Transmission of gray level images by multilevel dither techniques

Dither techniques were invented to display pictures using bilevel media, but yet give the visual appearance of many shades of gray. With the advent of several new display media which can show more than two gray levels at each sample (e.g. 4 or 8 levels), extension of the dither to multilevels is required. We present such an extension in this paper. We also show how dithered images with few levels (e.g. 4 or 8) can be converted into many gray levels for display on cathode ray tubes. Techniques for hierarchical transmission are developed in which the first stage results in a two-level dithered image at about 40% of and 20% of the total bits for four and eight levels ditheres images, respectively. A good quality image is obtained at one bit per pel if eight level dither is used with gray level reconstruction. This compares favorably with the state of the art predictive and transform coding systems.