Dear Johannes
Am 31.07.2017 um 04:18 schrieb johannes hanika:
yeah the solver sucks.
I think I was able to fix it. See
https://github.com/rabauke/darktable/tree/fix_colorchecker
I implemented for darktable the approach to calculate the expansion
coefficients which I outlined in an other e-mail before. As explained
in my very first e-mail on this topic the old version of the color look
up table module caused strange color changes even if source and target
colors where identical if the number of color patches is small. My new
version does no longer show this undesirable behavior.
Nevertheless, users may observe strong color changes if source and
target colors differ even for minor adjustments of the target colors if
the number of color patches is small, less than 5 let's say. This,
however, I would not consider a bug. It is an intrinsic feature of the
applied algorithm and has to be expected when interpolation is applied
with very few grid points (color patches). Possibly one should give in
the user manual a warning/hint that the source colors in the color
patches must be representative for the input image. It is not
sufficient to keep only those colors in the color checker that shall be
adjusted. This would lead usually to unintended strong color shifts.
I will test my code further and add some comments to the source code.
Some error handling for the case that the coefficient matrix still
becomes singular has to be included, too. When I think the code is
mature enough I will open a pull request.
Heiko
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