General purpose compose operations usually include a division by 255 of a number in the range 0 ... 65025. While fixing up Mozilla's alpha compositor I disturbed a sleeping hacker who has provided a (tested and working) macro which computes this operation much more quickly than any other solution tested, including GCC -On speed-ups using similar but different tricks. Of course, GCC cannot use this FAST_DIVIDE_BY_255 because it is safe only for 0 ... 65025, but I think it might be useful to Gimp, both in the core (which I presume has several alpha compositors) and in plug-ins, where alpha composition is also a popular operation. So, does Gimp already have something like this in core? Or should we consider borrowing this macro from wherever it originated and using it throughout Gimp (after 1.2, naturally) ? Nick.
