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A '''uniform color space''' ('''UCS''') is a color model that seeks to make the color-making attributes perceptually uniform, i.e. identical spatial distance between two colors equals identical amount of perceived color difference. 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.
==Background==
==Color appearance==▼
▲===Color appearance===
{{further|Color perception}}
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Therefore, if viewing conditions vary, the XYZ color model is not sufficient, and a color appearance model is required to model human color perception.
===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===
This section describes some of the color appearance phenomena that color appearance models try to deal with.
====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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Chromatic adaptation is a prime example for the case that two different stimuli with thereby different XYZ tristimulus values create an ''identical'' color ''appearance''. 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).
====Hue appearance====
Several effects change the perception of hue by a human observer:
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* '''[[Abney effect]]:''' The hue of monochromatic light changes with the addition of white light (which would be expected color-neutral).
====Contrast appearance====
[[File:Bartleson-Breneman effect.png|thumb|200px|Bartleson–Breneman effect]]
Several effects change the perception of [[Contrast (vision)|contrast]] by a human observer:
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* '''Bartleson–Breneman effect:''' Image contrast (of emissive images such as images on an LCD display) increases with the luminance of surround lighting.
====Colorfulness appearance====
{{further|Colorfulness#In color appearance models}}
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* '[[Hunt effect (color)|Hunt effect]]: Colorfulness increases with luminance.
====Brightness appearance====
There is an effect which changes the perception of brightness by a human observer:
* '''[[Helmholtz–Kohlrausch effect]]:''' Brightness increases with saturation.
====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 gray color). These phenomena are also known as [[optical illusion#Color and brightness constancies|optical illusions]]. Because of their contextuality, they are especially hard to model; color appearance models that try to do this are referred to as [[ICAM (Color Appearance Model)|image color appearance models (iCAM)]].
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