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Spatial phenomena only affect colors at a specific ___location of an image, because the human brain interprets this ___location in a specific contextual way (e.g. as a shadow instead of grey colour). These phenomena are also known as [[optical illusion#Colour and brightness constancies|optical illusions]]. Because of their contextuality, they are especially hard to model; colour appearance models that try to do this are referred to as [[ICAM (Colour Appearance Model)|image colour appearance models (iCAM)]].
==ColorColour appearance models==
Since the colorcolour appearance parameters and colorcolour appearance phenomena are numerous and the task is complex, there is no single colorcolour appearance model that is universally applied; instead, various models are used.
This section lists some of the colorcolour appearance models in use. The chromatic adaptation transforms for some of these models are listed in [[LMS colorcolour space]].
===CIELAB===
In 1976, the [[International Commission on Illumination|CIE]] set out to replace the many existing, incompatible colorcolour difference models by a new, universal model for colorcolour difference. They tried to achieve this goal by creating a ''perceptually uniform'' colorcolour space, i.e. a colorcolour space where identical spatial distance between two colorscolours equals identical amount of perceived colorcolour difference. Though they succeeded only partially, they thereby created the [[Lab color space#CIELAB|CIELAB (“L*a*b*”)]] colorcolour space which had all the necessary features to become the first colorcolour appearance model. While CIELAB is a very rudimentary colorcolour 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 colorcolour 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 colorcolour space (which had first appeared in the [[#LLAB|LLAB color appearance model]]) in conjunction with CIELAB.
===Nayatani et al. model===
The Nayatani et al. colorcolour appearance model focuses on illumination engineering and the colorcolour rendering properties of light sources.
===Hunt model===
The Hunt colorcolour appearance model focuses on colorcolour image reproduction (its creator worked in the [[Kodak#Kodak_Research_Laboratories|Kodak Research Laboratories]]). Development already started in the 1980s and by 1995 the model had become very complex (including features no other colorcolour appearance model offers, such as incorporating [[rod cell]] responses) and allowed to predict a wide range of visual phenomena. It had a very significant impact on [[#CIECAM02|CIECAM02]], but because of its complexity the Hunt model itself is difficult to use.
===RLAB===
===CIECAM97s===
After starting the evolution of colorcolour appearance models with [[#CIELAB|CIELAB]], in 1997, the CIE wanted to follow up itself with a comprehensive colorcolour appearance model. The result was CIECAM97s, which was comprehensive, but also complex and partly difficult to use. It gained widespread acceptance as a standard colorcolour appearance model until [[#CIECAM02|CIECAM02]] was published.
===IPT===
Ebner and Fairchild addressed the issue of non-constant lines of hue in their colorcolour space dubbed ''IPT''.<ref>
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The IPT colorcolour 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 colorcolour appearance model, but hard to achieve). It is therefore well-suited for [[Color management#Gamut mapping|gamut mapping]] implementations.
===ICtCp===
ITU-R BT.2100 includes a colorcolour space called ''[[ICtCp]]'', which improves the original IPT by exploring higher dynamic
range and larger colour gamuts.<ref>
{{Citation
===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) colorcolour appearance model.
===iCAM06===
[[ICAM (Color Appearance Model)|iCAM06]] is an [[ICAM (Color Appearance Model)|image colorcolour appearance model]]. As such, it does not treat each pixel of an image independently, but in the context of the complete image. This allows it to incorporate spatial colorcolour appearance parameters like contrast, which makes it well-suited for [[High-dynamic-range imaging|HDR]] images. It is also a first step to deal with [[#Spatial phenomena|spatial appearance phenomena]].
==Notes==
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