Adaptive threshold circuit for image processing

Abstract

An adaptive threshold circuit is disclosed wherein reflectance values are measure D in each of a number of scan lines on a document on which an image is scanned, digitized at a multiplicity of points and processed to create a processed digitized image signal that is stored or transmitted and then used to recreate the document accurately without objectional light and dark variations over the recreated documents and without being affected by the background color of the original document, variations in illuminating and the black/white makeup of the image. The unprocessed digitized data is first used to calculate a mean value of the digitized reflectance values for each of a number of columns on the document. The same data is then used to calculate a mean-above-mean value for the same columns. Each digitized reflectance value is compared to the mean value and only when it exceeds the mean value is the mean-above-mean value updated. A threshold value is then calculated for each column using the updated mean above mean values for a particular column and for the two contiguous columns. At the same time the most recent threshold value for each column is compared to each unprocessed digitized reflectance value taken in the associated column and values above the threshold value are assigned a first binary value indicating the sample is white and values equal to or below are assigned the opposite binary value indicating the sample is black.