NONLINEAR GLOBAL AND LOCAL DOCUMENT DEGRADATION MODELS

Two sources of document degradation are modeled: 1) perspective distor tion that occurs while photocopying or scanning thick, bound documents ; and 2) degradation due to perturbation in the optical scanning and d igitization process: speckle, blurr, jitter, and threshold. Perspectiv e distortion is modeled by studying the underlying perspective geometr y of the optical system of photocopiers and scanners. An illumination model is described to account for the nonlinear intensity change occur ring across a page in a perspective-distorted document. The optical di stortion process is modeled morphologically. First, a distance transfo rm on the foreground is performed; this is followed by a random invers ion of binary pixels in which the probability of flip is a function of the distance of the pixel to the boundary of the foreground. Correlat ing the flipped pixels is modeled by a morphological closing operation . (C) 1995 John Wiley & Sons, Inc.