Color constancy

The trichromatic theory, subsequendy confirmed by the existence of the three sets of cones, must be combined with the opponent theory, which is involved in the retinal pathway. A third approach, the appearance theory (2) or the retinex theory, must be added to explain color constancy and other effects. As one example of this last, consider an area perceived as red in a multicolored object such as a Mondrian painting when illuminated with white... 

Color Constancy M. Ebner 2007 John Wiley Sons, Ltd... 

Land and McCann (1971) then developed a computational theory for color constancy, the retinex theory. In their experiments, they used a stimulus similar to the famous paintings... 

Land and McCann (1971) came to the conclusion that color perception involves structures of the retina as well as the visual cortex. On the basis of these experiments, Land and McCann developed a computational theory of color constancy, the retinex theory. Land later developed additional versions of his retinex theory. The retinex theory of color vision is discussed in detail in Chapter 7. 

In principle, the algorithms for color constancy, which are described in the following chapters, can also be applied to the rotated coordinates (Figure 2.20). Land notes that, in practice, the transformation may not be that simple if the algorithm is nonlinear, i.e. contains a thresholding operation. We see later, e.g. in Section 6.6, Section 7.5, or Section 11.2, that some of the algorithms for color constancy are also based on a rotated coordinate system where the gray vector, i.e. the achromatic axis, plays a central role. 

Figure 2.20 The transformation performed by the color opponency mechanism may be a simple rotation of the coordinate system. In this case, the mechanism for color constancy may operate on the rotated coordinates. (Reprinted from Vision Res., Vol. 26, No. 1, Edwin H. Land, Recent advances in retinex theory, pp. 7-21, Copyright 1986, with permission from Elsevier.)...

A normal observer is able to exchange information between the left and the right hemisphere of the brain through the corpus callosum. A normal observer is therefore able to report on the correct color of the test region if the neutral-density filters are not used. In contrast, the subject with a cut corpus callosum can only report on the visual information processed by the left hemisphere of the brain. His color perception depends on whether a mirror setup is used. Therefore, the visual cortex is also involved in the process of color constancy. 

Macbeth 5000 K fluorescent, a Philips Ultralume fluorescent, and a Sylvania Cool White fluorescent tube. Some color constancy algorithms try to find out what type of illuminant produced the particular color sensation, which was measured by the sensor. If we measure the entire power spectrum for a particular patch of our object and also know the type of illuminant used, then it is easy to compute the BRDF. Let L(X) be the measured power spectrum and let E A.) be the power spectrum of the illuminant. Assuming a Lambertian surface, where the BRDF is independent of the normal vector of the patch Nobj, and also independent of the normal vector that points to the direction of the light source Ni, we... 

In developing color constancy algorithms, there are basically two roads to follow (Finlayson et al. 1994a). One is the accurate estimation of reflectances. This is of particular importance for object recognition. Object recognition becomes much easier if the reflectances are known. Another possibility would be to compute colors as they would appear under a canonical, e.g. white, illuminant. These are two different objectives and both are justified in their own right. In the first case (computation of reflectances), color... 

The second illuminant L can be considered to be an illuminant to which the eye has adapted itself. Necessary and sufficient conditions for von Kries chromatic adaptation to provide color constancy were derived by West and Brill (1982). 

Algorithms for Color Constancy under Uniform Illumination... 

Numerous algorithms have been proposed for color constancy. We now describe in detail how these algorithms work. We also see how the algorithms perform in practice. The performance of the algorithms will be shown on two sample images. One image shows a scene with a uniform illumination and the other shows a scene with a nonuniform illumination. 

ALGORITHMS FOR COLOR CONSTANCY UNDER UNIFORM ILLUMINATION... 

ALGORITHMS FOR COLOR CONSTANCY UNDER UNIFORM ILLUMINATION 109 For large n, we have... 

Forsyth (1988, 1992) developed the so-called gamut-constraint methods for color constancy. He assumes that the receptor response curves are disjoint and that they are only sensitive... 

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