[Gimp-developer] Reference on Opacity variations in merging modes

[David Gowers]

On Sat, Apr 25, 2009 at 12:05 AM, Alchemie foto\grafiche
<fotocomics at yahoo.it> wrote:
>
> I found the algorithms of many merging mode
>
> BUT all the the formula there do not take opacity into account so as now the blending may be done only at 100% opacity
> (btw is for a "blending filter" , that  allow to merge also in mode not available in Gimp
>  obviously the result, the output is the visible from the the merging as new layer )
>
> I'm sure somewhere in the Gimp code or documentation there is the answer:
>  how ,mathematically variation of opacity are taken into account, but i am really lost in front of page of codes and doc

You should understand, that the new GEGL-based layer modes are subtly
different from the legacy GIMP code in how they treat alpha.
That said, app/gegl/gimpoperationpointlayermode.c contains all code
dealing with layer compositing modes, and that is as simple as it
gets.
Yes, I find it confusing too.