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Implement LCH for Lab and YCbCr

Wolthera van Hövell requested to merge woltherav/krita:add-lch-to-visual-picker into master

I added LCH as the color conversion to be used by YCbCr and LAB in the Wide Gamut Color selector. It is somewhat underwhelming, but looks better than just driving the channels directly.

It is underwhelming because LAB is kinda really big compared to sRGB, and furthermore, our clipping is happening in the end result RGB, and not in CIELAB LCH itself. This is particularly noticeble with L* = 0 and L* = 100% showing blue and red and the like at the outer edges, if they were clipped differently, this wouldn't happen. However, that requires quite a bit of work, in the meanwhile, this does look better for Lab and YCbCr.

Test Plan

Make a LAB or YCbCr image, look at Wide Gamut selector.

In Krita Master this'll show the L*a*b* channels, in this patch, it looks a little bit more orderly.

NOTE: there's this weird thing where, if you load a lab colorspace image, it may not actually show the selector. I'm not sure what's causing this, or if it is a case of gremlins in my pc...

Formalities Checklist

  • I confirmed this builds.
  • I confirmed Krita ran and the relevant functions work.
  • I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
  • I made sure my commits build individually and have good descriptions as per KDE guidelines.
  • I made sure my code conforms to the standards set in the HACKING file.
  • I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.
  • Does the patch add a user-visible feature? If yes, is there a documentation MR ready for it at Krita Documentation Repository?

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