Am 07.03.2018 um 17:10 schrieb Matthieu Moy:
Now, what "subtract" should do with colors is debatable, especially in Lab.
On the L channel, something like max(in - out, 0) would be rather natural.
On the a an b channels, I'm not sure what to do. Just doing in-out would
mean the final image would be black&white if the input and the output pixels
have the same colors (the same a and b). They'd keep the same colors if the
output is black & white. But for example, if the input is magenta (positive a)
and the output is even more magenta (greater a), do we expect the output to
be black&white (a clipped to 0)? Or green (negative a)?

One way to avoid the question would be to do a difference only on the L
channel, but just copy a and b from the output.

Well, that's probably not a good way to go. We had this issue in the "difference" mode in the past. This way you would get images with let's say L=0 and a=-50 ("highly blue saturated pitch black"). Due to the practical way how these "unbounded" Lab values are converted into monitor RGB you get some strange color artifacts, not the "black" that one would normally expect.

Or perhaps another option would be to just deprecate and remove "difference"
if we assume no one's using it.


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