Abstract: We present a technique for adding a high-frequency pattern to JPEG images that is imperceptible to the unaided eye, but turns into a clearly readable large-letter warning if the image is recompressed with some different quality factor. Our technique aims to achieve a goal similar to copy-evident security-printing techniques for paper documents, namely to have an easily recognizable text message appear in lower-quality copies, while leaving the visual appearance of the original unharmed. We exploit non-linearities in the JPEG process, in particular the clipping of the result of the inverse discrete cosine transform.

  url          = {http://www.cl.cam.ac.uk/~abl26/info09-jpeg.pdf},
  journal      = {Lecture Notes in informatics},
  author       = {Andrew B. Lewis and Markus G. Kuhn},
  year         = {2009},
  title        = {Towards copy-evident {JPEG} images},