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April 2008
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2008.04.18 15:41 "Re: a question about creating transparency of the image in TIFF", by Bob Friesenhahn
On Fri, 18 Apr 2008, Edward Lam wrote: >> The TIFF specification does not prevent a colormapped file with >> transparency but the colors in the colormap would then need to be >> premultiplied, which could be a challenge to surmount. As a result, no one >> does this. > > Sorry, I don't understand this. We output TIFFs with premultiplied RGB colors > and associated alpha. How is this a "challenge"? I highly doubt that these are colormapped files. In that case, the values in the colormap also need to reflect pre-multiplied RGB. Pre-multiplied RGB depends on pixel position but colormapped images only use colormap indexes. Only a carefully crafted image with limited alpha values will work for this scenario. I am fairly sure that my software would decode such an image correctly, but few (if any) exist in the wild so there is nothing to test with. Bob ====================================== Bob Friesenhahn bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer, http://www.GraphicsMagick.org/