This appears common for many cameras and does not even remotely surprise me - back long ago, it was probably the simplest computationally, and camera manufacturers didn’t want people to complain about sudden massive changes in color rendering once more advanced curve approaches were feasible.
I’m 95% certain Sony’s JPEG engine still does per-channel tone curves (darktable’s basecurve module in its original pre-“preserve colors” form with a “Sony-like” curve was an almost 100% match to Sony JPEGs), and they continue to do this because while a vocal minority endlessly bitches about Sony colors due to some of the corner cases where hue twisting gets seriously problematic, most users are used to that behavior at this point.
Also of note: In the case of a camera underexposing significantly, it might be beneficial to look to see if there are any EXIF tags that hint at some form of DRO in play. Of course, if that’s the case, the camera might also be doing local tonemapping which will FUBAR any attempts to duplicate the behavior with a simple tone curve match.
would you be please so kind and point me to the DOCu that describe (or some video on yt) your workflow, I am unfortunately not that good with RT and I simply don’t understand well howto adopt your workflow, eg you’re saying:
you can get a much better match with the embedded JPEG if you match on the RGB channels separately.
Hi Dan, the match I was talking about was done outside RawTherapee purely for academical purposes, so it’s of little practical use unfortunately. The way in which the tone curves work in RT is not suitable for RGB-independent matching, but maybe we ought to support that at some point in the future. Definitely not something for the next release, I’m sorry to say.
I will repeat my earlier claim that when you switch to ‘Luminance’ mode instead of ‘Film-like’ you will usually get a bit closer to the embedded JPEG.
And finally, FWIW, the tone curve generated here is pretty sensible: the image is quite underexposed so there are very little midtones and highlights. Therefore the curve rises steeply and flattens off.
I am confused by this whole discussion. It seems to be working on the assumption that tone = color. I always understood the tone curve to change how light or dark a color is, but not to change the fundamental color. Fuji applies film simulations to its jpgs, which also alter the colors in addition to the shades. If you want to match one of these, you need to use a LUT and not just a tone curve.
I suspect that the way Fuji handles exposure compensation and whether that is read or used might also contribute to some of these issues…RT may not use this info when it matches against the JPG but Fuji does creating its JPG…
Update: Thanks to some awesome people like Iliah Borg, we now know the reason why Fuji RAW files appear darker. Turns out that Fuji has a special tag (0x9650) in its RAW files that highlights the necessary midpoint compensation for RAW files to interpret and make necessary changes. Below are the values for the Fuji X-T1:
ISO 200 / 0.72EV
ISO 400 / 0.72EV
ISO 800 / 0.72EV
ISO 1600 / 0.72EV
ISO 3200 / 1.38EV
ISO 6400 / 2.38EV
So keep the above in mind when looking at Fuji RAW files and comparing them to other cameras. If you are using a RAW converter from Adobe (and potentially other RAW converters), make sure to look at the above table for adjustments needed to make images appear as they should. Big thanks to Iliah Borg and the LibRaw team for discovering the Fuji tags and letting us know!
Beyond what is discussed there, many cameras implement a per-channel RGB tone curve that is well known to cause hue twisting - see the discussion of curve mode at Exposure - RawPedia
Maarten has a nice demonstration of these in the last half of his video with vectorscope representations in the primary colors showing the hue rotations and saturation changes…he plays them in sequence at the end of the video…I guess you could model on those for a reasonable approximation…
The vectorscope shots @priort provided happen to only be of a shot of that reference card, so any white balance offsets or the fact that pigments aren’t purely saturated would lead to the inputs for the primaries being not exactly on a primary, and thus being prone to hue twisting.
@Thanatomanic - when you mean “per-channel matching” - did you implement separate curves per channel, or a single curve, but matching the histograms on a per-channel basis?
I linked above pictures (like this one https://infophagia.com/ntz/paste/DSCF2782.tgz) where are not used film-sumulations … this is just “standard” FUJI photo without any simulation and yet the Colours Curve seems auto-guessed very wildly …
I don’t dispute that … FUJI certainly does something weird with pictures and exposure …
