The resulting index will usually contain a fractional part, so the output coordinate value is then generated by interpolating linearly between the appropriate entries in the table. If the index lies outside the range of the table, linear extrapolation is used based on the two nearest entries (i.e. the two entries at the start or end of the table, as appropriate).
If the lookup table entries increase or decrease monotonically, then the inverse transformation may also be performed.
AST A Library for Handling World Coordinate Systems in Astronomy