Color appearance model

Color originates in the mind of the observer; “objectively”, there is only the spectral power distribution of the light that meets the eye.

However, successful attempts have been made to map the spectral power distribution of light to human sensory response in a quantifiable way.

However, the XYZ color model presupposes specific viewing conditions (such as the retinal locus of stimulation, the luminance level of the light that meets the eye, the background behind the observed object, and the luminance level of the surrounding light).

If the color temperature of the illuminating light source changes, so do the spectral power distribution and thereby the XYZ tristimulus values of the light reflected from the white paper; the color appearance, however, stays the same (white).

The chromatic adaptation transforms for some of these models are listed in LMS color space.

One of the limitations of CIELAB is that it does not offer a full-fledged chromatic adaptation in that it performs the von Kries transform method directly in the XYZ color space (often referred to as “wrong von Kries transform”), instead of changing into the LMS color space first for more precise results.

The wrong transform also seems responsible for its irregular blue hue, which bends towards purple as L changes, making it also a non-perfect UCS.

Development already started in the 1980s and by 1995 the model had become very complex (including features no other color appearance model offers, such as incorporating rod cell responses) and allowed to predict a wide range of visual phenomena.

RLAB tries to improve upon the significant limitations of CIELAB with a focus on image reproduction.

It gained widespread acceptance as a standard color appearance model until CIECAM02 was published.

Ebner and Fairchild addressed the issue of non-constant lines of hue in their color space dubbed IPT.

ITU-R BT.2100 includes a color space called ICtCp, which improves the original IPT by exploring higher dynamic range and larger colour gamuts.

[5] ICtCp can be transformed into an approximately uniform color space by scaling Ct by 0.5.

This allows it to incorporate spatial color appearance parameters like contrast, which makes it well-suited for HDR images.

[9][10] CAM16 is used in the Material Design color system in a cylindrical version called "HCT" (hue, chroma, tone).

[12] As of September 2023, it is part of the CSS color level 4 draft[13] and it is supported by recent versions of all major browsers.