White patch retinex algorithm

The white patch retinex algorithm is basically just a simplified version of the retinex algorithm (Cardei and Funt 1999 Funt et al. 1998, 1996 Land and McCann 1971), which is described in the next chapter. The retinex algorithm relies on having a bright patch somewhere in the image. The idea is that, if there is a white patch in the scene, then this patch reflects the maximum light possible for each band. This will be the color of the illuminant, i.e. if R,(x, v) = 1 for all i e r, g, b) and G(x, y) = 1, then... 

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 6.3 Results for the white patch retinex algorithm where histograms were used to find a white patch. The histograms for all the three color bands of the input image are also shown. The cutoff value is marked with a vertical line. All the three cutoff values represent an estimate of the illuminant.

The gray world assumption as well as the white patch retinex algorithm are used frequently for automatic white balance. The popular draw utility written by Coffin (2004) scales each color channel using the average as an automatic white-balance option. After rescaling, the white point is set at the 99th percentile. In other words, all channels are scaled equally such that only the top 1% of all pixels are clipped. This assumes that there are only a few highlights. 

Again, the color of the output pixel is independent of the illuminant. Note that both the sample and the background are achromatic. Therefore, the sample will appear to be achromatic because all color channels will have the same value. Hence, the white patch retinex algorithm is not in agreement with the results obtained by Helson. 

If this map is applied to the color measured by the sensor in the center of the image, we obtain the same result as we obtained for the white patch retinex algorithm. The output color will be achromatic for all three cases. 

White patch retinex algorithm, maximum value per channel. 

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