On Sat, Sep 17, 2011 at 8:36 AM, Ofnuts <ofn...@laposte.net> wrote:

> **
> On 09/16/2011 02:20 AM, David Gowers (kampu) wrote:
> In GEGL, the layer mode code is autogenerated from formula strings -- see
> operations/generated/math.rb
> The opacity behaviour is not documented there, but my understanding is that
> the alpha of the underlying composite is preserved, and opacity only effects
> how much of the layer's COLOR channels are applied (per the standard
> blending formula).
> On Fri, Sep 16, 2011 at 7:09 AM, Ofnuts <ofn...@laposte.net> wrote:
>> I'll trying to find out what happens when a layer is in subtract mode
>> with a global opacity not one or zero (i.e. what computations are done
>> with the pixels values of the composite image of the layers below, the
>> pixel values of the layer, and the general opacity). I have a git
>> extract of the Gimp source code (and of gegl & babl) but got lost... Can
>> someone help?
> Well, I know that in "Normal" mode, I have in each RGB channel:
> Composite=Over*alpha+Under(1-alpha)
> (assuming "Under" is fully opaque), but in Subtract mode, I would have
> several possibilities
> No, you just subtract Over from Under with no multiplication, then you
perform a RGB (not RGBA) Composite of the result and Under, to produce the
final composite.
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