On Tue, Jul 18, 2000 at 09:41:49PM -0400, Garry R. Osgood wrote: > David Hodson wrote: > > Example: render an rgba image. (I was using some PovRay output; I > > presume it does a reasonable job.) Now create a flat colour > > background in the Gimp, lay the rgba image on top, and try to get > > a clean composite without black fringing. I don't think it can be > > done, > > It can't be done, in my opinion. What we surmise to be [R*A, G*A, > B*A, A] Gimp assumes to be [R, G, B, A] and reads the color > components as near black for near transparent regions. That makes > black fringing unavoidable. The way I got around this was to regenerate my own "Alpha" channel by re-rendering my PovRay scene in black-on-white. That is, white background with all objects solid black. If I recall correctly, it was just a matter of setting the "colour" of each object to black and rendering with a very low quality setting. (erm, or maybe I did it in white-on-black and inverted it ... in any case the same result) This process gave me a white mask (b) for the background which could then be multiplied with whatever image (i) I wanted "behind" the PovRay scene. Adding this (b*i) to the premultiplied alpha image from PovRay gave the correct visual result. Actually, come to think of it, I didn't render the high-quality scene with any alpha at all. The addition did it. Cheers, Tom -- -- Tom Rathborne tomr@xxxxxxxxxxxx http://www.aceldama.com/~tomr/ -- "We promise according to our hopes, and perform according to our fears." -- -- Francois, Duc de la Rochefoucauld
