The forensics of video watermarking

29 Jan 2015

A watermark is a recognizable image or pattern in paper used to identify authenticity – according to Wikipedia, paper-watermarking was first introduced in Italy in 1282. There were attempts to hone motion picture watermarking in the 1990s, as the logistics of delivering 35mm prints across the globe are daunting. Ever hand-carried a six-reel feature film in this format? I have, and it's not fun.

The obvious solution: transfer digital files to cinemas across the globe, eliminating shipping costs and associated logistics. And the obvious problem: ensuring that the intended recipient (and ONLY the intended recipient) has access to the file.

Because that file is a precious piece of intellectual property which can be perfectly copied. This changes the security profile from protection of a physical object (the original focus of the 13th century tech) to protection of a discrete chunk of data. Everyone from remote hackers to avaricious insiders has a potential motive to copy the file.

The 4K speedbump
MovieLabs is an organization whose "charter is to lead the evaluation, introduction and development of key distribution and usage technologies that may be critical to the studios and broader media market." According to an article in The Register, MovieLabs was "formed by Paramount, Sony, Twentieth Century Fox, Universal Studios, Disney and Warner Brothers and is funded by its members."

In late 2013, the organization issued a new specification for securing 4K streaming video content (4K is the next level in high-definition video). Included in the spec is forensic (digital) watermarking.

According to the Digital Watermarking Alliance (DWA), "Digital watermarking is the process by which identifying data is woven into media content such as images, movies, music or programming, giving those objects a unique, digital identity that can be used for a variety of valuable applications."

The idea is to encode a tracing element that doesn't interfere with the viewing experience. "Imperceptible to the human senses yet easily recognized by special software detectors," says the DWA website, "a digital watermark remains constant even through recording, manipulation and editing, compression and decompression, encryption, decryption and broadcast – without affecting the quality of the content."

Sounds great, in theory. But The Register article indicates that the quality of 4K (generally regarded as four times higher than 1080p, thus the name) means watermarking it is more effective compared to previous video formats. "By now forensic watermarking is becoming tougher and tougher to break," says the article. "In some systems, the watermarking process writes a unique device number into the content over a large number of frames in code, using key pixels, and in others it is only measurement against a pristine version of the file that reveals randomly placed pixels in a key coded sequence, which depict the device or stream identity."

This means would-be pirates must "spend a lot of money getting lots and lots of copies of the movie and comparing them and removing the differences, but then also changing it further – in the end you can only be sure you are not identifiable as a pirate after you have compromised the quality of the output, and in 4K that defeats the object."

Your quality is your safety
This situation – where the requisite quality can't be transmitted except by official channels – indicates 4K may have an edge. Back in analog days (some of us remember VHS videotape), each successive copy-generation introduced significant degradation of video quality, sometimes with laughable results. A first-generation copy might be watchable, but subsequent copies displayed smeared imagery more suited to a modern art gallery than a home video setup. And there was the infamous "tracking knob" that could be twirled to stabilize the picture – often in vain.

We've gone far beyond this type of first-gen home video, but introducing a watermark system that can theoretically bring a similar copy-degradation feature to 4K is an interesting development. Much work needs to be done to bring 4K to the mainstream market, but a digital watermark that deters piracy yet doesn't interfere with the viewing experience of legitimate product is desirable. As ever, the devil's in the details, and we'll see how things shake out as 4K edges closer to mass adoption.

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