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.