2004.05.20 23:42 "Re: [Tiff] Extra samples handling", by Chris Cox
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?