The colorfulness control of the color calibration module does an admirable job here. Well done. Makes sense to “prime” the signal before applying the per-channel curves (sigmoid) and then restore chrominance in the midrange areas. This is coincidentally very similar to what happens in AgX.
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A possible filmic v6 rendering. I set the white point manually, enabled reconstruction in filmic, and played with the params.
L1082187.DNG.xmp (10.8 KB)
Or, with a bit of bloom effect:
L1082187_01.DNG.xmp (11.3 KB)
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This is my usual sigmoid workflow, with the preserve hue slider in sigmoid down to zero and the blue ‘colorfulness’ slider in color calibration pulled back a bit - a good option for this sort of issue. Also tweaked the highlight reconstruction threshhold as needed with this camera.
I do find those blues hard though!
L1082187.DNG.xmp (9.0 KB)
There are at least a couple of issues at play in this post Ie highlights and the gamut of the blue light. I think one thing that is key to convey to the OP if it is not already apparent, is that there are many ways to handle this and many looks or outcomes… Some people totally blow it out and some people look for gradients. I have not explored it but I recall Aurelien said that one of the main uses for the guided Laplacian module was to reconstruct these gradients so this too could be experimented with esp now that it is much faster.
In the case of this image if you set the image defaults to be none so that you just have as shot legacy wb and use the default HLR which is now inpaint you will get this…
Simply changing the mode of HLR to LCH gives this…
Not far off a few of the edits… and still no tweaks to exposure or tone mapping etc
Switching back to default HLR and using CC with the gray tab and blue set to 1 and blended in average at about 50% opacity gives this… blending in normal gives a slightly different look and in case they can be tweaked by opacity…
Normal Blend
Or you can use the colorfulness tab… Blue -100%
EDIT:
I will upload the pictures later… I seem to be having issue with slow or cancelled uploads to Pixls the past week or more… Could be on my end… anyone seeing this too…
EDIT2 Pictures added…
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Linear, unbounded Rec2020 output (no filmic etc.), run through my gamut compressor experiment. This version was created using the modern chromatic adaptation method (white balance = camera reference, color calibration enabled, using the default gamut compression = 1).
And this one is just plain old white balance = as shot, without color calibration and of course gamut compression:
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BTW, @eobet , what is the computer in the foreground? It resembles a Commodore C64, but that only has 4 function keys, a 4-row keyboard (+space), and the power LED looks different, too:
Fairly good result. What’s your approach to the compression currently like?
How about a “new” Commodore C64?
Have fun!
Claes in Lund, Sweden
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I analyse the image, finding the max C (of LCh) for each out-of-gamut pixel. I then take the max. of the C_actual / C_at_gamut_boundary, and apply a curve (Tone Curve Math Question - #6 by Thanatomanic) that gradually scales the C values so they all get inside the gamut. For L, I use the same kind of curve, after determining the max brightness; the shoulder is chosen such that the brightest pixel’s L value ends up just under 1.
The code is here, but it’s not written in the style used in image processing. 
I tried to play it (type)-safe, didn’t care about performance. You’ll probably find it waaaay over-engineered (as it is).
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My Hungarian is rusty, but jól látta! 
It’s a Mini-ITX case based on the original Commodore injection molding tools. Someone in the UK bought them and made modifications so you can fit modern hardware in them (mine is one of the last made in the UK too, if you order one today you’ll get made in China). Instead of putting a PC in there, I put a in FPGA system (a guy in Spain makes a Mini-ITX kit) so I can run any computer/console from the last 40 years with everything from casette audio input to modern network support, both modern USB and old input support and both modern and old video output support as well (you can see a bit of a pro video CRT monitor in the background that I use with it for that real old school feel).
Sorry for the long and nerdy answer, but since you spotted nerdy details, I thought you might want some back! 
(Translation for Photography nerds: Imagine a hobbyist hardware kit camera that could take any lens from the last 40 years and simulate any sensor from the last 40 years, and optionally shoot both digital and film.) 
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@Jiyone thanks for sharing your xmp file. I tried to work out what made your edit so good. It seems that the guided laplacians for the highlights reconstruction module was the main secret. I have personally never had any success with this option, but I see you turned the threshold down and when I moved this slider, suddenly I too got a good result.
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Yes the laplacian hl reconstruction is the right module to use in that case to smooth highlights.
I had to lower the threshold because I saw that highlights were clipped on the histogram (and in the picture obviously) but they were not selected by the mask.
Then I choose the best suited diameter, and change the iteration number until desired results.
Excuse my ignorance … where did you find the laplacian hi reconstruction.
Looking at the .xmp I only see the inpaint version!
OK … I see it now in the filmic operation …
It was in the one I downloaded…make sure you loaded it…
It’s only available for Bayer sensors (that is, it’s unavailable for Fuji X-Trans). Most of the time it’s not much good, and is very slow.
https://darktable-org.github.io/dtdocs/en/module-reference/processing-modules/highlight-reconstruction/
The one in filmic is not really about recovery, but rather about ensuring a smooth transition. It does not know what pixels are clipped at the sensor level; rather, it uses the filmic white relative exposure as the threshold.
https://darktable-org.github.io/dtdocs/en/module-reference/processing-modules/filmic-rgb/#reconstruct
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Strangely it creates quite an effect with my Fuji X-Trans!
There seems to be a confusion. There is the guided laplacians method in highlight reconstruction, which is not available for X-Trans. You yourself wrote:
To that I responded by pointing out that the method is:
- not available for X-Trans; I even provided the link to the documentation
- different from the method in filmic.
And now you say ‘it’ (which from the context (It’s only available for Bayer sensors) one would interpret as the guided laplacian) creates ‘quite an effect’. It does not. Here is one of your X-Trans images, for which guided laplacian is not available:
Here is one of my Bayer images, for which it is:
Again: the reconstruction operation in filmic does not deal with clipped sensor data, you can even use it with JPGs. All it cares about is the white relative exposure.