On 07/03/18 11:32, Matthieu Moy wrote:
Before considering a fix: any idea how the "substract" operator should
act in the Lab color space?
I have no idea ;-).

Actually, my guess is that "subtract" is useful to blend two totally
different images together (e.g. blend a black&white text onto an image
for a dodge&burn effect). For darktable where blending is done between
the original image and the image transformed by the module, I'm not
even sure "subtract" is a relevant thing to do. The fact that it's been
broken for years without anyone noticing is probably a sign that no
one ever found it useful.

I am guilty of never using it in darktable, but the usage could be to see _where_ in the image the applied module is acting. I mean, for example, if you really are subtracting, and you are applying an high pass, you should see a black image (no variation) with just the edge highlighted.

Now, I have tried with a grayscale image and it seems to work, so I am a bit surprised... with a color image it is not working, I agree.


Romano Giannetti

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