Fix found, thanks Alex! If anyone else comes across this, it's caused by premultiplied alpha. The fix for this case is just to change the return line in the CI Filter to:
return premultiply(vec4(sample(image, samplerCoord(image)).rgb,alpha)); Chris 2008/10/16 Chris Wood <[EMAIL PROTECTED]> > I'm in need of a basic 'image fade' filter, using transparency, and I seem > to have come across a pretty strange bug. > What I'm doing is: > > 1. Create an image. > 2. Pass it through a CI filter. The filter just samples the image, sets the > alpha value in the 0.0 - 1.0 range, and returns the new pixel. The > transparency is the same across the whole image. > 3. Pass the modified image to a billboard, with the transparency set to > 'over'. > > What 'should' happen: the image should fade in + out, depending on the > alpha value. > What does happen: at alpha 1.0 the image is normal. At alpha 0.0, it's > transparent but appears to be doing additive blending, not over blending. At > 0.0, it looks identical to additive blending set on the billboard, infact. > > I've attached a composition showing the bug in action. There are two > billboards, showing a gaussian gradient. The one on the left has > transparency set in the gradient colours: transparency works perfectly. The > one on the right has an image with no transparency, and transparency is > applied in the CI filter. Transparency doesn't work as it should. There are > published parameters to control the left/right transparency. > > Am I making some silly mistake here? Or is it a bug? I find it hard to > believe nobody has done this before :) > > Chris >
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