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September 2006
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2006.09.16 20:47 "Re: alpha in Grayscale or Palette", by Bob Friesenhahn
On Sat, 16 Sep 2006, Joris wrote: > > > Associated alpha makes no sense since Pallete entries are not related > > to pixel position but associated alpha is related to pixel position. > > I'd agree, but you should note PNG thinks otherwise. If I remember > correctly, PNG does support alpha in palette color specification. They > call it 'poor man's transparency', though I've never understood why that > name should apply as you need to be a very rich man to find the time to > think through all implications and fully integrate the concept into your > image processing library. PNG does not support associated (pre-multiplied) alpha. It only supports unassociated (simple) alpha. In this case, supporting alpha from within the colormap makes sense. For example, you may assign one color to represent all completely transparent areas (like GIF). TIFF does support unassociated (simple) alpha but the TIFF specification does not define how it should be applied to the image to obtain a viewable result. Bob ====================================== Bob Friesenhahn bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer, http://www.GraphicsMagick.org/