Color appearance model: Difference between revisions

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{{More citations needed|date=September 2020}}

A '''color appearance model''' ('''CAM''') is a mathematical model that seeks to describe the [[perception|perceptual]] aspects of human [[color vision]], i.e. viewing conditions under which the appearance of a color does not tally with the corresponding physical measurement of the stimulus source. (In contrast, a [[color model]] defines a [[coordinate space]] to describe colors, such as the [[RGB color model|RGB]] and [[CMYK color model|CMYK]] color models.)

A '''uniform color space''' ('''UCS''') is a color model that seeks to make the color-making attributes perceptually uniform. A CAM under a fixed viewing condition results in a UCS; a UCS with a modeling of variable viewing conditions results in a CAM. A UCS without such modelling can still be used as a rudimentary CAM.

==Color appearance==

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==Color appearance parameters==

The basic challenge for any color appearance model is that human color perception does not work in terms of XYZ tristimulus values, but in terms of '''appearance parameters''' ([[hue]], [[lightness]], [[brightness]], [[colorfulness|chroma, colorfulness and saturation]]). So any color appearance model needs to provide transformations (which factor in viewing conditions) from the XYZ tristimulus values to these appearance parameters (at least hue, lightness and chroma).

==Color appearance phenomena==

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===CIELAB===

In 1976, the [[International Commission on Illumination|CIE]] set out to replace the many existing, incompatible color difference models by a new, universal model for color difference. They tried to achieve this goal by creating a ''perceptually uniform'' color space (UCS), i.e. a color space where identical spatial distance between two colors equals identical amount of perceived color difference. Though they succeeded only partially, they thereby created the [[Lab color space#CIELAB|CIELAB (“L*a*b*”)]] color space which had all the necessary features to become the first color appearance model. While CIELAB is a very rudimentary color appearance model, it is one of the most widely used because it has become one of the building blocks of [[color management]] with [[ICC profile]]s. Therefore, it is basically omnipresent in digital imaging.

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. ICC profiles circumvent this shortcoming by using the [[LMS color space#CIECAM97s, LLAB|Bradford transformation matrix]] to the LMS color space (which had first appeared in the [[#LLAB|LLAB color appearance model]]) in conjunction with CIELAB.

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===RLAB===

RLAB tries to improve upon the significant limitations of [[#CIELAB|CIELAB]] with a focus on image reproduction. It performs well for this task and is simple to use, but not comprehensive enough for other applications.

Unlike CIELAB, RLAB does allow for dealing with viewing conditions in the von Kries step by allowing a customized ''D'' value. "Discounting-the-illuminant" can still be used by using a fixed value of 1.0.<ref>10.1002/9781118653128.ch13</ref>

===LLAB===

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===Other models===

The following is a short list of other uniform color spaces:

;[[OSA-UCS|{{vanchor|OSA-UCS}}]]

: A 1947 color spaceUCS with generally good properties and a conversion from CIEXYZ defined in 1974. The conversion to CIEXYZ, however, has no closed-form expression, making it hard to use in practice.

;SRLAB2

:A 2009 modification of CIELAB UCS in the spirit of RLAB (but with discounting-the-illuminant). Uses CIECAM02 chromatic adaptation matrix to fix its hue issue.<ref name=Levien>{{cite web |title=An interactive review of Oklab |url=https://raphlinus.github.io/color/2021/01/18/oklab-critique.html |first1=Raph |last1=Levien |language=en |date=18 January 2021}}</ref>

;{{vanchor|JzAzBz}}

:A 2017 CAMUCS designed for HDR color. Has J (lightness) and two chromaticities.<ref>{{cite journal |last1=Safdar |first1=Muhammad |last2=Cui |first2=Guihua |last3=Kim |first3=Youn Jin |last4=Luo |first4=Ming Ronnier |title=Perceptually uniform color space for image signals including high dynamic range and wide gamut |journal=Optics Express |date=26 June 2017 |volume=25 |issue=13 |pages=15131 |doi=10.1364/OE.25.015131|doi-access=free }}</ref>

;XYB

:A family of color spacesUCS used in [[Guetzli]] and [[JPEG XL]], with a main goal in compression. Better uniformity than CIELAB.<ref name=Levien/>

;{{vanchor|Oklab}}

:A 2020 CAMUCS designed for normal dynamic range color. Same structure as CIELAB, but fitted with improved data (CAM16 output for lightness and chroma; IPT data for hue). Meant to be easy to implement and use, just like CIELAB and IPT were, but with improvements to uniformity.<ref>{{cite web |last1=Ottosson |first1=Björn |title=A perceptual color space for image processing |url=https://bottosson.github.io/posts/oklab/ |language=en}}</ref>

A uniform color space only becomes a CAM after the addition of viewing condition modeling, but they are useful in their own right.

==Notes==