2018-03-07 16:13 GMT+01:00 Romano Giannetti <romanof...@rgtti.com>:
> 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.
[OT] I usually do this with difference blend mode. For example when I want
to know in which areas affects equalizer I change temporary to difference
blend mode and can see it.
> Romano Giannetti
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