High efficiency coding method for still natural images mingled with bi-level images

摘要

An original grayscale image or color image to be coded is processed by a bi-level image mingled still image coding system to produce a differential image. The original image and the differential image are compared for each pixel block. Only those of the pixel blocks for which the bi-level image mingled still image coding system will be more efficient than a natural image coding system are encoded by the bi-level image mingled still image coding system, and the other pixel blocks are encoded by the natural image coding system. The coding method of the present invention is directed to a grayscale or color image containing an area where pixel values undergo an abrupt change, such as an edge of a letter, or a document with multi-color and natural color images intermingled therein. The method of the present invention obtains a less deteriorated coded image even from an image in which the edge area is very blunt, and moreover, the invention permits high efficiency coding.