| This dissertation studies how to generate computer images of mosaic art with high quality. In traditional mosaic art made from bricks, tiles, or counted cross-stitch patterns, artists would need to divide the original image into small parts of reasonable sizes and shapes and represent the colors of each part using just one color selected from a given color palette. Using standard image processing methods to automate this process, the resulting mosaic image may contain undesirable visual artifacts of patches and color bandings. Error-diffusion dithering algorithms can reduce such artifacts. Based on the observation that image parsing directions are critical for diffusing errors, we present a new error-diffusion scheme called "Four-Way Block dithering" (FWB) to correct certain artifacts caused by existing methods, including the directional and latticed appearance produced by Floyd and Steinberg's dithering (FSD). FWB divides the input image into blocks of equal size with each block consisting of four sub-blocks such that the size of each sub-block is suitable for an underlying error-diffusion algorithm. Scanning the blocks from left to right and from top to bottom, for each block being scanned, FWB starts from the center of the block and diffuses errors along four directions on each sub-block. This dissertation shows that FWB can better retain the original structure and reduce unstructured artifacts. We show that FWB dithering produces much better peak signal-to-noise ratios on mosaic images over those generated by FSD. We build a web site called "Art the Mosaic Tile" (AMT), which is an e-commerce Web application system for making mosaic art images generated by FWB-FSD error diffusion from regular pictures. AMT makes it possible for ordinary users to produce high-quality mosaic art images with tiles without special training. |
