Nonuniform illuminant

Qualitative Picture of Processes Taking Place at a Semiconductor-Electrolyte Interface under Its Nonuniform Illumination... 

In the other scheme the photosensitive interface operates in the photo-galvanic pair regime (Goryachev et al, 1970 Goryachev and Paritsky, 1973). The method is based on the occurrence of a potential difference between illuminated and nonilluminated areas on the surface of a semiconductor electrode in a solution (cf. Section 1 lb). As a result of nonuniform illumination, local anodes and cathodes arise on the surface and this, in turn, leads to nonuniform deposition of metal onto the surface. 

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. 

Figure 6.2 The white patch retinex algorithm is able to perform some color adjustments for the image shown in (a) but the image still looks very yellow. The white patch retinex algorithm does not work very well for the image shown in (b) because of the nonuniform illuminant.

Figure 6.26 Comprehensive color normalization iteratively normalizes all pixels and then rescales the color channels using the gray world assumption. Therefore, intensity information is lost. Since it is based on the gray world assumption, it does not work if we have a nonuniform illumination.

Algorithms for Color Constancy under Nonuniform Illumination... 

To handle nonuniform illumination, a threshold must be applied to each ratio along the path. If the logarithm of the ratio is larger than the threshold, it is reset to zero. In this case, the output color in Land s retinex theory is calculated as follows (Land 1983 Land et al. 1983). Let (x) be a step function. 

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