Out of gamut colors, matrix, hsv

Software darktable
2

No. No. And no again. Do you even understand what HSL is ? It’s bloody RGB 3D space turned into polar coordinates.

It sucks. It deeply sucks. For soooo many reasons:

  1. not perceptually uniform, aka 10% hue shift in blue will be 200% bigger than a 10% hue shift in red,
  2. 100% dependent on the base RGB space, aka sRGB HSL ≠ Rec2020 HSL,
  3. it’s not hue-invariant, aka reduce HSL “saturation” by 50% and you have changed the (really perceptual) hue too,
  4. not chromaticity-related, aka no connection with light spectrum as well,
  5. the very name “hue-saturation-value/lightness” is misleading because it has actually nothing to do with psychological color perception at all.

HSL spaces go back to the 1980’s. If they were good for gamut mapping, by now we would know it. And yet, CIE is still looking for the perfect gamut-mapping space : CIECAM02 failed, CIECAM16 doesn’t do HDR, IPT-HDR is still not perfectly hue-invariant, JzAzBz is kind of ok but still experimental.

Just because HSL/HSV produces ok-ish results in some cases doesn’t make it a reliable working space.

Please read https://last.hit.bme.hu/download/firtha/video/Colorimetry/Fairchild_M._Color_appearance_models__2005.pdf

There are only 2 ways to do image processing:

  1. the psychological/perceptual way, with color adaptation models like Lab, IPT, CIECAM02, CIECAM16, Hunt-Lab and so on. Problem → they all fail in HDR
  2. the physical/radiometric way, with linear RGB code values as a metric for a light emission.

HSV/HSL is none of the above.

Gamut mapping is a well-known problem of shrinking color spaces and remap source colors to the closest available color in the target space. Again, you can either choose a perceptually based strategy (minimizing the delta E or any other metric of visual color distance) or a chromaticity based strategy (minimizing the wavelength spectral difference).

But HSL/HSV allows none of the above.

The only valid case of HSL use is to “simplify” the GUI of artistic RGB operations. But then, translate the params from HSL to RGB before applying the actual pixel operation.

3 Likes