Apparatus and methods for digital halftoning

  • US 5,166,809 A
  • Filed: 05/08/1990
  • Issued: 11/24/1992
  • Est. Priority Date: 11/08/1988
  • Status: Expired due to Term
First Claim
Patent Images

1. A digital signal processing method for producing one or more continuous tone color separations having a predetermined screen angle and a predetermined screen ruling by sequentially processing digitally encoded continuous tone intensity values, each of which represent the color content within an associated spatial region of an image to be reproduced by said one or more continuous color separations, each said spatial region of said image to be reproduced being identifiable by a set of coordinate values, said method comprising the steps of:

  • storing said continuous tone intensity values in memory with each said continuous tone intensity value being stored at a memory address (xi, yj), i=0, 1, 2, 3, . . . , p;

    j=0, 1, 2, 3, . . . , q;

    so that each memory address at which a continuous tone intensity value is stored is associated with a set of coordinate values of said spatial regions of said photographic image;

    establishing a dither matrix that includes a plurality of dither threshold values that are individually identifiable by a dither matrix row index and a dither matrix column index;

    processing a first one of said memory addresses to obtain a set of dither space coordinates and rounding each dither space coordinate of said set of dither space coordinates to obtain an integer dither matrix row index and an integer dither column index for the continuous tone intensity value stored at said first memory address; and

    sequentially obtaining a set of dither space coordinates, an integer dither matrix row index and an integer dither matrix column index for continuous tone intensity values stored at other addresses of said memory, each set of dither space coordinates being obtained by incrementing the dither space coordinates that correspond to another set of spatial region coordinates by a set of predetermined addressing increments; and

    for each resulting set of dither space coordinates, comparing the dither threshold value associated with said integer dither matrix row and column index with the continuous tone intensity value stored at the memory address that is associated with said set of dither space coordinates to determine whether a corresponding spatial region of said continuous tone color separation should be blackened.

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