Interpol Unscrambles Doctored Photo In Manhunt

jackpot777 writes in with an AP story out of Paris reporting that Interpol has distributed photos of a man suspected of sexually exploiting children. The images were recovered from pictures taken off the Internet in which the man's face had been blurred using something like Photoshop's Filter > Distort > Twirl tool. German police were able to recover recognizable images of the man, whose identity and nationality are not known. Interpol would not discuss the techniques used to recover the images. jackpot777 writes: "It does show one interesting facet of internet privacy that has also been noted with topics ranging from reading blurred check numbers in images to Google's plan to blur out license plate and face data for Street View. And that is: blurring is not the same as completely obscuring. As computers become more adept at extrapolating data of different types, your identity isn't safe unless you completely cover all those identifying features."

Comments

Pictures

[avij]

The pictures can be seen on Interpol's site.

Re:Pictures

[mikael]

A convolution filter would allow you to sharpen or blur the image, or maybe even pick out high frequency detail.

The spiral distort effect is simply a mathematical function to map one point in a rectangular mesh to another. You basically convert integer pixel coordinates into a floating-point coordinate system with the origin at the centre, apply a rotation based on the distance from the origin, convert back into integer pixel coordinates and transfer the pixel data.

Consequently, since every pixel is remapped to a new position, the transformation can be reversed.

Interpol not the ones to descramble

[packeteer]

From the interpol web page it says:

These pictures have been produced by specialists from Germany's federal police force, the Bundeskriminalamt, working from originals found on the Internet, which had been digitally altered to disguise the man's face.

The actual AP article

[avij]

.. can be read here.

Blurring different from twirling...

[Max Romantschuk]

A twirl is essentially shifting pixels around an image, and is designed to keep as much information as possible.

A blur on the other hand, especially a gaussian blur, will mix pixels together in such a way that any recovered image will be one of many possible outcomes.

Then again, removing information, by pixellating for example, would be best.

blurring != obscuring; true, but...

[Animaether]

true, blurring isn't the same as obscuring. That said, a twirl/swirl filter isn't a blur filter either. A twirl/swirl filter relocates pixels from position A to position B. The original pixels are still largely there, you just have to move them back from B to A. That's what Interpol did here - kudos to them for figuring that out. But a blur filter doesn't just relocate pixels - it blends a bunch together. Now don't get me wrong - there's certainly deconvolution methods to reduce blur - especially motion blur - ( one example software: http://www.focusmagic.com/ ) but you're not going to be able to just take any heavily non-motion blurred image and get a supersharp result back. Other techniques, such as pixelization, are even worse to restore - you may as well not try.

-That- having been said.. yes, obscuring does tend to be better.. as long as it's a proper obscuring and not some half-hearted attempt by a news station where an interviewer / whatever has said to want to be inrecognizable, and then you just get a dark silhouette of the person where you can 1. still make out the silhouette, 2. their voice goes unaltered, 3. bump up the brightness enough and you can even make out a face or, in the case of yea olde license plate, a black bar that is supposed to 'track' the license plate properly, but the person applying the bar is a lazy-ass tracker and it 'swims' over the plate, revealing tiny bits of the bottom/top on certain frames - not too much guesswork involved to figure out the proper license plate, as even with multiple possibilities, only one is likely to match the type/color of the car when looked up on the interwebs.

Now then... Let the "what if somebody photoshopped somebody else's head on there first, then applied the filter, now some poor innocent sap is framed!" replies begin.

Comment removed

[account_deleted]

Comment removed based on user account deletion

Re:Super-sekr1t unblurring techniques

[AmiMoJo]

Your method is not entirely foolproof. The problem is that someone can repeat the steps you took and compare the results. They could photograph a person in the same pose, apply the same or very similar filters and if the results match, there is a good chance the source images were also quite similar.

A similar technique was used to guess blurred out numbers on cheques, passports, car number plates etc. Simply run through all possible combinations of letters and numbers, applying a mosaic each time until the mosaics match.