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May 2004
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2004.05.20 23:42 "Re: Extra samples handling", by Chris Cox
At 11:05 AM +0000 5/20/04, mayankg@iiitb.ac.in wrote: > Hi, > > Please confirm if i am correct with the extra samples handling in TIFF images. > > If TIFFTAG_EXTRASAMPLES has: > > Type as 1(associated alpha): then the color values have to be taken as it is. > also the number of extra channels will always be one only. No, associated alpha means that it is transparency, and images will have to be un-premultiplied. > Type as 2(unassociated alpha): then all the color values have to be multiplied > by the alpha channel value. No - unassociated means that it is NOT transparency. And in no case would you multiply values when reading a TIFF -- the values in the TIFF file are premultiplied only if they have an associated alpha (transparency) channel. > Question 1: Are multiple unassociated alpha channels possible? If > yes, then how > are the final color values calculated? Yes - unassociated alpha channels don't contribute anything to the color or transparency. > Type as 0(unspecified data): then the extra channels have to be > simply ignored. No, they can be loaded (as in Photoshop). > Question 2: Are combinations of Type 2 and Type 0 possible for a particular > TIFF image? Yes, of course. You could even have all 3 types. > If yes, do we need to ignore the alpha channel/multilply the color > values with the vaues of the color channel accordingly? See above. Chris