Digital image processing technique including improved gray scale

Abstract

An improved technique for gray scale compression of document image data is disclosed. The method comprises evaluating subportions of an image array as to relative levels of sameness and attempting to create sub-arrays having the same pixel value. Successively smaller sub-arrays are evaluated as to gray scale sameness unti 2.times.1 pixel arrays are encountered. At that time a code unique to each possible 2.times.1 pixel arrangement is stored. A high speed, real-time compression implementation is facilitated through described hardware. Document images are subdivided into 8 pixel by 512 pixel slices. As one document slice is stored in a first buffer RAM, a previously stored slice is compressed. As the pixel information is stored, tests are performed for 8.times.8 and 4.times.4 array sameness and one bit data stored according to the results of those tests. High speed compression is facilitated by querying the stored data as to the 8.times.8 and the 4.times.4 array sameness prior to further processing. Pixel gray scale information is provided as input to look up tables which provide coded information as to gray scale value in a variable length coded output. Status of 8.times.8 and 4.times.4 blocks as to sameness controls sequencing of the input to the look up tables. The microprocessor controller handles no pixel data but merely processes coded data for placement on appropriate bus and storage in an appropriate memory medium.