HCL color space

HCL (Hue-Chroma-Luminance) is a color space model designed to accord with human perception of color. HCL has been adopted by information visualization practitioners to present data without the bias implicit in using varying saturation.

HCL is designed to have characteristics of both cylindrical translations of RGB color space such as HSL and HSV and L*a*b* color space. HSL and HSV color spaces have the benefit of being perceptually uniform translations of the RGB color space, but their luminance variation does not match the way humans perceive color. Perceptually uniform translations of RGB colorspace do have the benefit of outperforming RGB in cases such as high noise environments. L*a*b colorspace does correspond to the three channels of human perception, but have poor hue constancy, especially in the blue range.

HCL uses the CIELAB model defined by the International Commission on Illumination (CIE) in 1976, translated into polar coordinates. HCL preserves the L (luminance) axis of L*a*b*, but transforms ab to polar coordinates, where the distance from zero is the chroma (an alternative measure of colorfulness), and the phase (angle) is our familiar hue. The older Munsell color system is based on different mathematics, but has some similarity to HCL.

The dimensions of the HSL and HSV geometries—simple transformations of the not-perceptually-based RGB model—are not directly related to the photometric color-making attributes of the same names, as defined by scientists such as the CIE or ASTM. Nonetheless, it is worth reviewing those definitions before leaping into the derivation of our models.

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