"HCL" designed in 2005 by Sarifuddin and Missaou, which is a transformation of whatever type of RGB color space is in use.
A name for a cylindrical transformation of CIELuv employed by Ihaka and adopted by Zeileis et al.. This name appears to be the one most commonly used in information visualization. Ihaka, Zeileis, and co-authors also provide software implementations and web pages to promote its use.
A name for , employed by chroma.js.
Derivation
Color-making attributes
HCL concerns the following attributes of color appearance: ; Hue: The "attribute of a visual sensation according to which an area appears to be similar to one of the perceived colors: red, yellow, green, and blue, or to a combination of two of them". ; Lightness, value: The "brightness relative to the brightness of a similarly illuminated white". ; Luminance : The radiance weighted by the effect of each wavelength on a typical human observer, measured in SI units in candela per square meter. Often the term luminance is used for the relative luminance, Y/Yn, where Yn is the luminance of the reference white point. ; Colorfulness: The "attribute of a visual sensation according to which the perceived color of an area appears to be more or less chromatic".
Sarifuddin 2005
The HSL and HSV color spaces are more intuitive translations of the RGB color space, because they provide a single hue number. However, their luminance variation does not match the way humans perceive color. Perceptually uniformcolor spaces outperform RGB in cases such as high noise environments. Sarifuddin, noting the lack of blue hue consistency of CIELab—a common complaint among its users—decided to make their own color space by mashing up some of the features. The result, being a simpler transformation from sRGB than the other color spaces, may have more potential in machine learning.
From RGB
The luminance is essentially a combination of the HSL "L" with a correction factor. Q is a tuning parameter that varies luminosity between a highly saturated color and white: Chroma also uses the correction factor to maintain linearity. It also reshapes the color space into a cone superficially similar to the one seen in HSV. The Hue calculation, like Chroma, somewhat resembles the circular/nonhexagonal variant of HSL.
Color difference
The CH correction factor has been described as unrealistic. Unlike what is claimed in the paper, it does not appear to be an improvement over established color difference metrics.
To RGB
See the long-form corrected report.
CIE color spaces
CIE-based Lch color spaces are transformations of the two chroma values into the polor coordinate. See the respective articles for how the underlying coordinates are derived.
Implementations
CIE Lch has been implemented in a wide range of ways: as programmatic code for generating color swatches in statistics tools, as standalone tools for designing and testing swatches, or as libraries that allow other programs to use the color space. Some implementations include:
