gamut_mapping: add tonal gamut-mapping function

Based on a radically new approach of transforming hue leaves into a new bespoke colorspace called "tone saturation hue", which is designed to represent colors in a hue-leaf-relative representation.

Every color with a tone (T) parameter above 0.0 is subjectively brighter than the maximally saturated color of this hue, while every color with a tone below 0.0 is subjectively darker. This means that all colors crossing the T=0.0 plane are basically on the same intensity as the maximally saturated color for that hue.

Similarly, the saturation (S) parameter has been normalized so that S=1.0 always corresponds to the maximally saturated color of that hue leaf. So all of the vertices of the RGB cube have S=1 T=0, with differing values of hue.

This is combined with a simple quadratic rolloff to protect the in-gamut region from being warped, resulting in a nice, smooth colorimetric mapping.

Provided in both a hue-preserving and hue-shifting variant. Usefulness TBD.

Maybe this solves some of the issues people were having with perceptual?

Actually, one thing this function can do very well is apply bidirectional mapping, i.e. increase saturation of standard gamut content on wide gamut monitors. (In this case I would prefer the non-hue-shifting variant)

WIP, need to fix some clipping issues

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• b85f9666 - wip tonal shift

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marked this merge request as draft

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• b85f9666...e6a89f31 - 2 commits from branch videolan:master
• 60f6a2ab - gamut_mapping: fix hue shift margin direction
• 852a8822 - gamut_mapping: factor out hue shifting code
• 7270e772 - gamut_mapping: safety clamp on desat_bounded()
• 30506060 - gamut_mapping: protect against UB in softclip()
• b721bd3c - gamut_mapping: make strength of hue shift tweakable
• 1d1c87f6 - gamut_mapping: add tonal gamut-mapping function
• 26c791e6 - gamut_mapping: slightly reduce hue shifting strength

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• a8e19f51 - gamut_mapping: add tonal gamut-mapping function
• 5392aea5 - gamut_mapping: slightly reduce hue shifting strength

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added 6 commits

• f53badbb - 1 commit from branch videolan:master
• 0c17fc02 - gamut_mapping: fix hue shift margin direction
• 049a8689 - gamut_mapping: factor out hue shifting code
• f40bd853 - gamut_mapping: safety clamp on desat_bounded()
• 5307129b - gamut_mapping: protect against UB in softclip()
• 167f1da2 - gamut_mapping: add tonal gamut-mapping function

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Okay, after re-analyzing this I figured out why it failed/clipped, and fixed it properly (with RGB softclip pass). Now it passes all my expected tests. I also re-nuked the drop of hue shift strength to 0.8, it doesn't benefit anything anymore (after the mentioned bugs were fixed)

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The reverse tone-mapping seems a bit weak at reds, sadly. Saturation mapping still does a better job here. It would be ideal if I could get a sort of saturation mapper that also protects skin tones.

One way to make it a lot stronger is to switch the mixing function from quadratic to linear. I'll do some testing to see if it messes up skin tones too much.

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• dd3beb16 - gamut_mapping: add tonal gamut-mapping function

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After careful testing, I decided that the best replacement for the mixing function is a pl_smoothstep() with endpoints 0.2 and 0.8. This provides a nice, gradual step, allowing saturated colors to be mapped while in-gamut colors are protected.

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• 4b4a1c56 - gamut_mapping: add tonal gamut-mapping function

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Slight fix to the softclip peak calculation, and re-upped the smoothstep endpoint from 0.8 to 1.0 to reduce excessive desaturation of very strongly saturated highlights

So, I'm considering axing the 'tonal' name and instead making this the new perceptual, with a rebrand of the old perceptual to softclip.

The reason is that I simply think this is the best quality by far, works bidirectionally (and looks great while doing so), fixes some issues perceptual had, makes most highlights much less 'desaturated', and overall comes closer in spirit to what a 'perceptual' mode should be.

mentioned in merge request !501 (closed)

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reopened

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• b7c9e9c2 - 1 commit from branch videolan:master
• 96d62742 - gamut_mapping: fix hue shift margin direction
• ba50373c - gamut_mapping: factor out hue shifting code
• c3ea00e6 - gamut_mapping: safety clamp on desat_bounded()
• 29aeb94e - gamut_mapping: protect against UB in softclip()
• 98f46bad - gamut_mapping: add tonal gamut-mapping function

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mentioned in merge request !516 (merged)

closed