Method for generating digital halftone images

In a processor including a central processing unit, a memory and a process means for generating output images, a method, executable on said processor for generating bi-level output images from continuous tone input images, comprising the steps of:

• providing an input array of image intensity values representing a continuous tone input image;
• determining a range of image intensity values in said input array;
• determining from said range of image intensity values a threshold matrix size value;
• generating a threshold matrix of a size corresponding to said threshold matrix size value for thresholding said input array, said threshold matrix being generated by the steps of:
• defining a two-dimensional array of memory storage positions for storing threshold values;
• dividing said two-dimensional array into four quadrants of equal size, each quadrant having a quadrant origin, a quadrant perimeter and four subquadrants disposed around the quadrant origin;
• assigning threshold values to the memory storage positions of said quadrants, said threshold values being arranged in a first pair of diagonal quadrants in order of increasing magnitude from the quadrant origin toward the quadrant perimeter, and said threshold values being arranged in a second pair of diagonal quadrants in order of increasing magnitude from the quadrant perimeter toward the quadrant origin;
• said threshold values being arranged in said quadrants in order of increasing magnitude in alternating pairs of diagonally opposing subquadrants to provide bi-lateral symmetry in each quadrant;
• and
• comparing threshold values of the threshold matrix to corresponding intensity values of the input array, and if said intensity values exceed said threshold values, generating a first set of bi-level image values, otherwise, if said intensity values do not exceed said threshold values, generating a second set of bi-level image values, said first and second sets of bi-level image values defining light and dark image components of an output bi-level image.

A digital matrix filter for converting a continuous tone input image to a bi-level output image, comprising:

• a data storage medium providing a two-dimensional array of storage positions for storing intensity threshold values;
• first process means for dividing said two-dimensional array into four quadrants of equal size, each quadrant having a quadrant origin, a quadrant perimeter and four subquadrants disposed around the quadrant origin;
• second process means for arranging threshold values in said storage positions of said quadrants, said threshold values being arranged in a first pair of diagonal quadrants in order of increasing magnitude from the quadrant origin toward the quadrant perimeter, and said threshold values being arranged in a second pair of diagonal quadrants in order of increasing magnitude from the quadrant perimeter toward the quadrant origin;
• and
• third process means for arranging said threshold values in said quadrants in order of increasing magnitude in alternating pairs of diagonally opposing subquadrants to provide bi-lateral symmetry in each quadrant.

A computer program product operable in a computer system including a central processing unit and a memory, for generating digital halftone matrices of selected size for use in an image processing procedure in which bi-level output images are formed from continuous tone input images using the digital halftone matrices, said computer program product comprising:

• a data storage medium for storing digital information;
• means recorded on said data storage medium for causing said computer system to generate a two-dimensional array of storage positions in said memory for storing threshold values;
• means recorded on said data storage medium for causing said computer system to divide said two-dimensional array into four quadrants of equal size, each quadrant having a quadrant origin, a quadrant perimeter and four subquadrants disposed around the quadrant origin;
• means recorded on said data storage medium for causing said computer system to assign threshold values to the storage positions of said quadrants, said threshold values being arranged in a first pair of diagonal quadrants in order of increasing magnitude from the quadrant origin toward the quadrant perimeter, and said threshold values being arranged in a second pair of diagonal quadrants in order of increasing magnitude from the quadrant perimeter toward the quadrant origin, said threshold values being arranged in said quadrants in order of increasing magnitude in alternating pairs of diagonally opposing subquadrants to provide bi-level symmetry in each quadrant.