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I have a question about how the resulting alpha is computed for different blend modes in gimp.

The manual [1] describes the color channels, but not how the opacity is processed (I am also sort of interested in a rationale).

[1] https://docs.gimp.org/en/gimp-concepts-layer-modes.html

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    I think this is a question for the GIMP development community, not a graphic design community. – Scott Sep 8 '20 at 19:29
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For all but specific modes (Normal, Dissolve...), the alpha of the result is the alpha of the bottom layer(*). And when it comes to the alpha of the top layer, these two methods produce the same result:

Method #1: Apply Top (blend) layer at x% opacity

Method #2: Apply Top (blend) layer at 100% opacity, get the result (Layer>New from visible), hide the "blend" layer and set the result layer to x% opacity.

(*) Within a layer group... because layer groups act like parentheses in an equation, the blend mode of a layer only applies to the layers below it in the same group.

  • Yes, with exception of displve it works like this. – joojaa Sep 9 '20 at 8:03

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