Browse Prior Art Database

Document Security for Copiers

IP.com Disclosure Number: IPCOM000087481D
Original Publication Date: 1977-Feb-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 2 page(s) / 35K

Publishing Venue

IBM

Related People

Hildenbrandt, LH: AUTHOR

Abstract

In black-white copying systems, the original scanner output is broken into a multitude of picture elements (PELS) to which is attached a value indicative of the reflectance or optical density of that PEL. This value may be expressed in analog or digital form. At the printer, the decision to print with toner or ink in a particular PEL is a binary decision. The mapping of the continuous tone or intermediate scanned values into the binary output values is referred to as "thresholding".

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Document Security for Copiers

In black-white copying systems, the original scanner output is broken into a multitude of picture elements (PELS) to which is attached a value indicative of the reflectance or optical density of that PEL. This value may be expressed in analog or digital form. At the printer, the decision to print with toner or ink in a particular PEL is a binary decision. The mapping of the continuous tone or intermediate scanned values into the binary output values is referred to as "thresholding".

The figure shown is an example of a scanner whose output has been digitized into eight distinct levels. Its gain has been adjusted to output 000 for any optical density of .01 or less and to output 111 for any optical density of 1.0 or greater. Levels between these two values are presumed linear with corresponding digital outputs.

A typical typed document has background with an optical density approximately .02 or less and characters printed with an optical density of approximately .85 to 1.2, thus the PELS representing characters or background fall into the areas as indicated. In this example then, the PELS from a character will either be 110 or 111 at the scanner output, whereas the PELS from the background will either be 000 or 001 at the scanner output. A simple thresholding algorithm may be to print only if the output is greater than 100. This is an example of a simple "fixed" threshold algorithm, and clearly a more complicated algorithm is needed to...