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2006.09.16 21:50 "Re: alpha in Grayscale or Palette", by Joris Van Damme
Bob, Bob Friesenhahn wrote: > 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. I have another take on this. The difference between pre-multiplied (associated) and ordinarry non-multiplied alpha, is merely a multiplication (except when alpha indicates complete transparency). That's just a maths operation. So in my mind, the fundamental quirck about having alpha inside palette entries is independent of whether you code it as pre-multiplied or non-multiplied. The simple fact remains you're making fixed associations between colors and alpha, as opposed to coding alpha as per actual pixel position. It's this simple fact that makes a huge impact on the code required. For example, dithering from full-color-with-alpha to palette-including-alpha is the challenge, and whether those palette entries are coded pre-multiplied or non-multiplied doesn't have an impact in this regard. Best regards, Joris Van Damme info@awaresystems.be http://www.awaresystems.be/ Download your free TIFF tag viewer for windows here: http://www.awaresystems.be/imaging/tiff/astifftagviewer.html