[Gimp-developer] [wish] provide transparent color

[Chris Moller]

Daniel Hornung wrote:
> 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
>
>   

Yeah, I expect it's the same function, but adding a transparent layer,
painting it, switching to subtract mode, and merging down, is more
cumbersome than just painting in subtract mode would be. 

cm


> Daniel
>   
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