Bit-depth extension using spatiotemporal microdither based on models of the equivalent input noise of the visual system

摘要

Continuous tone, or "contone", imagery usually has 24 bits/pixel as a minimum, with eight bits each for the three primaries in typical displays. However, lower-cost displays constrain this number because of various system limitations. Conversely, higher quality displays seek to achieve 9-10 bits/pixel/color, though there may be system bottlenecks limited at 8. The two main artifacts from reduced bit-depth are contouring and loss of amplitude detail; these can be prevented by dithering the image prior to these bit-depth losses. Early work in this area includes Roberts' noise modulation technique, Mitsa's blue noise mask, Tyler's technique of bit-stealing, and Mulligan's use of the visual system's spatiotemporal properties for spatiotemporal dithering. However, most halftoning/dithering work was primarily directed to displays at the lower end of bits/pixel (e.g., 1 bit as in halftoning) and higher ppi. Like Tyler, we approach the problem from the higher end of bits/pixel/color, say 6-8, and use available high frequency color content to generate even higher luminance amplitude resolution. Bit-depth extension with a high starting bit-depth (and often lower spatial resolution) changes the game substantially from halftoning experience. For example, complex algorithms like error diffusion and annealing are not needed, just the simple addition of noise. Instead of a spatial dither, it is better to use an amplitude dither, termed microdither by Pappas. We have looked at methods of generating the highest invisible opponent color spatiotemporal noise and other patterns, and have used Ahumada's concept of equivalent input noise to guide our work. This paper will report on techniques and observations made in achieving contone quality on ～100 ppi 6 bits/pixel/color LCD displays with no visible dither patterns, noise, contours, or loss of amplitude detail at viewing distances as close as the near focus limit (～120 mm). These include the interaction of display nonlinearities and their role of generating a low-spatial frequency flicker from mutually high-pass spatial and temporal noise, as well as the temporal response symmetries.