The contrast of the image processing system and method for expanding the histogram equalization

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In contrast enhancement during image processing, a histogram equalization process or apparatus divides an equalization image screen into overlapping windows and performs histogram equalization on each window. Using windows improves contrast of pixel values that are rare throughout the entire image and reduces or prevents loss of information represented by the rare pixel values. Overlapping the windows reduces discontinuities at the boundaries of the windows. The size and overlap of the windows can be adjusted according to the time available for equalization and can be optimized according to the desired image quality improvement. The histogram equalization can be selectively performed on an entire image screen or any part thereof to reduce image degradation in the entire or part of an image. As a further aspect of the invention, to reduce the number of operations required for equalization, a histogram for a current window is obtained starting from the histogram for a previous window. To further improve image quality, a histogram equalization apparatus uses low-pass- or band-pass-filtered pixel data as a lookup table address to output a histogram-equalized value. This helps remove amplified thermal or quantization noise.