Alteration of dither matrix size for full color dithering

Details Alteration of dither matrix size for full color dithering Alteration of dither matrix size for full color dithering

1996-04-26

1998-11-03

Rogers, Scott

Image analysis

Histogram processing

For setting a threshold

382270, 382275, 358533, 358535, G06K 1500, G06T 500, H04N 152, H04N 158

Patent

active

058321853

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to the display of images by use 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 (i) random dither, (ii) clustered dot ordered dither and (iii) dispersed dot dither. Of these, (ii) clustered dot dither and (iii) dispersed dot dither are the most prevalent.


PRIOR ART DRAWINGS

The prior art will now be described with reference to FIGS. 1 and 2 in which:
FIG. 1 illustrates an example of a traditional dithering system; and
FIG. 2 illustrates the process of tiling an image with a dither matrix.
Referring now to FIG. 1, there is shown an example of a traditional dithering system 1. The dithering system relies on the utilization of a dither matrix 2 in conjunction with an image 3 which is desired to be printed out. Each pixel of the image 3 is presented to a comparator 4 along with a corresponding dither matrix value from dither matrix 2. The comparator 4 compares its two inputs and if the pixel value from image 3 is greater than the dither matrix value from dither matrix 2 the comparator outputs a "1" to a printing device 5. Otherwise, a "0" is output to the printing device. The printing device 5 produces an output mark on pixel locations when it receives a "1" from the comparator 4 and it produces no output marks when it receives a "0" from the comparator 4.
The correspondence between a particular pixel value in image 3 and its corresponding dither matrix value in image 2 is normally obtained by means of the lower order address bits of the pixel location in image 2. Those skilled in the art will understand that this is equivalent to utilizing modulo arithmetic on the pixel address of image 3 to derive a corresponding dither matrix element from dither matrix 2.
Referring now to FIG. 2, those skilled in the art will further realize that the use of modulo arithmetic is equivalent to "tiling" the image 3 by multiple copies of the dither matrix 1.
Unfortunately, the process of tiling of an image can lead to unwanted artifacts, detectable by the human eye, especially where an image 3 consists of large areas of slowly varying intensities. The production of artifacts can be somewhat alleviated by utilizing larger dither matrices. However, the use of large dither matrices does not result in total elimination of these artifacts in an image.
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 dither values. The difference between the two techniques is that in clustered dot dithering the lower threshold values are centred in a cluster pattern whereas in dispersed dot dithering they are scattered in the dispersed pattern. As such, the clustered-dot technique has a central dot that increases in size as the signal level increases and the dispersed-dot technique has small scattered dots that increase in number as the input signal level increases. In both techniques, the number of levels that can be normally represented by the dither matrix is normally equal to the size of the screen pattern, e.g. an 8.times.8 screen can produce 64 unique levels.
Unfortunately the use of larger dither matrices does not result in the total elimination of artifacts in an image. This is especially the case when a dither matrix is used in the field of colour printing where multiple passes are made using different colour inks to produce a final image. One common form of colour printing is to have subtractive colour printing whereby separate Cyan, Magenta, Yellow and Black passes are utilized in forming an image.
Patterns can

REFERENCES:
patent: 4651293 (1987-03-01), Kato
patent: 4698778 (1987-10-01), Ito et al.
patent: 4809063 (1989-02-01), Moriguchi et al.

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