United States Patent   Patent Number: 4,673,977
Stelzenmuller  Date of Patent: Jun. 16, 1987
 METHOD OF SPATIALLY THRESHOLDING A DISCRETE COLOR IMAGE
 Inventor: William K. Stelzenmuller, Morrisville, N.C.
 Assignee: International Business Machines Corporation, Armonk, N.Y.
 Appl. No.: 747,094
 Filed: Jun. 20, 1985
 Int. CV H04N 7/12; H04N 1/00;
 U.S. CI 358/133; 358/261;
 Field of Search 358/75, 133, 260, 261;
 References Cited
U.S. PATENT DOCUMENTS
4,189,711 2/1980 Frank 382/22 X
4,476,495 10/1984 Fujisawa et al 358/261 X
4,501,016 2/1985 Persoon et al 382/22
4,542,411 9/1985 Imanaka et al 382/56 X
Primary Examiner—E. A. Goldberg
Assistant Examiner—Patrick W. Foster
Attorney, Agent, or Firm—Gerald R. Woods
A method is disclosed for thresholding the luminance component of color picture elements (pels) prior to compression. The method is most effective when used with discrete color images, such as documents having areas of nominally constant color. The method uses spatial information rather than histogram information to threshold the image data. Each scan line is defined as having a series of pel runs. A pel is considered to be in a run if its luminance is within a certain range of an average luminance based on preceding pels. Each run may end in a transition region wherein the luminance of each pel differs from the luminance of the preceding pel by more than a predetermined amount. Where a transition region is found, a local threshold is set based on the average luminance of the preceding run and of the following pel. All pels in the transition region are assigned a luminance value equal either to the average luminance of the preceding run or the actual luminance of the following pel.
8 Claims, 10 Drawing Figures