A technique is presented for coding images that are bilevel in nature but have been captured in continuous-tone format. Following various stages of image processing, a three-level image is generated, and compressed to about 0.1 to 0.2 b/pixel. The technique has been implemented in the IBM freeze-frame videoconferencing system.>

The processing algorithm described herein processes multi-level scans of mixed documents, those containing continuous tone images as well as text and line art, and produces good quality binary documents which compress well when operated on by run-length based coding schemes such as the CCITT standard MH (Modified Hoffman, ref 1). The binary documents produced feature sharp edges in text areas and an accurate representation of tone, free of contours, in continuous tone areas of the processed documents. In addition, the overall grayness of the original documents can be recovered from the halftoned documents, which permits them to be converted to other representations more appropriate for specific output devices. The simplicity of the algorithm allows it to work in a real time environment and it can be easily implemented on various signal processors and micro-processors.