Method for binarizing scanned document images containing gray or light colored text printed with halftone pattern

A computer program product comprising a computer usable non-transitory medium having a computer readable program code embedded therein for controlling a data processing apparatus, the computer readable program code configured to cause the data processing apparatus to execute a process for binarizing a gray-scale document image which has been generated by scanning a paper-based document, the process comprising:

• (a) identifying text characters in the gray-scale document image, including:
• performing an initial binarization of the gray-scale image to generate an initial binary image;
• and
• extracting connected image components in the initial binary image as text characters;
• (b) classifying each text character identified in step (a) as either a halftone text character which is a character formed by a halftone pattern or a non-halftone text character based on a topological analysis of the text character which determines a number of holes in a connected image component corresponding to the text character, including:
• calculating an Euler number for each text character;
• and
• classifying a text character as halftone text if the Euler number for the text character is below a predetermined value, and classifying a text character as non-halftone text if the Euler number of the text character is equal to or above the predetermined value;
• and
• (c) binarizing halftone text characters using pixel value characteristics obtained from only halftone text characters classified in step (b).

A scanner comprising:

• a scanning section for scanning a hard copy document to generate a gray-scale document image;
• and a data processing apparatus for processing the gray-scale document image to generate a binary map of the gray-scale document image,
• wherein the processing of the gray-scale document image includes:
• (a) identifying text characters in the gray-scale document image, including performing an initial binarization of the gray-scale image to generate an initial binary image, and extracting connected image components in the initial binary image as text characters, (b) classifying each text character identified in step (a) as either a halftone text character which is a character formed by a halftone pattern or a non-halftone text character based on a topological analysis of the text character which determines a number of holes in a connected image component corresponding to the text character, including calculating an Euler number for each text character, and classifying a text character as halftone text if the Euler number for the text character is below a predetermined value, and classifying a text character as non-halftone text if the Euler number of the text character is equal to or above the predetermined value, and (c) binarizing halftone text characters using pixel value characteristics obtained from only halftone text characters classified in step (b).