Re: Finding out what happens when compositing layers

Ofnuts <ofnuts@xxxxxxxxxxx> · Sat, 17 Sep 2011 01:06:37 +0200



    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 <ofnuts@xxxxxxxxxxx>
        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

    
    Composite=Under(1-alpha)-Over*alpha

    or

    Composite=Under(1-alpha)+(1-Over)*alpha

    or

    Composite=Under(1-alpha)+(1-Over*alpha)

    
    and $deity knows what else.

    
    math.rb isn't taking the opacity in account (but is an interesting
    read...)

    
    Thx.

    
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