Invented by Kevin J. Parker, PhD and Theophano Mitsa, PhD

Printing devices have long used the technique of halftoning to render the appearance of shades of gray using dot patterns. But high-quality halftones required an exasperating amount of time for printout. Drs. Kevin Parker and Theophano Mitsa developed Blue Noise Mask (BNM), a novel approach to providing high-quality halftones many times faster than the best algorithms available in the late 1980s (download the interactive Tutorial for an engaging overview). Dr. Parker, an expert in the field of medical imaging, had noticed that printouts from his team's diagnostic equipment were slow and plagued by distracting "noise" patterns called image artifacts. The researchers were unable to tell whether a spot on a picture represented an incipient tumor or an artifact added to the image during printing. For a faster, more accurate way to render the pictures, they conceptualized pictures as being composed of a fine mosaic of black and white dots. This fine mosaic pattern was mathematically constructed into a Blue Noise Mask. After a computer calculates the optimum mask for a printing device, the pre-built mask is stored in the printer's software to produce halftones almost instantly. For color printers, the mask also halves the number of bits needed to produce a high-quality image, permitting major savings in printing speeds and computer memory.

Researchers at the University of Rochester designed the mask with substantial computer power and several years of work. The BNM was introduced to graphics and other industries in 1991. At that time BNM was a leap forward in halftoning technology. BNM was the first method to combine high quality with virtually instantaneous halftoning.

Worldwide patents and copyrights protect the technology. "Blue noise" refers to an unstructured pattern with negligible low frequency noise components that produce a fine, visually appealing arrangements of dots. BNM is the best technology available for eliminating the wavy moire patterns that tend to occur using previous methods, particularly in color images. BNM is more isotropic (directionallly uniform) than error diffusion, and produces less periodic (recurrent) patterns than ordered dither methods. Error diffusion techniques tend to show scanning and start-up artifacts, while BNM is free from these. The technology can be implemented either as a software filter in digital equipment, or as a plastic or glass screen in the optical processes widely used in photocopiers and newspaper publishing. BNM provides high-quality images at faster speeds using software that eliminates the computations previously required for digital halftoning in such devices as photocopiers, scanners, laser printers, and fax machines. No calculations are required because the BNM pattern is pre-built. Only a trivial comparison of the scanned pixels to the mask is required to generate a halftone. In a psychovisual test, BNM halftones were rated 2.7 times more pleasing than clustered-dot halftones, 1.6 times more pleasing than dispersed-dot-dither halftones, and about as pleasing as error-diffusion halftones which took 45 times longer to generate.

Research continues into new BNM applications, such as data compression and encryption, error masking, and color transmission.

   

Copyright 1997 - 2011, University of Rochester
Hopeman Engineering Building 203, P.O. Box 270126
Rochester, NY 14627-0126
email: parker@seas.rochester.edu

Last updated: January 21, 2011