The challenge here is to remove that nasty blue color band that borders the back of the performers (as well as the entire left side of the frame). I don’t know which module to use (I have tried color balance) and most importantly, how to build an appropriate parametric mask.
I have used here Darktable 3.2.1
License: Creative Commons, By-Attribution, Share-Alike.
20191201_NIK1145.nef (46.9 MB)
20191201_NIK1145.nef.xmp (23.6 KB)
Always those nasty blue lights that look much better in real life…
I’m not too familiar on how to do this efficiently in darktable, but I assume my result from RawTherapee can be replicated quite easily.
filmic preserve chrominance luminance Y and raw denoising on the B channel
Mine is a terrible rendition, using tools in ways Mother Nature did not attend… 
With this profile hangup I’ve got, wanted to see how it’d handle this. Don’t have a D810 dataset, so I used my Z6 profile. That made the blue more gradated, but it was still Blue.
Got to thinking, I’d recently put a blue threshold in my whitebalance tool so I could use it on indoors images with outdoor-looking windows. I put one in past filmic, dialed up the red multiplier, and set the blue threshold so it was only working on the blue:
There’s still blue, but it’s tame…
Well, three times I tried to export out of darktable and three times it stopped responding, so I give up. Instead, I’ll tell you my method. In channel mixer drop B slider in B channel to 0. This will give the image a yellow cast and fixing it by increasing R or G sliders ruins the skin tones. So leave them alone, and instead go to your parametric mask, G channel, choose input, and take it out of all but the darkest of shadows. In filmic you might have to drop mid tone saturation to 0.
How about this:
I’m of the opinion that the blues should not be removed. There are blue lights that illuminate the scene so blue casts will be visible. That nasty looking (led) cast/glow should go!
I think it it is too much tuning for defective input profile.
To mitigate the blue cast, there is also the ACES trick that @anon41087856 used in some thread, but I am not able to find it.
@gaaned92 I think you are talking about these settings:
Red channel: 1.0 / -0.18 / +0.18
Green channel: -0.2 / 1.0 / +0.2
Blue channel: +0.05 / -0.05 / 1.0
Tried, but didn’t work for me in this case.
It is mentioned in this topic:
Yes that’s it. I think @anon41087856 said it has to be applied in REC2020 working space.
You can try that
R channel: 0.88, 0.06, 0.265
G channel: 0.033, 0.799, 0.362
B channel: 0.033, 0.033, 1
Changed the input profile to “linear prophoto RGB” and used color zones. Tried to avoid darkening of the blue areas which looks unnatural especially in her hair.
Edit: And here is one, where the blue is only tamed (-70% mix in color zones)
Do you mean you think this is why it wouldn’t export? Darktable doesn’t like parametric masking in channel mixer?
Wow, thanks everyone for the excellent examples! I’m now going through a few of the xmp that have been posted above… I’m particularly keen on understanding color zones so that I can apply the same technique to other cases. I need something simple that I can learn & remember without having to do too much mental gymnastics (I’m obviously referring to the channel mixer trick that @gaaned92 and @Jade_NL are discussing above… I need to re-read the post by Aurelien “hardcore dt” first!)
Alright, so I went through a few of the xmps posted above and … I don’t understand them. At all!
First of all, I went through the settings posted by @sovereign; the process that removed the blue color cast is a single channel mixer with destination red and settings R+0.95 G-0.1 B0. Ok, if I replicate this on my original photo I get weird yellowish colors. I understand this may be due to the different white balance settings, but what I don’t see is how did he get to that point. There’s nothing intuitive here! Is there a reasoning behind? Did you fix the white balance first or what? How did you get to those parameters in the channel mixer?
Second issue, there was a discussion on input profiles above. I may be missing something but what is the problem with input profiles? The defaults are input=standard color matrix, working=linear rec 2020 rgb. Should they be changed? Why @gaaned92 made that comment about “too much tuning for defective input profile”? why defective? How defective? how can you tell it’s defective? Picture me with a big question mark and a dumb expression on my face.
Third, color zones as used by @yteaot. Finally, a single module that I can probably use it, no problems! … well turns out that I have problems also on this!!! I see how it’s been used (by dropping a big hole in the saturation panel right where the blues are) but again – I’m unable to draw that sharp rectangle .
Probably less ‘defective’, more about ‘deficient’…
3x3 matrix profiles are quite fine for the majority of images, but in the cases where there are significant numbers of colors out of the destination gamut, they tend to just pile up all those colors at the gamut edge. A camera profile that uses a LUT can be made to make those color mappings have more gradation than just ‘pile-up-at-the-fence’. That’s what’s going on with your blues…
Hi @aadm, you failed to notice I modified all the three color channels in the channel mixer module.
Just click on the destination and toggle between red, green and blue channels to see the values applied.
The rest of the edit is trivial and it revolves around exposure and tone equalizer.
