Color appearance model: Difference between revisions

Revision as of 08:38, 25 July 2018

Revision as of 08:41, 25 July 2018

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The basic challenge for any colour appearance model is that human colour perception does not work in terms of XYZ tristimulus values, but in terms of '''appearance parameters''' ([[hue]], [[lightness]], [[brightness]], [[colourfulness|chroma, colourfulness and saturation]]). So any colour 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).

==ColorColour appearance phenomena==

This section describes some of the colorcolour appearance phenomena that color appearance models try to deal with.

===Chromatic adaptation===

[[Chromatic adaptation]] describes the ability of human colorcolour perception to abstract from the [[white point]] (or [[colorcolour 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 colorcolour appearance phenomena, and therefore a '''chromatic adaptation transform''' ('''CAT''') that tries to emulate this behavior is a central component of any colorcolour appearance model.

This allows for an easy distinction between simple tristimulus-based colorcolour models and colorcolour appearance models. A simple tristimulus-based colorcolour model ignores the white point of the illuminant when it describes the surface colorcolour of an illuminated object; if the white point of the illuminant changes, so does the colorcolour of the surface as reported by the simple tristimulus-based colorcolour model. In contrast, a colorcolour appearance model takes the white point of the illuminant into account (which is why a colorcolour appearance model requires this value for its calculations); if the white point of the illuminant changes, the colorcolour of the surface as reported by the colorcolour 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'' colorcolour ''appearance''. If the colorcolour 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 colorcolour ''appearance'', however, stays the same (white).

===Hue appearance===

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* '''[[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 colorcolour-neutral).

===Contrast appearance===

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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.

===ColorfulnessColourfulness appearance===

There is an effect which changes the perception of colorfulnesscolourfulness by a human observer:

* '''Hunt effect:''' ColorfulnessColourfulness increases with luminance.

===Brightness appearance===

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===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 graygrey colorcolour). These phenomena are also known as [[optical illusion#ColorColour and brightness constancies|optical illusions]]. Because of their contextuality, they are especially hard to model; colorcolour appearance models that try to do this are referred to as [[ICAM (ColorColour Appearance Model)|image colorcolour appearance models (iCAM)]].

==Color appearance models==