Thanks! I could find little information on HCL, but based on this blog post [0] it seems that it is a cylindrical version of CIE LUV. This is the color space that HUSL is based on. What HUSL does is it replaces the chroma component with a saturation component that is defined as a percentage ranging from 0 to the maximum chroma for the given hue and lightness. To put it in another way, it stretches the chroma component to make it fit into a nice rectangle. This makes all components bounded and lets us pick color without stepping outside of the gamut.
UPLab looks cool, thanks for sharing.
UPLab is not a “slight tweak” of CIELAB. It’s a lookup table for going back and forth between CIELAB and a slightly rotated version of Munsell space, https://en.wikipedia.org/wiki/Munsell_color_system
You can look at it that way. You can also see it as a modification of cylindrical LAB.
Explain your "slightly rotated" comment?
From Lindbloom’s site: “The mapping was done such that Munsell hue 2.5RP has a UP Lab hue angle of zero degrees. This particular rotation was chosen to allow greater chroma encoding of blue colors in the lower right quadrant of the a/b plane.”
Also, to be completely precise, it should be noted that the lightness dimension of UPLab was left the same as CIELAB’s lightness, rather than using Munsell value. Only a and b were adjusted to reflect Munsell hue/chroma.
There is also uniform perceptual LAB: http://www.brucelindbloom.com/index.html?UPLab.html which is a slight tweak to LAB to reduce the 'blue -> purple' effect that happens when you tweak chroma.