Abstract: The standard JPEG compression algorithm was only designed to process images as they were acquired by camera sensors. In practice, however, photographic images are often compressed and decompressed multiple times. Here, image quality can be preserved better with special recompression algorithms designed for input that has been compressed and decompressed before. Our new JPEG recompressor represents the exact arithmetic operations of the preceding decompression as an overdetermined system of equations. It then uses interval arithmetic and iterative refinement to invert decompression steps in a way that takes into consideration all possible rounding effects. This results in a set of possible compression results, which will include the actual result of the last compression. We have also extended our recompression algorithm into a forensic tool that identifies those regions of a raw image that cannot possibly have been the result of a previous JPEG decompression step.

  author       = {Andrew B. Lewis and Markus G. Kuhn},
  url          = {http://www.cl.cam.ac.uk/~abl26/abl26-recompression.pdf},
  year         = {2008},
  title        = {Exact {JPEG} recompression and forensics using interval arithmetic (poster/slides)},