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A '''colourcolor appearance model''' (abbreviated '''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 colourcolor does not tally with the corresponding physical measurement of the stimulus source. (In contrast, a [[color model]] defines a [[coordinate space]] to describe colourscolors, such as the [[RGB color model|RGB]] and [[CMYK color model|CMYK]] color models.)
==ColourColor appearance==
[[ColourColor]] originates in the mind of the observer; “objectively”, there is only the [[spectral power distribution]] of the light that meets the eye. In this sense, ''any'' colourcolor perception is subjective. However, successful attempts have been made to map the spectral power distribution of light to human sensory response in a quantifiable way. In 1931, using [[Psychophysics|psychophysical]] measurements, the [[International Commission on Illumination|International Commission on Illumination (CIE)]] created the [[CIE 1931 colourcolor space|XYZ colourcolor space]]<ref>“XYZ” refers to a colourcolor ''model'' and a colourcolor ''space'' at the same time, because the XYZ colourcolor space is the only colourcolor space that uses the XYZ colourcolor model. This differs from e.g. the RGB colourcolor model, which many colourcolor spaces (such as [[sRGB]] or [[Adobe RGB color space|Adobe RGB (1998)]]) use.</ref> which successfully models human colourcolor vision on this basic sensory level.
However, the XYZ colourcolor 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 colourcolor ''appearance'' for a human observer. If some conditions change in one case, two identical stimuli with thereby identical XYZ tristimulus values will create ''different'' colourcolor ''appearances'' (and vice versa: two different stimuli with thereby different XYZ tristimulus values might create an ''identical'' colourcolor ''appearance'').
Therefore, if viewing conditions vary, the XYZ colourcolor model is not sufficient, and a colourcolor appearance model is required to model human colourcolor perception.
==ColourColor appearance parameters==
The basic challenge for any colourcolor appearance model is that human colourcolor perception does not work in terms of XYZ tristimulus values, but in terms of '''appearance parameters''' ([[hue]], [[lightness]], [[brightness]], [[colourfulnesscolorfulness|chroma, colourfulnesscolorfulness and saturation]]). So any colourcolor 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).
==ColourColor appearance phenomena==
This section describes some of the colourcolor appearance phenomena that color appearance models try to deal with.
===Chromatic adaptation===
[[Chromatic adaptation]] describes the ability of human colourcolor perception to abstract from the [[white point]] (or [[colourcolor 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 colourcolor appearance phenomena, and therefore a '''chromatic adaptation transform''' ('''CAT''') that tries to emulate this behavior is a central component of any colourcolor appearance model.
This allows for an easy distinction between simple tristimulus-based colourcolor models and colourcolor appearance models. A simple tristimulus-based colourcolor model ignores the white point of the illuminant when it describes the surface colourcolor of an illuminated object; if the white point of the illuminant changes, so does the colourcolor of the surface as reported by the simple tristimulus-based colourcolor model. In contrast, a colourcolor appearance model takes the white point of the illuminant into account (which is why a colourcolor appearance model requires this value for its calculations); if the white point of the illuminant changes, the colourcolor of the surface as reported by the colourcolor appearance model remains the same.
Chromatic adaptation is a prime example for the case that two different stimuli with thereby different XYZ tristimulus values create an ''identical'' colourcolor ''appearance''. If the colourcolor 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 colourcolor ''appearance'', however, stays the same (white).
===Hue appearance===
* '''[[Bezold–Brücke shift|Bezold–Brücke hue shift]]:''' The hue of monochromatic light changes with [[luminance]].
* '''[[Abney effect]]:''' The hue of monochromatic light changes with the addition of white light (which would be expected colourcolor-neutral).
===Contrast appearance===
* '''Bartleson-Breneman effect:''' Image contrast (of emissive images such as images on an LCD display) increases with the luminance of surround lighting.
===ColourfulnessColorfulness appearance===
There is an effect which changes the perception of colourfulnesscolorfulness by a human observer:
* '''Hunt effect:''' ColourfulnessColorfulness increases with luminance.
===Brightness appearance===
===Spatial phenomena===
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 greygray colourcolor). These phenomena are also known as [[optical illusion#ColourColor and brightness constancies|optical illusions]]. Because of their contextuality, they are especially hard to model; colourcolor appearance models that try to do this are referred to as [[ICAM (ColourColor Appearance Model)|image colourcolor appearance models (iCAM)]].
==ColourColor appearance models==
Since the colourcolor appearance parameters and colourcolor appearance phenomena are numerous and the task is complex, there is no single colourcolor appearance model that is universally applied; instead, various models are used.
This section lists some of the colourcolor appearance models in use. The chromatic adaptation transforms for some of these models are listed in [[LMS colourcolor space]].
===CIELAB===
In 1976, the [[International Commission on Illumination|CIE]] set out to replace the many existing, incompatible colourcolor difference models by a new, universal model for colourcolor difference. They tried to achieve this goal by creating a ''perceptually uniform'' colourcolor space, i.e. a colourcolor space where identical spatial distance between two colourscolors equals identical amount of perceived colourcolor difference. Though they succeeded only partially, they thereby created the [[Lab color space#CIELAB|CIELAB (“L*a*b*”)]] colourcolor space which had all the necessary features to become the first colourcolor appearance model. While CIELAB is a very rudimentary colourcolor 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 colourcolor 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 colourcolor space (which had first appeared in the [[#LLAB|LLAB color appearance model]]) in conjunction with CIELAB.
===Nayatani et al. model===
The Nayatani et al. colourcolor appearance model focuses on illumination engineering and the colourcolor rendering properties of light sources.
===Hunt model===
The Hunt colourcolor appearance model focuses on colourcolor 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 colourcolor 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 colourcolor appearance models with [[#CIELAB|CIELAB]], in 1997, the CIE wanted to follow up itself with a comprehensive colourcolor appearance model. The result was CIECAM97s, which was comprehensive, but also complex and partly difficult to use. It gained widespread acceptance as a standard colourcolor appearance model until [[#CIECAM02|CIECAM02]] was published.
===IPT===
Ebner and Fairchild addressed the issue of non-constant lines of hue in their colourcolor space dubbed ''IPT''.<ref>
{{Citation
| last1 = Ebner
</ref>
The IPT colourcolor 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 colourcolor 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 colourcolor 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) colourcolor appearance model.
===iCAM06===
[[ICAM (Color Appearance Model)|iCAM06]] is an [[ICAM (Color Appearance Model)|image colourcolor 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 colourcolor 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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