This step applies only to color index and stencil index groups. If the index is a floating-point value, it is converted to fixed-point, with an unspecified number of bits to the right of the binary point. Indices that are already integers remain so; any fraction bits in the resulting fixed-point value are zero.

The fixed-point index is then shifted by
bits,
left if and right otherwise.
In either case the shift is zero-filled.
Then,
the signed integer offset ` INDEX_OFFSET` is added to
the index.

Sat Mar 29 02:23:21 PST 1997