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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