Image analysis – Image enhancement or restoration – Edge or contour enhancement
Patent
1996-10-22
1998-10-13
Lee, Thomas D.
Image analysis
Image enhancement or restoration
Edge or contour enhancement
382254, 382263, 382266, 358456, 358458, G06K 940, H04N 140
Patent
active
058224696
ABSTRACT:
A post-filtering method and apparatus for increasing the high-frequency and edge information of halftoned images is described. If a continuous-tone (contone) image is processed by a halftoning algorithm into a binary or multi-level image, some of the high frequency information is lost. However, some of the lost information can be restored by judiciously rearranging some of the halftone picture elements (pixels). Each horizontally adjacent pair of pixels in the contone image are compared with each other. If the difference between their luminance is greater than a threshold, then the corresponding pair of adjacent pixels in the halftone image are compared. If the difference between the halftone image pixels has the opposite sign of the differences between the contone image pixels (e.g., the image changes from dark to light, but the halftone pixels change from light to black), the two halftone pixels are exchanged with each other. The procedure is then repeated in the vertical direction. The procedure can be applied as a post filter to any halftoned image, and can substantially improve the image quality. The procedure works with binary, multiple gray level and color images. The procedure is also reasonably fast to compute.
REFERENCES:
patent: 4926267 (1990-05-01), Shu et al.
patent: 4992955 (1991-02-01), Yabuuchi et al.
patent: 5068914 (1991-11-01), Klees
Hewlett--Packard Company
Lee Thomas D.
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