On Wed, Mar 19, 2003 at 03:49:20PM -0800, Daniel Rogers wrote: > There seems to be a fairly good agreement that post-multiplied alpha is > the way to go. It is more accurate, and there are currently features of > gimp that depend on it. > > I think for those reasons gegl should use post-multiplied alpha (afaik, > it currently doesn't). The loudest dissenters tend to be a little > flamey, but I think the most accurate way to go should be where we > start, since we want the gimp to look good. After the gimp looks good, > we can optimize without sacrificing image quality. I suspect there > might even be a way to get gegl to decide when pre multipling is a good > plan and do it, which would be cool. It should be able to do both probably. Gimp favors an un-premultiplied version of composite for painthits on layers. But for Gimp's Layer stack you could probably get away with a version of a premultiplied over (You would just premultiply the incoming color from the layer before using the premultiplied version). So far I have only coded pre-multiplied versions of the compositing ops for simplicity. > In the same vein, I noticed that to multiply two integer samples you use > a macro called INT_MULT that does a non-fp division (a bit shift and > addition) that approximates the formula: Alvy Ray Smith talks about this formula some in "Image Compositing Fundamentals" as well as un-premultiplied and premultiplied versions of composite: http://alvyray.com/Memos/default.htm While we are here, his "Sprite Theory of Image Computing" is interesting too, his description of the compositing system he wrote called altimira composer. Calvin
