Many photo-sharing websites decompress and compress uploaded images, to enforce particular compression parameters. This recompression degrades quality. Some web proxies can also recompress images/videos, to give the impression of a faster connection.

In Towards copy-evident JPEG images (with Markus Kuhn, in Lecture Notes in Informatics), we present an algorithm for imperceptibly marking JPEG images so that the recompressed copies show a clearly-visible warning message. (Full page demonstration.)

Original image: After recompression:

(If you can’t see the message in the recompressed image, make sure your browser is rendering the images without scaling or filtering.)

Richard Clayton originally suggested the idea of trying to create an image which would show a warning when viewed via a recompressing proxy server. Here is a real-world demonstration using the Google WAP proxy.

Our marking technique is inspired by physical security printing, used to produce documents such as banknotes, tickets, academic transcripts and cheques. Photocopied versions will display a warning (e.g. ‘VOID’) or contain obvious distortions, as duplication turns imperceptible high-frequency patterns into more noticeable low-frequency signals.

Our algorithm works by adding a high-frequency pattern to the image with an amplitude carefully selected to cause maximum quantization error on recompression at a chosen target JPEG quality factor. The amplitude is modulated with a covert warning message, so that foreground message blocks experience maximum quantization error in the opposite direction to background message blocks. While the message is invisible in the marked original image, it becomes visible due to clipping in a recompressed copy.

The challenge remains to extend our approach to mark video data, where rate control and adaptive quantization make the copied document’s properties less predictable. The result would be a digital video that would be severely degraded by recompression to a lower quality, making the algorithm useful for digital content protection.