The emerging international standard for compression of bilevel images and bi-level documents, JBIG2,provides a mode dedicated for lossy coding of halftones. The encoding procedure involves descreening of the bi-levelimage into gray-scale, encoding of the gray-scale image, and construction of a halftone pattern dictionary.The decoder first decodes the gray-scale image. Then for each gray-scale pixel looks up the corresponding halftonepattern in the dictionary and places it in the reconstruction bitmap at the position corresponding to the gray-scale pixel. The coding method is inherently lossy and care must be taken to avoid introducing artifacts in the reconstructed image. We describe how to apply this coding method for halftones created by periodic ordered dithering, by clustered dot screening (offset printing), and by techniques which in effect dithers with blue noise, e.g., error diffusion. Besides descreening and construction of the dictionary, we address graceful degradationand artifact removal.
|Journal||Journal of Electronic Imaging|
|Publication status||Published - 2000|