METHOD AND APPARATUS FOR ESTABLISHING THRESHOLD LEVEL IN A BINARY MODE DOCUMENT SCANNER

In a document scanner that is operating in the binary mode, the magnitude of the signal that is generated for each picture element (PEL) of each document scan line is compared to the magnitude of a threshold signal. Either a binary 0 or a binary 1 is generated as a result of this comparison. The threshold signal magnitude is dynamically established by a method and an apparatus that provides a stored history or histogram of the various signal magnitudes that are generated for each PEL of a plurality of the document scan lines. Since the signal magnitude(s) that is provided by scanning the background, non-image, area of the document occurs the majority of the time within this histogram, the magnitude of this majority signal(s) is used to set the threshold signal magnitude. As a result, the PELS of the document's visible image are converted to a binary electronic image with a minimum loss of visual image content. The threshold signal magnitude is continuously established in this manner during the scanning of a document.