1. A computer-implemented method for processing a grayscale digital image, comprising: receiving, at a computer programmed with appropriate software, pixel data of a plurality of pixels comprising a grayscale digital image, the pixel data including intensity data representative of a brightness of each pixel, the intensity data including a set of quantized intensity values, each quantized intensity value representing a corresponding brightness level of the pixel; transforming the pixel data into an intensity matrix, each matrix element in the intensity matrix representing a pixel in the grayscale digital image, a value of each matrix element in the intensity matrix being the intensity value associated with the corresponding pixel; comparing a value of each matrix element in the intensity matrix to values of all available nearest neighbor matrix elements, assigning a texture to the pixel corresponding to the matrix element in accordance with the values of the nearest neighbor matrix elements, and categorizing the pixel according to its brightness and its texture; transforming the intensity matrix into a brightness/texture matrix, each matrix element in the brightness/texture matrix corresponding to a matrix element in the intensity matrix and having one of a predefined set of brightness/texture values representing the categorization of the corresponding pixel, each brightness/texture value having one of a specified corresponding shading levels associated therewith, the transformation of the intensity matrix into the brightness/texture matrix being performed by the computer in a single pass through the intensity matrix; assigning one of the specified shading levels to each pixel based on the value of the corresponding matrix element in the brightness/texture matrix; and generating a segmented image, each pixel in the segmented image having a shading level corresponding to its brightness/texture value; wherein each pixel is categorized as being one dark and uniform, dark and variable, bright and uniform, and bright and variable, and wherein each brightness/texture matrix element is assigned one of four brightness/texture values corresponding to the pixel's categorization, each of the four brightness/texture values corresponding to one of four specified shading levels.