A new technique is presented for quickly identifying global affine transformations applied to tabular document images, and to correct for those transformations. This technique, based on the Fourier-Mellin transform, is used to register (align) a set of tabular documents to each other. Each component of the affine transform is handled separately, which dramatically reduces the total parameter space of the problem. This method is robust, and deals with all components of the affine transform in a uniform way. The Fourier-Mellin transform is also extended to handle shear, which can approximate a small amount of perspective distortion, and to not need Blackman windowing for document images. In order to limit registration to foreground pixels only, and to eliminate Fourier “edge effects”, a novel, locally adaptive foreground-background segmentation algorithm is introduced, based on the median filter. An original method is also presented for automatically obtaining blank document templates from a set of registered document images. Finally, image registration is demonstrated as a tool for compression of document images which share the same template.
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