HP Labs Technical Reports
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The LOCO-I Lossless Image Compression Algorithm: Principles and Standardization into JPEG-LS
Weinberger, Marcelo; Seroussi, Gadiel; Sapiro, Guillermo
Keyword(s): lossless image compression; standards; Golomb codes; geometric distribution; context modeling; near- lossless compression
Abstract: LOCO-I (LOw COmplexity LOssless COmpression for Images) is the algorithm at the core of the new ISO/ITU standard for lossless and near-lossless compression of continuous-tone images, JPEG-LS. It is conceived as a "low complexity projection" of the universal context modeling paradigm, matching its modeling unit to a simple coding unit. By combining simplicity with the compression potential of context models, the algorithm "enjoys the best of both worlds". It is based on a simple fixed context model, which approaches the capability of the more complex universal techniques for capturing high-order dependencies. The model is tuned for efficient performance in conjunction with an extended family of Golomb-type codes, which are adaptively chosen, and an embedded alphabet extension for coding of low-entropy image regions. LOCO-I attains compression ratios similar or superior to those obtained with state-of- the-art schemes based on arithmetic coding. Moreover, it is within a few percentage points of the best available compression ratios, at a complexity level estimated at an order of magnitude lower. We discuss the principles underlying the design of LOCO-I , and its standardization into JPEG-LS.
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