Some extensions of the ordering techniques for compression of two-level facsimile pictures

We present extensions of our earlier published ordering techniques for efficient coding of two-level (black and white) facsimile pictures. Ordering techniques use the two-dimensional correlation present in spatially close picture elements to change the relative order of transmission of elements in a scan line so as to increase the average length of the runs of consecutive black or white elements in the ordered line, making the data more amenable to one-dimensional run-length coding. The extensions that we consider allow us to use different run-length codes to match the statistics of different parts of the ordered data, and to drop certain runs from transmission. Computer simulations using the eight standard CCITT pictures, which have a resolution of approximately 200 dots/inch, indicate that these extensions can result in transmission bit rates which are about 11 to 21 percent lower than the ordering schemes described in our earlier work. The entropies vary between 0.021 and 0.125 bits/pel for the eight pictures.