The process of digitizing images to create halftones inherently reduces sharpness through the averaging of grayness values within cell areas. Within the context of a resolution independent page description language, overcoming this reduced sharpness is conventionally addressed by adjusting the grayness values of cells to create larger or smaller halftone dots where edges are present. Such an approach does not take full advantage of the capabilities of the output device. The objective of this project was to design and implement a method of sharpening digital images by altering the shape and position, rather than the size, of halftone dots. Such a method can more accurately represent the original image and more closely emulate the characteristics of photomechanically produced halftones. Within the PostScript page description language, the generation of halftone dots is controlled by the spot function. A particular type of spot function, the Tug function, was developed to control the shape and position of halftone dots based on the grayness value of each cell and its surrounding neighbors. Because the standard PostScript imaging operators are not designed to allow halftone dot shapes to be redefined on a cell-by-cell basis, an alternate method of generating images was created. A computer program in the PostScript programming language was written to perform the requisite image analysis and halftone dot structuring to drive a laser printing device for viewing the effects of the Tug function. Three representative photographic images were processed in this manner. A panel of judges compared the resulting prints with control prints processed without the Tug function but by the same imaging method. The judges' subjective preferences are presented, and the relative merits of the Tug function are discussed.
Library of Congress Subject Headings
Computer graphics--Technique--Evaluation; Image processing--Digital techniques--Evaluation; PostScript (Computer programming language); Photoengraving--Halftone process
Department, Program, or Center
School of Print Media (CIAS)
Hoffenberg, Steven, "The Tug function: A Method of context sensitive dot structuring for digital halftones" (1990). Thesis. Rochester Institute of Technology. Accessed from
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