# Sudoku saves photographers from copyright theft

A new watermarking technology based on a system akin to the permutation rules used to solve the numeral puzzles known as Sudoku has been developed by computer scientists in Malaysia. Writing in the International Journal of Grid and Utility Computing the team reports how their system could resist attempts to "crop" the watermark in more than nine times out of ten cases.

Images, photos and graphics on the web are easy pickings for plagiarists and those who might ignore copyright rules. Photographers and others often add a watermark to their images to reduce the risk of their images being lifted for use on others' sites without permission. However, those intent on leeching an image might simply crop the watermark in some cases.

With the proliferation of digital on the internet, content owners and service providers require robust technology to protect their work. Digital watermarking is commonly used to embed specific information into the media to be protected, such as a company's logo or product serial number. Such information can later be extracted and used to detect forgery and unauthorized usage and to prove authenticity and provenance. Importantly, a digital watermark must not distort or disrupt display of the image when used in its rightful place and so needs to be imperceptible in use.

Now, Shamsul Khalid of the Universiti Tun Hussein Onn Malaysia, in Johor, and colleagues, explain how a valid 9x9 Sudoku solution - comprising a pixelated second image - is used to create the watermark so that it is evenly distributed within an image and so that it resists automated cropping and noise additions by bots and other tools that scrape images from websites and add them to an illicit database for unlicensed resale or distribution to other websites that require a range of unique images but do not wish to pay for the privilege.

The approach uses the permutations of rows and columns in Sudoku solutions to create and detect an invisible digital watermark that is overlaid on an image with a random distribution. If the image pirate crops part of the image, then the chances are that enough of the watermark will remain elsewhere in the image that the complete watermark might be retrievable provided that the precise and correct Sudoku solution is given.

The team's initial tests showed that with 81 9x9 Sudoku solutions they could defeat more than 94% of attempts at cropping. They are currently implementing 256 16x16 Sudoku, which they suggest will be even stronger. The best "anti-cropping" watermarks used previously achieved only 75% resistance. Moreover, the Sudoku approach does not require investigators or the authorities to have access to the original image. Based on the relationship between full and partially recovered watermarks, the Sudoku approach will be able to discern whether a pirated image has the copyright owner's watermark.

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More information: "Anti-cropping digital image watermarking using Sudoku" in Int. J. Grid and Utility Computing, 2013, 4, 169-177

Feedback to editors

Sep 05, 2013
94% is actually pretty good (there is no 100% method).
You have to realize that watermarks must be insensitive to cropping, color changes, stretching, mirroring, rotation, (color)smoothing, conversion to different format, and a whole host of other techniques (individually or combined) that don't alter the image perceptibly but might destroy the watermark.

Sep 06, 2013
Surely if an image is cropped, then you can always prove it's yours since you have that missing piece that fits onto and extends it - I can't see how that could be faked in almost all cases.

Sep 06, 2013
That alone won't help - as you can always take the same (uncropped) picture from somewhere else and claim it's yours. While that would prove that the one who cropped it doesn't own the original it wouldn't prove to whom the original belongs.

A watermark must be invisible and indelible and only extractable/verifiable by the owner. The invisible part is especially important if we're dealing with stuff like x-ray images or CT,MR volumes that are used for diagnostics (don't want the watermark to skew the diagnosis). That's why many watermarks work by changing the blue pixels minimally and/or adding information in the frequency domain (fourier transformed image)