Techniques for Coding Dithered Two-Level Pictures

This paper consider several methods for the efficient coding of two-level pictures dithered to give the appearance of multiple amplitude levels. In the dithering technique, a multilevel image signal is compared with a position-dependent set of thresholds (called a dither matrix), and, if the image value exceeds the threshold, the two-level output signal is taken to be “white,” otherwise it is taken to be “black.” Spatial correlation present in the original image is not preserved in the two-level picture due to spatial variation of the value of the threshold; and, therefore, standard techniques for coding two-level pictures, such as run-length coding, lose their efficiency. We show how some of our recently developed techniques for coding two-level pictures can be modified to code two-level dithered images. Our computer simulations on a few representative two-level dithered pictures indicate that an entropy between 0.2 to 0.3 bit/pel is possible using our technique. A comparison with some recently proposed techniques by Judice indicates that those schemes result in about 10 to 60 percent higher entropy than our schemes.