Controls the writing of individual bits in the color index buffers.
The least significant
n bits of its value, where
n is
the number of bits in a color index buffer, specify a mask. Where a 1
appears in the mask, it is possible to write to the corresponding bit
in the color index buffer (or buffers). Where a 0 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
drawBuffer).
Initially, all bits are enabled for writing.