glStencilMaskSeparate

glStencilMaskSeparate: man3/glStencilMaskSeparate.xml

glStencilMaskSeparate controls the writing of individual bits in the stencil planes. The least significant n bits of mask, where n is the number of bits in the stencil buffer, specify a mask. Where a 1 appears in the mask, it's possible to write to the corresponding bit in the stencil buffer. Where a 0 appears, the corresponding bit is write-protected. Initially, all bits are enabled for writing. There can be two separate mask writemasks; one affects back-facing polygons, and the other affects front-facing polygons as well as other non-polygon primitives. glStencilMask sets both front and back stencil writemasks to the same values, as if glStencilMaskSeparate were called with face set to GL_FRONT_AND_BACK.