1. A method implemented in a data processing apparatus for binarizing a gray-scale document image which has been generated by scanning a paper-based document, the method comprising: (a) identifying text characters in the gray-scale document image; (b) classifying each text character identified in step (a) as either a halftone text character or a non-halftone text character based on a topological analysis of the text character; and (c) binarizing halftone text characters using pixel value characteristics obtained from only halftone text characters classified in step (b).