Lossless Image Compression Format

Dropbox developed and open-sourced Lepton, a streaming image compression format that achieves 22% lossless savings for JPEGs by predicting coefficients and using an arithmetic coder. It compresses at 5MB/s and decodes at 15MB/s, saving multiple petabytes of space. Lepton is used for archival and serving, with rigorous testing for determinism and security via seccomp. The format encodes AC coefficients using unary, sign, and standard binary representations, and optimizes DC coefficient compression by serializing them last and predicting them based on neighboring blocks and gradients.