Request a Stencil Buffer
If using stencil, request sufficient bits of stencil
Implementations may support from zero to 32 bits of stencil
8, 4, or 1 bit are common possibilities
Easy for GLUT programs: glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH | GLUT_STENCIL); glutCreateWindow(“stencil example”);