[Gimp-developer] [wish] provide transparent color

[Chris Moller]

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.
> Kind regards,
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>   


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