On Wednesday 24 September 2008, Chris Moller wrote: > Maciej Pilichowski wrote: > > So there is obstacle, indeed. And till now I have no idea how to solve > > this. > > Might be a bit of a wild branch on this topic, but I'm wondering if all > the effects described here could be implemented as a /subtractive/ process. > > I haven't looked at gimp's implementation, but generally speaking colors > combine as Cr = Ce * ( 1- o) + Cb * o, where Cr is the resultant color > (clamped full saturation of each of it's components), Ce is the existing > color, Cb is the brush color, and a is the opacity. What if, instead, > you made Cr = Ce * (1 - o) - Cb * o, with Cr clamped at 0? All this > doesn't fiddle with the alpha channel at all, but might get the desired > effect. Isn't that pretty much what the 'subtract' mode does? And yes, it doesn't involve the alpha channel. http://docs.gimp.org/en/gimp-concepts-layer-modes.html Daniel
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