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This program is part of Netpbm.
pgmdeshadow removes gray shadows from an image. This is useful for an image containing text, such as a scanned book pages, where a shadow typically appears near the book crease or near one side of the image. pgmdeshadow recognizes a gray shadow as an area of smoothly changing color, starting from the outer edges of the image. The program uses a simple image reconstruction algorithm to determine the local shadow gray level, then divides each pixel's gray level by the local shadow gray level.
The algorithm is the "fast hybrid grayscale reruction" algorithm from Luc Vincent, "Morphological Grayscale Reruction in Image Analysis: Applications and Efficient Algorithms.
pgmdeshadow was added to Netpbm in Version 10.35 (August 2006).