[Cin] oops apparently 'subtract' mode is broken!

[Andrea paz]

I attach the new image of the overlays to replace "normal.png" in the
manual. It seems to me that the original image corresponding to
"Subtract" is wrong: it corresponds to the Porter-Duff type overlay
called "SrcOut". The test I did using the green and red of the
original image seems to me to lead to a correct result.

Green (ColorPicker):

R = 0.1020
G = 0.8078
B = 0.2980


Red (ColorPicker):

R = 0.7333
G = 0.1686
B = 0.2980

Subtract:

  Alpha      Chroma
(Sa - Da); (Sc - Dc)

(0 - 0);   ([0.1020-0.7333], [0.8078-0.1686], [0.2980-02980])
   0   ;    (0, 0.6392, 0)

The ColorPicker indicates that the green resulting from "Subtract" is
exactly (0, 0.6392, 0)

I don't really understand this whole overlays thing, especially if
they have to do with plugins that use the alpha channel (Reroute and
Invert Video, for example). I hope IgorBeg can say something, because
he has gone into this topic a lot.
Andrew's proposal to add in the pulldown a new section with CinCV
overlays sounds great to me because if we can't reproduce the results
of the tutorial, something must be wrong. Even just the fact that we
cannot use YUVA, but only RGBA.

PS: the definition of "Subtract" given in the manual seems to me to
match the definition on the "rippletraining" website that you posted.
I would not modify it...
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