glClearIndex.3G man page
glClearIndex — specify the clear value for the color index buffers
void glClearIndex(GLfloat c);
Specifies the index used when the color index buffers are cleared. The initial value is 0.
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.
GL_INVALID_OPERATION is generated if glClearIndex is executed between the execution of glBegin() and the corresponding execution of glEnd().
glGet() with argument GL_INDEX_CLEAR_VALUE
glGet() with argument GL_INDEX_BITS
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