Required Fidelity In Proof

Video surveillance, generally speaking, is recorded* for later review. We do so for a number of obvious reasons; we want to know who dented my car in the lot, who or what caused the window to break, who stole my wallet, laptop, lawnmower and so on. In such situations we don’t (usually) expect to know the person who is digging through our pockets. And then there are situations where unexpected intrusion is not a problem. This could be a factory floor where a stranger would quickly be observed and apprehended by the staff, in those cases you might be monitoring people you do know .

If you have a bad photograph from a family vacation, you know – out of focus, slightly shaken, you are still very capable of identifying the guy in the red Speedos (2 sizes too small), in fact you would probably testify in court that you know who that guy is, even if you were not present when the snapshot was taken.  But what if you were presented with unclear, smudged photos of total strangers – and then you are asked to drive through town, locating someone who looked just like that 20×32 pixel grab that I just handed you? Now we’d be hard pressed to testify with absolute certainty that we’ve got the right guy (unless he is wearing small red Speedos).

We don’t need record video to realize that something happened, we record because we want to know what or who caused the incident. In fact we are often motivated to go through the archives exactly because we, on our own accord, noticed that something sinister had happened. A smudged, pixelated frame of a man digging through our pockets tell us what we already know. We need to have enough fidelity that we can recognize the perp, even if he is a total stranger, and certainly enough fidelity that someone who knows the perp will be able to.

In more serious cases, the frame or video will be shown to a large amount of people, and we then hope that someone is able to recognize the person from the lo-fi video (and is willing to spill the beans too). But in most cases, the loss of you wallet will not make it to Americas Most Wanted, and you suddenly realize that the money spent on video only provided you with information that you could have deduced otherwise (albeit after some searching of the usual places for the missing wallet  – but you do the ransacking before you review the video anyhow 🙂 )

There are certainly situations where you do not need high fidelity. Long term surveillance of staff or a previously identified person, at that point you do know who you are looking at, so identification is not the issue, but their actions might be.

Most systems allow you to “not record if nothing happens”. This is an extremely crude form of compression, frame 1 is almost exactly like frame 2, so we discard frame 2 completely. But think about the situation where someone fraudulently claims you are liable for something that happened to them. Is if proof to say “I have no recordings, so nothing happened”? Proving that “nothing” happened requires almost no fidelity – even a 160×120 feed from a storage room is good enough to prove that no-one entered between 5 am and 10 am. If someone DID enter, 160×120 is almost certainly not good enough to identify who (assuming it’s a stranger).

What this means, is that the recorder should not only have a “live and recorded” mode, but also be able to change the feed when “things” happen.

H.264 and MPEG4 using variable bit rate offers some of what I am requesting. In a static environment the P-frames are relatively small, but it presents other problems. We’ve discussed the long GOP problem before, and it requires a lot of computational power to alter the frame rate and resolution of H.264, so pruning is usually out of the question for these formats. Then there are issues of browsing on low bandwidth devices where transcoding is needed.

What to record, when, and what you can do with the recordings is absolutely not trivial. Perhaps massive storage solutions will render the problem moot, perhaps extremely fast CPUs will, but cracking the issue now would offer an advantage to the first ones to get it right.

*Although I have encountered situations where video was not recorded at all, those have been rare, and usually nothing is recorded for political reasons.


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