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

Stencil Test

The stencil test takes place only if there is a stencil buffer, which you need to request when your window is created. (If there is no stencil buffer, the stencil test always passes.) Stenciling applies a test that compares a reference value with the value stored at a pixel in the stencil buffer. Depending on the result of the test, the value in the stencil buffer can be modified. You can choose the particular comparison function used, the reference value, and the modification performed with the glStencilFunc() and glStencilOp() commands.

void glStencilFunc(GLenum func, GLint ref, GLuint mask);void glStencilFuncSeparate(GLenum face, GLenum func,                         GLint ref, GLuint mask);

Sets the comparison function (func ...