to disable ...