Color appearance model

Color appearance Color 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 color 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 color space which successfully models human color vision on this basic sensory level. However, the XYZ color 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 color appearance for a human observer. If some conditions change in one case, two identical stimuli with thereby identical XYZ tristimulus values will create color appearances (and vice versa: two different stimuli with thereby different XYZ tristimulus values might create an color appearance). 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, 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. This allows for an easy distinction between simple tristimulus-based color models and color appearance models. A simple tristimulus-based color model ignores the white point of the illuminant when it describes the surface color of an illuminated object; if the white point of the illuminant changes, so does the color of the surface as reported by the simple tristimulus-based color model. In contrast, a color appearance model takes the white point of the illuminant into account (which is why a color appearance model requires this value for its calculations); if the white point of the illuminant changes, the color of the surface as reported by the color 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 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: • Bezold–Brücke hue shift: The hue of a stimulus of constant chromaticity changes with luminance. • Abney effect: The hue of monochromatic light changes with the addition of white light (which would be expected color-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) increases with the luminance of surround lighting. Colorfulness appearance There is an effect which changes the perception of colorfulness by a human observer: • 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. Not modeled by CIECAM02. • Contrast appearance effects (see above), modeled by CIECAM02. 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 illusions. Because of their contextuality, they are especially hard to model; color appearance models that try to do this are referred to as image color appearance models (iCAM). ==Color appearance models==