Homomorphic filtering

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Homomorphic filtering is a generalized technique for image enhancement. It simultaneously normalizes the brightness across an image and increases contrast.

Homomorphic filtering is used to remove multiplicative noise. Illumination and reflectance are not separable, but their approximate locations in the frequency domain may be located. Hence, the components are separated by processing the natural logarithm of the image in the frequency domain. To make the illumination of an image more even, the high-frequency components are increased and low-frequency components are decreased because the high-frequency components are assumed the reflectance in the scene (the amount of light reflected off the object in the scene), whereas the low-frequency components are assumed the illumination in the scene. Low pass filtering is used to repress low frequencies, which is assumed the illumination component, and high pass filtering is used to amplify high frequencies, which is assumed the surface reflection component.

Overview of homomorphic filtering