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JPEG-LSJPEG-LS is not a JPEG lossless mode. This is the separate international standard, ISO/IEC IS 14495 "Lossless and near-lossless compression of continuous-tone still images". JPEG-LS uses adaptive prediction described earlier in this lesson, surrounded by several blocks of additional adaptation to local image characteristics. For example if longer run of the same value (so called flat region) is detected, coder switches to special run-length coding mode; which may be explained as sending pairs of two numbers: the value and the number of samples with this value. This solution is very effective for coding of some non-natural images, for example computer screenshots. Entropy coding in JPEG-LS is based on Golomb-Rice coding also adapted to local image statistics. Compression ratios obtained with JPEG-LS are about 10 – 20% higher comparing to JPEG lossless base mode.  JPEG-LS coder structure picture from paper The LOCO-I Lossless Image Compression Algorithm: Principles and Standardization into JPEG-LS (pdf) |