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April 2008
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2008.04.18 15:56 "Re: a question about creating transparency of the image in TIFF", by Edward Lam
Hi Bob, Sorry, I didn't realize what you mean by "colormapped". Yes, I was thinking of RGB full color images. Thanks for the clarification. -Edward Bob Friesenhahn wrote: > 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/ >