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{{further|Color perception}}
[[Color]] originates in the mind of the observer;
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). Only if all these conditions stay constant will two identical stimuli with thereby identical XYZ [[CIE 1931 color space#Tristimulus values|tristimulus]] values create an identical '''color appearance''' for a human observer. If some conditions change in one case, two identical stimuli with thereby identical XYZ tristimulus values will create {{em|different}} color appearances (and vice versa: two different stimuli with thereby different XYZ tristimulus values might create an {{em|identical}} color appearance).
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This section describes some of the color appearance phenomena that color appearance models try to deal with.
====
{{Main|Chromatic adaptation}}
Chromatic adaptation describes the ability of human color perception to abstract from the [[white point]] (or [[color temperature]]) of the illuminating light source when observing a reflective object. For the human eye, a piece of white paper looks white no matter whether the illumination is blueish or yellowish. This is the most basic and most important of all color appearance phenomena, and therefore a ''[[chromatic adaptation transform]]'' (CAT) that tries to emulate this behavior is a central component of any color appearance model.
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Several effects change the perception of hue by a human observer:
* '''[[Bezold–Brücke shift|Bezold–Brücke hue shift]]:''' The hue of
* '''[[Abney effect]]:''' The hue of monochromatic light changes with the addition of white light (which would be expected color-neutral).
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There is an effect which changes the perception of brightness by a human observer:
* '''[[Helmholtz–Kohlrausch effect]]:''' Brightness increases with saturation. Not modeled by CIECAM02.
* Contrast appearance effects (see above), modeled by CIECAM02.
====Spatial phenomena====
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This section lists some of the color appearance models in use. The chromatic adaptation transforms for some of these models are listed in [[LMS color space]].
===
{{Main|CIELAB color space}}
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 [[CIELAB color space|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.
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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 uses a proper von Kries step. It also allows for tuning the degree of adaptation by allowing a customized ''D'' value. "Discounting-the-illuminant" can still be used by using a fixed value of 1.0.<ref>{{cite book | doi=10.1002/9781118653128.ch13 | chapter=The RLAB Model | title=Color Appearance Models | date=2013 | pages=243–255 | isbn=9781119967033 }}</ref>
===LLAB===
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===CIECAM97s===
After starting the evolution of color appearance models with [[#CIELAB|CIELAB]], in 1997, the CIE wanted to follow up
===
{{Main|ICtCp#In IPT}}
Ebner and Fairchild addressed the issue of non-constant lines of hue in their color space dubbed ''IPT''.<ref>
{{Citation
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| series = Proc. IS&T 6th Color Imaging Conference
| place = Scottsdale, AZ
| pages = 8–13
| year = 1998
}}
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| first = Christopher
| title = US Patent 8,437,053, Gamut mapping using hue-preserving color space
| url =
| access-date = 9 February 2016
}}
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The IPT color appearance model excels at providing a formulation for hue where a constant hue value equals a constant perceived hue independent of the values of lightness and chroma (which is the general ideal for any color appearance model, but hard to achieve). It is therefore well-suited for [[Color management#Gamut mapping|gamut mapping]] implementations.
===
{{Main|ICtCp}}
ITU-R BT.2100 includes a color space called ''[[ICtCp]]'', which improves the original IPT by exploring higher dynamic
range and larger colour gamuts.<ref>
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| year = 2016
}}
</ref> ICtCp can be transformed into an approximately uniform color space by scaling Ct by 0.5. This transformed color space is the basis of the Rec. 2124 wide gamut color difference metric ΔE<sub>ITP</sub>.<ref>{{cite web |title=Recommendation ITU-R BT.2124-0 Objective metric for the assessment
===
{{Main|CIECAM02}}
After the success of [[#CIECAM97s|CIECAM97s]], the CIE developed [[CIECAM02]] as its successor and published it in 2002. It performs better and is simpler at the same time. Apart from the rudimentary [[#CIELAB|CIELAB]] model, CIECAM02 comes closest to an internationally agreed upon “standard” for a (comprehensive) color appearance model.
Both CIECAM02 and CIECAM16
===iCAM06===
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===CAM16===
The CAM16 is a successor of CIECAM02 with various fixes and improvements. It also comes with a color space called CAM16-UCS. It is published by a CIE workgroup, but is not CIE standard.<ref>{{cite journal |last1=Li |first1=Changjun |last2=Li |first2=Zhiqiang |last3=Wang |first3=Zhifeng |last4=Xu |first4=Yang |last5=Luo |first5=Ming Ronnier |last6=Cui |first6=Guihua |last7=Melgosa |first7=Manuel |last8=Brill |first8=Michael H. |last9=Pointer |first9=Michael |title=Comprehensive color solutions: CAM16, CAT16, and CAM16-UCS |journal=Color Research & Application |date=December 2017 |volume=42 |issue=6 |pages=703–718 |doi=10.1002/col.22131}}</ref> CIECAM16 standard was released in 2022 and is slightly different.<ref>{{Cite web |title=The CIE 2016 Colour Appearance Model for Colour Management Systems: CIECAM16 {{!}} CIE |url=https://cie.co.at/publications/cie-2016-colour-appearance-model-colour-management-systems-ciecam16 |access-date=2022-09-16 |website=cie.co.at |language=en}}</ref><ref>{{Cite web |title=PR: Implement support for "CIECAM16" colour appearance model. by KelSolaar · Pull Request #1015 · colour-science/colour |url=https://github.com/colour-science/colour/pull/1015 |access-date=2022-09-16 |website=GitHub |language=en}}</ref>
CAM16 is used in the [[Material Design]] color system in a cylindrical version called "HCT" (hue, chroma, tone). The hue and chroma values are identical to CAM16. The "tone" value is CIELAB L*.<ref>{{cite web |last1=O'Leary |first1=James |title=The Science of Color & Design |url=https://material.io/blog/science-of-color-design |website=Material Design |language=en}} [https://github.com/material-foundation/material-color-utilities source code]</ref>
===OKLab===
{{Main|Oklab color space}}
As of September 2023, it is part of the [[CSS color]] level 4 draft<ref>{{cite web |title=CSS Color Module Level 4 |url=https://www.w3.org/TR/css-color-4/#resolving-oklab-oklch-values |website=www.w3.org}}</ref> and it is supported by recent versions of all major browsers.<ref>{{cite web |title=oklab() (Oklab color model) |url=https://caniuse.com/mdn-css_types_color_oklab |website=Can I use... |access-date=27 September 2023}}</ref>
===Other models===
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:A 2009 modification of CIELAB in the spirit of RLAB (with discounting-the-illuminant). Uses CIECAM02 chromatic adaptation matrix to fix the blue 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 UCS 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=
;XYB
:A family of UCS used in [[Guetzli]] and [[JPEG XL]], with a main goal in compression. Better uniformity than CIELAB.<ref name=Levien/>
▲:A 2020 UCS 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 (especially from sRGB), 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> As of December 2022, it is part of the [[CSS color]] level 4 draft<ref>{{cite web |title=CSS Color Module Level 4 |url=https://www.w3.org/TR/css-color-4/#resolving-oklab-oklch-values |website=www.w3.org}}</ref> and it is supported by [[Safari (web browser)|Safari]] 15.4 and newer.<ref>{{cite web |title=oklab() (Oklab color model) |url=https://caniuse.com/mdn-css_types_color_oklab |website=Can I use... |access-date=2 December 2022}}</ref>
==Notes==
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