A colour appearance model (abbreviated CAM) is a mathematical model that seeks to describe the perceptual aspects of human color vision, i.e. viewing conditions under which the appearance of a colour does not tally with the corresponding physical measurement of the stimulus source. (In contrast, a color model defines a coordinate space to describe colours, such as the RGB and CMYK color models.)
Colour appearance
Colour 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 colour 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 psychophysical measurements, the International Commission on Illumination (CIE) created the XYZ colour space[1] which successfully models human colour vision on this basic sensory level.
However, the XYZ colour 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 tristimulus values create an identical colour appearance for a human observer. If some conditions change in one case, two identical stimuli with thereby identical XYZ tristimulus values will create different colour appearances (and vice versa: two different stimuli with thereby different XYZ tristimulus values might create an identical colour appearance).
Therefore, if viewing conditions vary, the XYZ colour model is not sufficient, and a colour appearance model is required to model human colour perception.
Colour appearance parameters
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, 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).
Colour appearance phenomena
This section describes some of the colour appearance phenomena that color appearance models try to deal with.
Chromatic adaptation
Chromatic adaptation describes the ability of human colour perception to abstract from the white point (or colour 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 colour appearance phenomena, and therefore a chromatic adaptation transform (CAT) that tries to emulate this behavior is a central component of any colour appearance model.
This allows for an easy distinction between simple tristimulus-based colour models and colour appearance models. A simple tristimulus-based colour model ignores the white point of the illuminant when it describes the surface colour of an illuminated object; if the white point of the illuminant changes, so does the colour of the surface as reported by the simple tristimulus-based colour model. In contrast, a colour appearance model takes the white point of the illuminant into account (which is why a colour appearance model requires this value for its calculations); if the white point of the illuminant changes, the colour of the surface as reported by the colour 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 colour appearance. If the colour 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 colour appearance, however, stays the same (white).
Hue appearance
Several effects change the perception of hue by a human observer:
- 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 colour-neutral).
Contrast appearance
Several effects change the perception of contrast by a human observer:
- Stevens effect: Contrast increases with luminance.
- Bartleson-Breneman effect: Image contrast (of emissive images such as images on an LCD display) increases with the luminance of surround lighting.
Colourfulness appearance
There is an effect which changes the perception of colourfulness by a human observer:
- Hunt effect: Colourfulness 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 grey colour). These phenomena are also known as optical illusions. Because of their contextuality, they are especially hard to model; colour appearance models that try to do this are referred to as image colour appearance models (iCAM).
Colour appearance models
Since the colour appearance parameters and colour appearance phenomena are numerous and the task is complex, there is no single colour appearance model that is universally applied; instead, various models are used.
This section lists some of the colour appearance models in use. The chromatic adaptation transforms for some of these models are listed in LMS colour space.
CIELAB
In 1976, the CIE set out to replace the many existing, incompatible colour difference models by a new, universal model for colour difference. They tried to achieve this goal by creating a perceptually uniform colour space, i.e. a colour space where identical spatial distance between two colours equals identical amount of perceived colour difference. Though they succeeded only partially, they thereby created the CIELAB (“L*a*b*”) colour space which had all the necessary features to become the first colour appearance model. While CIELAB is a very rudimentary colour appearance model, it is one of the most widely used because it has become one of the building blocks of color management with ICC profiles. 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 colour 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 Bradford transformation matrix to the LMS colour space (which had first appeared in the LLAB color appearance model) in conjunction with CIELAB.
Nayatani et al. model
The Nayatani et al. colour appearance model focuses on illumination engineering and the colour rendering properties of light sources.
Hunt model
The Hunt colour appearance model focuses on colour image reproduction (its creator worked in the Kodak Research Laboratories). Development already started in the 1980s and by 1995 the model had become very complex (including features no other colour 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, but because of its complexity the Hunt model itself is difficult to use.
RLAB
RLAB tries to improve upon the significant limitations of 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.
LLAB
LLAB is similar to RLAB, also tries to stay simple, but additionally tries to be more comprehensive than RLAB. In the end, it traded some simplicity for comprehensiveness, but was still not fully comprehensive. Since CIECAM97s was published soon thereafter, LLAB never gained widespread usage.
CIECAM97s
After starting the evolution of colour appearance models with CIELAB, in 1997, the CIE wanted to follow up itself with a comprehensive colour appearance model. The result was CIECAM97s, which was comprehensive, but also complex and partly difficult to use. It gained widespread acceptance as a standard colour appearance model until CIECAM02 was published.
IPT
Ebner and Fairchild addressed the issue of non-constant lines of hue in their colour space dubbed IPT.[2] The IPT color space converts D65-adapted XYZ data (XD65, YD65, ZD65) to long-medium-short cone response data (LMS) using an adapted form of the Hunt-Pointer-Estevez matrix (MHPE(D65)).[3]
The IPT colour 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 colour appearance model, but hard to achieve). It is therefore well-suited for gamut mapping implementations.
ICtCp
ITU-R BT.2100 includes a colour space called ICtCp, which improves the original IPT by exploring higher dynamic range and larger colour gamuts.[4]
CIECAM02
After the success of 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 model, CIECAM02 comes closest to an internationally agreed upon “standard” for a (comprehensive) colour appearance model.
iCAM06
iCAM06 is an image colour 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 colour appearance parameters like contrast, which makes it well-suited for HDR images. It is also a first step to deal with spatial appearance phenomena.
Notes
- ^ “XYZ” refers to a colour model and a colour space at the same time, because the XYZ colour space is the only colour space that uses the XYZ colour model. This differs from e.g. the RGB colour model, which many colour spaces (such as sRGB or Adobe RGB (1998)) use.
- ^
Ebner; Fairchild (1998), Development and Testing of a Color Space with Improved Hue Uniformity, Proc. IS&T 6th Color Imaging Conference, Scottsdale, AZ, pp. 8–13.
{{citation}}: CS1 maint: ___location missing publisher (link) - ^ Edge, Christopher. "US Patent 8,437,053, Gamut mapping using hue-preserving color space". Retrieved 9 February 2016.
- ^ ICtCp Introduction (PDF), 2016
References
- Fairchild, Mark D. (2013). Color Appearance Models. Wiley-IS&T Series in Imaging Science and Technology (3 ed.). Hoboken: John Wiley & Sons. ISBN 978-1-119-96703-3.