Homomorphic filter
Homomorphic filter an image enhancement technique based upon the illuminationreflectance model. The homomorphic filter assumes the image function f (x, y) is the combination of two functions i(x, y) and r(x, y). By taking the natural log of the images the components are separated and can be processed separately, the reflectance of an image usually contains high frequency components while the illumination component tends to vary slowly (the low frequencies), thus, the filter applied to the logarithm image should affect the low and high frequency components differently. The most common use for the filter is to enhance images by compressing the brightness range while increasing contrast, the filter applied in this case is a circularly symmetric high pass filter with the stop band magnitude < 1 and pass band magnitude > 1.
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Author of the text: Ed. P.A. Laplante Boca Raton: CRC Press LLC, 2000
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