I just wanted to point this out, because in RawTherapee we had summation errors up to 10% range for some algorithms (long ago) because we did not take care of thisā¦
For wide dynamic range, additive device like a camera, a ā2.5ā D LUT is probably the right approach. This means creating a table in which the per channel luminance lookup is independent (i.e. orthogonal) to the chrominance lookup. If your luminance linearization is done outside the colorspace conversion, then the table could be purely 2D. This would have an advantage over a matrix in allowing for the embedding of spectral sample statistics. i.e. you can weight areas of the table for the best conversion for the likely real world spectra associated with each region in the chromaticity plane.
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Itās just funny how the āwithout the mathsā entry is already at the point of discussing commutativity of matrix operations and error accumulation in limited precision floating point operations. 
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So I would be happy if you could review my code, because my tests donāt show that property. Also, what solver did you use ?
Hello Aurelien,
My point is general, not specific to a particular piece of software (plus I am not a coder): long ago I took a raw capture of a CC24, white balanced the raw data by multiplying it with diag(WB_mult), converted to RGB by keeping the R and B channels as-is and averaging the greens of each quartet, then fed them to the optimization routine to obtain CCM(1); then I repeated the process but without the white balancing step, to obtain CCM(2). For practical purposes CCM(2) was equal to CCM(1)*diag(WB_mult), as theory suggests.
I donāt remember if I used Matlabās fminsearch (Nelder-Mead) or lsqnonlin (trust-region-reflective) for a solver, with CIEDE2000 as the minimization criterion.
When theory meets c programming practice, practice wins 
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plus Ƨa change, plus cāest la mĆŖme chose 
It would be possible to add a mixed white balance, 50% CAT and 50% native rgb?
This should gives a more robust white balance
Thatās possible - color calibration alows masking.
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Thanks aurelienpierre for this new feature. I used it together with Spydercheckr to replicate some paintings and that I needed matching colours for. And thanks johnny-bit for recommending Spydercheckr for me.
Anyone tried this since 4.0.0? Normalization values donāt move anymore, even if I increase the slider in the exposure module.
If your saying what I think your saying it changed at some point and the values given are the values that you should set for exposure and black level to be accurate. it doesn t change like before, ie its not an offset anymore but the actual value for exposure.
No difference in the 3.8 manual from the 4.0 manual. Has this been mentioned at Github?
Ya I will try to dig it up and I think it is mentioned in one of APās videosā¦maybe the one called something like getting the most out of color calibration or something like thatā¦
Here you goā¦took me a momentā¦
Thanks!
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This is an old thread. I am curious how much of it holds true today.
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As per AP - originally calibration is done on 2 steps - the old WB (White Balance) module and the new Color Calibration. I am trying to understand the manual darktable 4.4 user manual - color calibration but I am not perceiving that WB module is used anymore (in terms - not as a color picker) - it is left on the reference setting. Am I perceiving it correctly?
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My understanding is that the āNormalization valuesā a guidance for the exposure and black level correction. And the user is guided how to change them. With few trials - I was able to adjust āblack offsetā to zero but even with big offsets - I was unable to make āexposure compensationā zero. Is the user simply expected to put the number written (and not to expect that it will change to zero when the profile is re calculated)?
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To have a fairly universal profile - the suggestion is to create a preset that is based on the āas shot in cameraā after applying the profile so the āmatrix adaptation spaceā is applied on top of it.
On initial thought I understand why this would be the case. But then this is going to take the WB as recorded by the camera. And it can vary - Auto WB or fixed (by measurement - graycard or by estimate. Suppose the user used Auto WB - is the user expected to further estimate the WB by measuring the scene?
Also - the universal profile is based on natural light (good quality). Is there any difference when a profile is being created if the test shot is done using direct sunlight or cloudy or overcast?
What is the approach going to be if the shot is done using artificial light (like fluorescent / energy savers etc. not photo grade) - is the user expected to change back to āas shot in cameraā or this would be not a needed step?
There are 3 icons at the bottom right of the color calibration
The re calculate and apply are self explanatory but what is ācheck output delta Eā used for?
Very quick repliesā¦
Point 1 yes leave WB on reference if using CC module for WB/illuminant selection
Point 2 Initially those values were offsets to add to existing exposure etcā¦this was later changedā¦ those are the values at which the profile is accurate as reported so you would enter themā¦
Point 2 ā¦As shot is just really what ever value the camera used for the shot and is passed to DT. Your matrix will be added to thatā¦ The profile will be a varying value at any lighting other than when the shot was takenā¦ the closest to a general use is a daylight shot profile saved to apply to the as-shot cameraā¦ Its set to as shot in the preset so that it only applies the matrix to your image and the current wb and not some totally wrong value encoded at the time of the preset creationā¦
That button I believe is a check is so you can see where your current profile deltaE is when you start and then you can see from there where you move toā¦
I believe this is accurateā¦ There is a tweak to this in the recent code allowing some modules to get access to the wb coefficients as that is a better set of reference values for them and the D65 part is now handled a little differently but that is all in the backgroundā¦
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My experience is that these values move (the normalization value).
However - the black offset can be driven to zero while the exposure compensation cannot.
Unless - I am misunderstanding and I should simply apply the values initially seen in the exposure module and then either not re calculate or re calculate but ignore the second set of values (because re calculation makes the values move).
I am finding that it is possible to create a style that is based on āas shotā but further refine it based on the white card. So - if the user has a style then they can use the white card after applying the style. This can be useful as the style can take a while to prepare.
https://www.datacolor.com/spyder/products/spyder-checkr-photo-sck310/
I was surprised to learn that the 18% card is for film exposure (on the info section).
Another interesting detail is that these patches are expected to last about 2y. My guess is that this would be of bigger value for somebody that uses it commercially.
It is a pleasant surprise that the tool is listed in the module so all the patches can be used together.
And about price - it is cheaper than xrite/calibrite.
I believe itās using the tonal patches to determine that exposure value it gives. I can look up the reference but it basically means this is what you should set your exposure to to be correct. The compensation one always jumps around and overall I think AP said not to keep trying to zero it. Of course the exposure changes scene to scene as well so its just another variable to manage if you are hoping for a āuniversalā profile. It may be useful to try this approach but really I think at times you might just introduce a further change to the scene. I think itās a great feature to use on a shot where you have a color checker in the shot and then edit a set of images in similar lightingā¦ as for a global or universal profile Iām less confident of the overall value
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