Method of producing a dither matrix by dividing an array into a

Details Method of producing a dither matrix by dividing an array into a Method of producing a dither matrix by dividing an array into a

1996-05-28

1999-11-16

Rogers, Scott

Image analysis

Histogram processing

For setting a threshold

382237, 382270, 358535, 358457, 358459, 358466, 358298, G06K 1502, H04N 152, H04N 1405

Patent

active

059872195

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to the display of images by usage of the halftoning technique of dithering and in particular, relates to the display of images having multiple colour components.


BACKGROUND ART

Digital halftoning for the purpose of generating colour images with multiple levels is a well known art. For an extensive review of the field of halftoning, reference is made to "Digital Halftoning" by R. Ulichney, MIT Press, Cambridge, Mass. The area of halftoning is generally divided into the fields of dithering and error diffusion. The field of dithering is generally divided into the areas of random dither, clustered dot ordered dither and dispersed dot dither. Of these, clustered dot dither and dispersed dot dither are the most prevalent.
Both clustered dot dither and dispersed dot dither are based on the creation of a threshold screen pattern or dither matrix, for example an 8.times.8 array of image pixels. This dither matrix is compared, pixel by pixel, with inputted digital image values, with the dither matrix being treated in a `modulo` or `wrap-around` sense. If the inputted value is greater than or equal to a current dither matrix value, the output at that point is set to be "on" and if it is less than the output, it is set to be "off".
The difference between the two techniques of clustered dot dither and dispersed dot dither is that in clustered dot dithering the lower threshold values are centred in a clustered pattern. Whereas in the dispersed dot dithering screen pattern the lower threshold values are scattered. As such, the clustered-dot technique has a central dot that increases in size as the signal level increases whilst the dispersed-dot technique has small scattered dots that increase in number as the signal level increases. In both techniques, the number of levels that can normally be represented is equal to the size of the dither matrix. For example, an 8.times.8 dither matrix is generally designed to produce 64 unique levels. If more levels are required then larger dither matrix sizes can be used. However, for a given type of display equipment. this will mean a reduction in the effective spatial resolution that can be achieved with clustered dot dithering because the dither matrix size will be larger, thereby taking up more space for the printing of each individual pixel. By way of example, high quality offset printers are generally able to print approximately 2400 dots per inch. However, images are normally printed in the order of 150 pixels to the inch. Since 2400/150=16, this provides a 16.times.16 array of dots for each individual pixel, and allows for 256 different possible intensity levels.


DESCRIPTION OF THE PRIOR ART DRAWINGS

In relation to the prior art,
FIG. 1 illustrates the problems associated with dot gain for a single pixel;
FIG. 2 illustrates the problem of dot gain for multiple dots clustered together;
The need to use a clustered dot technique arises from the occurrence of dot gain. Dot gain is normally a consequence of the printing technology used whereby there is a non-linear response function of the amount of ink or toner used in relation to the size of the dots produced through the usage of that amount of ink or toner. Referring now to FIG. 1, there is shown an example of dot gain. It is desired to lay down a certain standard size dot 1 on a piece of paper. However, as a consequence of the printing process, it is often found that the ink used can actually produce, in some circumstances, a smaller dot 2 or, in other circumstances, a larger dot 3. This results in a large error factor or "dot gain" for each individual dot (where the dot shrinks the "gain" is negative). The opacity profile of each individual dot can also vary across the dot with the variation often being approximately gaussian in nature.
The occurrence of dot gain is normally minimised through the use of a clustered dot ordered dither technique whereby, in each dither cell the dots are clustered together. Referring now to FIG. 2, there is shown an example of this process whereby nine dots 4-1

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"Optimization By Simulated Annealing" by S. Kirkpatrick, et al., Science, 220: pp. 671-680 (1983).

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