Extending DACLIC for near-lossless compression with postprocessing of greyscale images
Summary form only given. A lossless/near lossless coding scheme, DACLIC, is presented. The proposed scheme attempts to remove redundancy in a given image in the spatial domain. The redundancy removal is achieved by block direction prediction and context-based error modeling. The block direction operation in DACLIC first partitions an image into blocks. Pixels within each incoming block are analyzed resulting in a best directional prediction for that block. The best direction is chosen from a given set that results in the minimum prediction error. Removal of redundancy by the block direction technique is not possible for removing all possible redundancy in a given image. Another decorrelation part of the DACLIC scheme is the context-based error modeling which exploits context-dependent DPCM error structures. The DACLIC scheme is primarily used as a lossless image compression technique. However, the scheme can be easily extended to near-lossless compression applications by introducing a small quantization loss. This small quantization loss is restricted to an absolute error not exceeding a prescribed value n for all pixels in a given image. Application of block direction and context modeling reduces a given image into residuals. This residual typically has a lower entropy than the given image. A quadtree Rice coder (QRC) is proposed as an entropy coder of DACLIC. An arithmetic coder is also given as an option. The QRC operates on residual blocks with low computing complexity that compares favorably with the residual coding method used by LOCO-I as the proposed QRC is two-dimensional in nature. For near-lossless compression with a larger value of n, banding artifacts are visible in the decoded image. In the DACLIC system, a postprocessing technique is proposed to remove the banding artifacts.