Technology : Hidden watermark traps picture pirates

2019-02-27 12:19:10

By Barry Fox COPYRIGHT pirates who publish photographs without paying the picture agency or photographer could soon find themselves in trouble. Thorn EMI’s research laboratories in Hayes, Middlesex, have developed a hidden electronic “watermark” for photographs and videos that will reveal the owner of the image. The watermark survives when the original is copied onto a CD, transmitted over the Internet or even digitally compressed. Unlike previous attempts to mark copyright material, the watermark persists even if the photograph is converted from digital code into an analogue image and back into digital code again. The watermark will not just detect picture piracy. It will also reveal cases where the picture has been digitally manipulated. When the London Evening Standard published a picture of John Prescott, the Labour deputy leader, at a function last month, it removed a beer bottle from the shot in order to portray the politician as a “champagne socialist”. The newspaper quickly owned up, but with a watermarked photograph, say Thorn EMI’s researchers, the absence of a watermark where the beer bottle should have been would show that the picture had been tampered with. Several companies, including British-based computer company ICL, are working on ways to watermark analogue images. But Thorn is the first to demonstrate a working system and explain how it works. Thorn’s system converts the photograph into a digital image, which it breaks up into tiny rectangles containing a few hundred pixels. Each of the rectangles is then analysed to see if it contains a great deal of detail—depicting a face, say—or is featureless, like a patch of cloudless sky. The system then adds a pattern to the digitised picture. Each owner has a unique pattern. And the strength of the pattern is geared to the detail in each rectangle so that it is always imperceptible to the eye. The pattern can be read with a decoder, which converts an analogue image into digital code and scans it, looking for a repetitive pattern—the watermark that identifies the copyright owner. But even under close visual examination, photographs treated by the process are indistinguishable from the original prints. David Monteith of Thorn EMI says: “If you strip out the whole picture, you can see the marking, but when the picture is present it disappears.” The same system can also be used to watermark films and videos. Each individual frame is separately processed,