glClearIndex specifies the index used by glClear to clear the color index buffers. c is not clamped. Rather, c is converted to a fixed-point value with unspecified precision to the right of the binary point. The integer part of this value is then masked with 2 m - 1, where m is the number of bits in a color index stored in the frame buffer.
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glClearIndex: man2/glClearIndex.xml
glClearIndex specifies the index used by glClear to clear the color index buffers. c is not clamped. Rather, c is converted to a fixed-point value with unspecified precision to the right of the binary point. The integer part of this value is then masked with 2 m - 1, where m is the number of bits in a color index stored in the frame buffer.