Browse Prior Art Database

Bilevel or Trilevel Displays Representative of Gray Scale Images

IP.com Disclosure Number: IPCOM000050769D
Original Publication Date: 1982-Dec-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 3 page(s) / 25K

Publishing Venue

IBM

Related People

Tseng, SC: AUTHOR

Abstract

This article relates to the generation of bilevel or trilevel images from gray scale images using complementary sequences. A computer output device, such as a printer or a display, is primarily a bilevel device, i.e., it gives only "on" and "off" outputs. In certain printers, such as thermal printers, the printer head can be made to print three or four levels in intensity.

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Bilevel or Trilevel Displays Representative of Gray Scale Images

This article relates to the generation of bilevel or trilevel images from gray scale images using complementary sequences. A computer output device, such as a printer or a display, is primarily a bilevel device, i.e., it gives only "on" and "off" outputs. In certain printers, such as thermal printers, the printer head can be made to print three or four levels in intensity.

On the other hand, gray scale images are usually 256 levels. In order to print such a gray scale (256-level) image on a device which can only provide 2 or 3 levels, a corresponding bilevel or trilevel image must be generated which closely looks like the gray scale image to the human eye. A means to generate such a bilevel or trilevel image from a given gray scale image is described hereinbelow.

The fact that a complementary set of sequences can be used to perform a unitary transform has been described in the publication by S.C. Tseng in IEEE Transactions on Information Theory IT-18, 644-651 (September 1972). For the sake of explanation, the simplest case of a complementary set of sequences will be used herein, which is a pair of sequences. Let H1 and H2 be a pair of complementary sequences of the same length whose elements are +1 or -1. (I) 1-D bilevel process

A scan line of a gray scale image will be used to generate a scan line of a bilevel image in this case.

Let one scan line of the gray scale image be G(i), i=1,N, and let the complementary sequence pairs be H1(j)=+1 or -1 and H2(j)H1 or -1 for j=1,M.

Then, the kth pel (picture element) of the two outputs gl(k) and g2(k) from the complementary filters H1 and H2 will be g1(k)=E/i=1/M H1(i)*G(k-i+1) g2(k)=E/i=1/M H2(i)*G(k-i+1)

Let the bilevel scan line, which is generated, be B(k), k=1,N.

The bilevel (high or low) scan line is generated one pel at a time. Assuming we have already generated the binary pels up to the (k-1)th pel and are about to generate B(k), the kth pel, we have to decide whether this B(k), the kth...