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My tl;dr understanding: Drones send video back to operator. Video is typically compressed so that it only updates part of the picture that have changed. Even if

My tl;dr understanding: Drones send video back to operator. Video is typically compressed so that it only updates part of the picture that have changed. Even if the video is encrypted, the researchers are able to measure the bitrate. Thus when the researchers make a significant change like putting a board against a window, and seeing if the traffic increases, the researchers can determine whether the drone is looking at that window.

Sargos 9y ago
I'm assuming this is predicated on the drone being absolutely still and that nothing in the scene (leaves on trees) is moving? Is that even realistic?
- godelski 9y ago
  If you do a regular sequence of major changes, you could pick that signal out. This would create a larger and more regular signal in the data than the random noise from leaves. I don't think you could accurately predict observation with just one set of environmental changes.
- Kalium 9y ago
  Generally speaking, inter-frame video compression uses an amount of data directly related to the amount that things change from one frame to the next. So while noise like leaves moving shows up as small changes moment to moment, larger changes are still realistically visible. My younger years spent screwing around with video for a hobby are coming back to me now.
- kmm 9y ago
  Shouldn't simple linear movement of a scene still compress really well?
  - godelski 9y ago
    Yes. The compression works by analyzing changes in the screen. A simple example of this would be if you are displaying data. Say you are taking real time data and it is scrolling across the screen. Your display moves the data to the left, drops off what moved out of the frame, and adds what is new. Pictures are more complex, but the idea is the same. Only take what is new.
- natosaichek 9y ago
  Well, It assumes that the amount of delta you can create in the transmitted image is sufficiently large that you can discern it from other sources of random variation in the image. So if the drone is moving around dramatically and there are huge deltas in the image, making your windows opaque / translucent repeatedly may not be sufficient signal. But if the drone is observing a dark compound and you flash lights everywhere repeatedly, it might not matter that the drone is moving around a lot - there might still be a dramatic change in the data transmitted by the drone. Ultimately you're putting some signal into a potentially noisy channel and trying to reconstruct whether or not your observations contain that signal. The example is simple enough that normal human eyeballs looking at the output can detect it. Using more sophisticated techniques might be able to tease out smaller signals from noisier channels.
wyc 9y ago
Couldn't this be bypassed by regulating a drone's bitrate to a constant steady flow? The sacrifice will be either video quality or freshness of data, but it would be a straightforward way get past that protection.
- oreo81 9y ago
  Yes, just like anything...but it comes with a trade off.
- ucaetano 9y ago
  Or and even easier way: just have the drone fill up the stream with random data so that the data stream never goes "quiet" when the image is mostly unchanged. No sacrificing video quality or freshness required.
  - Drakim 9y ago
    So you compress the video to a smaller size, and then fill in the bandwidth you saved with random data? Why compress at all? Wouldn't just having a pixel perfect stream solve the same issue and give better quality video?
    - ucaetano 9y ago
      > Why compress at all? Wouldn't just having a pixel perfect stream solve the same issue and give better quality video? Because compression reduces the amount of data that needs to be transmitted, reducing network and battery usage, especially if the radio channel is congested. Instead of filling the stream and having a constant stream size, you could just code the drone do add random bursts of data, making any prediction from the observer useless.
      - Roujo 9y ago
        > Instead of filling the stream and having a constant stream size, you could just code the drone do add random bursts of data, making any prediction from the observer useless. If it's actually random, wouldn't the average bitrate of "still scene + occasional random data" still be lower than "active scene + occasional random data"? Given enough samples, of course.

Sargos 9y ago

I'm assuming this is predicated on the drone being absolutely still and that nothing in the scene (leaves on trees) is moving? Is that even realistic?

godelski 9y ago
If you do a regular sequence of major changes, you could pick that signal out. This would create a larger and more regular signal in the data than the random noise from leaves. I don't think you could accurately predict observation with just one set of environmental changes.
Kalium 9y ago
Generally speaking, inter-frame video compression uses an amount of data directly related to the amount that things change from one frame to the next. So while noise like leaves moving shows up as small changes moment to moment, larger changes are still realistically visible. My younger years spent screwing around with video for a hobby are coming back to me now.
kmm 9y ago
Shouldn't simple linear movement of a scene still compress really well?
- godelski 9y ago
  Yes. The compression works by analyzing changes in the screen. A simple example of this would be if you are displaying data. Say you are taking real time data and it is scrolling across the screen. Your display moves the data to the left, drops off what moved out of the frame, and adds what is new. Pictures are more complex, but the idea is the same. Only take what is new.
natosaichek 9y ago
Well, It assumes that the amount of delta you can create in the transmitted image is sufficiently large that you can discern it from other sources of random variation in the image. So if the drone is moving around dramatically and there are huge deltas in the image, making your windows opaque / translucent repeatedly may not be sufficient signal. But if the drone is observing a dark compound and you flash lights everywhere repeatedly, it might not matter that the drone is moving around a lot - there might still be a dramatic change in the data transmitted by the drone. Ultimately you're putting some signal into a potentially noisy channel and trying to reconstruct whether or not your observations contain that signal. The example is simple enough that normal human eyeballs looking at the output can detect it. Using more sophisticated techniques might be able to tease out smaller signals from noisier channels.

wyc 9y ago

Couldn't this be bypassed by regulating a drone's bitrate to a constant steady flow? The sacrifice will be either video quality or freshness of data, but it would be a straightforward way get past that protection.

oreo81 9y ago
Yes, just like anything...but it comes with a trade off.
ucaetano 9y ago
Or and even easier way: just have the drone fill up the stream with random data so that the data stream never goes "quiet" when the image is mostly unchanged. No sacrificing video quality or freshness required.
- Drakim 9y ago
  So you compress the video to a smaller size, and then fill in the bandwidth you saved with random data? Why compress at all? Wouldn't just having a pixel perfect stream solve the same issue and give better quality video?
  - ucaetano 9y ago
    > Why compress at all? Wouldn't just having a pixel perfect stream solve the same issue and give better quality video? Because compression reduces the amount of data that needs to be transmitted, reducing network and battery usage, especially if the radio channel is congested. Instead of filling the stream and having a constant stream size, you could just code the drone do add random bursts of data, making any prediction from the observer useless.
    - Roujo 9y ago
      > Instead of filling the stream and having a constant stream size, you could just code the drone do add random bursts of data, making any prediction from the observer useless. If it's actually random, wouldn't the average bitrate of "still scene + occasional random data" still be lower than "active scene + occasional random data"? Given enough samples, of course.