I tried with an image from Imaging Resource (not that one, but another test scene with a color chart in it shot at base ISO at f8). I believe their lighting is closer to D50 and not D65, but I don’t know if that matters much.
Well, to be honest. I tried the thing aurelienpierre shows in the video: To use the color-calibration colorchecker mode to set the R/G/B tabs. Not to create a real profile from it.
… but, that doesn’t help at all. Once again, the problem is not getting more accurate colors, the problem is the built-in input profile Darktable uses for my camera, which destroys things for color-balance if the white balance is too far off.
I can try again if I manage to get a real profile from it.
But I think the fact is still this: The input profile adds a lot of red, apparently my camera/sensor needs that compensation in daylight.
But if I shoot an image where the camera picks +/- 2800k as white balance, but you set the white-balance module to reference (+/-6500k) the image is very orange. The input profile then adds even more red (which affects the entire image).
Then, CC tries to neutralize the image and messes up the reds now (since the input profile messed them up before).
Starting the whole process with the white-balance module set more to reality (+/- 2800k), and the image is way more neutral going into the input-profile module, which then does NOT messes up the colors but actually does what it is supposed to. Then CC comes on top and works fine (and can now even benefit from more accuracy by use of a R/G/B calibration).
Ok, I fetched the image you linked (well, the raw version of it).
Followed the dcamprof way to go from (rawtherapee save as reference image → argyii scanin → dcamprof make-profile → dcamprof make-icc) to an input profile.
Then, used the same picture to do the ‘white balance preset’ way: Disable everything, get a white balance picker on one of the grey patches on the colorchecker to get my ‘reference white balance’.
Then did the ‘use color-calibration to get even more accurate colors’ thing, so using the colorchecker-profiler baked into color-calibration module to set R/G/B values. This is with my new ‘reference white balance’ preset set, and using my own ICC created my dcamprof.
Got a profile rating of ‘very good’.
Then, tried using it to edit the picture linked in my opening post again.
Still the same purple thing (although I get the feeling it’s a bit less now?).
Then redid the edit, but with white balance not set to the reference-preset but to a more neutral ‘suitable’ one by sampling the lower-left, still using my own custom ICC, then using the custom calibrated color-calibration preset created earlier, and then sampling the same lower-left part in CC (ie, what I did before to workaround the problem → not use a reference white balance in the white balance module).
The difference is still very stark.
The idea of a single reference white balance is just not possible with this camera…
I redid the edits with the Darktable built-in camera profile, and the normal reference white-balance vs neutral white-balance. The difference is clearly much worse, so going through the whole calibration pipeline helped a bit, but didn’t (really) fix it:
_DSC4665 - test neutralwb.ARW.xmp (18.5 KB)
_DSC4665 - test referencewb.ARW.xmp (12.9 KB)