glIndexMask.3G man page
glIndexMask — control the writing of individual bits in the color index buffers
void glIndexMask(GLuint mask);
Specifies a bit mask to enable and disable the writing of individual bits in the color index buffers. Initially, the mask is all 1's.
glIndexMask controls the writing of individual bits in the color index buffers. The least significant n bits of mask, where n is the number of bits in a color index buffer, specify a mask. Where a 1 (one) appears in the mask, it's possible to write to the corresponding bit in the color index buffer (or buffers). Where a 0 (zero) appears, the corresponding bit is write-protected.
This mask is used only in color index mode, and it affects only the buffers currently selected for writing (see glDrawBuffer()). Initially, all bits are enabled for writing.
GL_INVALID_OPERATION is generated if glIndexMask is executed between the execution of glBegin() and the corresponding execution of glEnd().
glGet() with argument GL_INDEX_WRITEMASK
glColorMask(), glDepthMask(), glDrawBuffer(), glIndex(), glIndexPointer(), glStencilMask()
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