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2008.11.21 21:45 "Re: RGB to palette images", by Kevin Myers
Richard Nolde wrote: > You cannot do it in one pass because the entries in a palettized image are > indices into the palette, which you have not built unless you have seen > all the entries. In the special case that you know all the possible > values ahead of time, you could do it in one pass. You could also do it in > one pass if you knew in advance the mapping you wanted to use between the > input values and the output values, eg for a USGS topo map, there are only > a fixed number of possible colors and you can be sure that only a specific > set of RGB values will appear in the data stream. In that case, build you > palette table at the top of the code, and convert RGB triplets to palette > indices on the fly. But, while scanning an image, if I come across a new unique color (a color that hasn't already been added to the palette), then I do know the index into the palette, because I know that this is the nth new color that I have added to the palette, right? So, I still think that this could be done in a single pass, building the palette on the fly, if desired, but maybe I am overlooking something. s/KAM