4 ms·

It's known as the Rolling Shutter effect. It's most commonly observed when attempting to take still imagery of very fast moving objects like rotors on airplanes

It's known as the Rolling Shutter effect. It's most commonly observed when attempting to take still imagery of very fast moving objects like rotors on airplanes, but affects pretty much any image sensor that isn't designed specifically to work around it. The following Wikipedia article goes into far more depth, but the basic problem is that an image sensor doesn't sample every pixel instantly, but instead reads them out serially, and thus there's a tiny, tiny fraction of a second in-between each pixel being sampled. It's small enough to not matter for most purposes, but it's still there, and can be useful / detremental depending on the application. https://en.wikipedia.org/wiki/Rolling_shutter https://en.wikipedia.org/wiki/Rolling_shutter

Sharlin 8y ago
And it is why mechanical shutters are still a thing in dedicated cameras. It is still much faster to move two pieces of metal or carbon fiber over the sensor than it is to read out a multi-megapixel CMOS sensor. When shooting video mechanical shutters cannot be used so the rolling shutter effect can cause artifacts such as fast-moving subjects appear slanted.
- im3w1l 8y ago
  Borrowing from rendering, it seems like a solution would be for every pixel to have "double buffers", one for the measurement in progress and one for the past measurement. Then you just need to make sure you can read all the past values in less than a frame and that all the buffer swaps happen in sync.
  - zeta0134 8y ago
    I believe this is the premise behind image sensors which support global shutter (as opposed to rolling shutter) but other than being aware that this feature might exist, I'm no expert on the subject.
- namibj 8y ago
  Err, modern global shutters like those made by CMOSIS/ams are _much_ faster than that. Their fast one is an APS-C sensor with 4k*3k Px, a shutter closing time of 1s/~120000 and a minimum shutter open time of ~1s/50000. The shutter closing time might be even faster, just reconstruction from frame overhead time values I remember, and adjusting for the share the row skew had. Check the datasheet if you like to. If you know a mechanical shutter that can do such, I'd like to know. This, by chance, allows laser-flash illumination of objects that are behind a close wall of fog, as you can keep the shutter closed while the light travels to the further away object of interest. You will have the blur, but no longer the massive constrast loss due to light pollution. If it's not as bad as fog, and just e.g. normal rainfall or such, you lack the reflection artifacts that would be common, and only retain the refraction artifacts from the light passing through it by necessity. These sensors do lack a little dynamic range, but you can compensate with some slight trickery, see the datasheet, to get ~15 stops out of this particular device. Rolling shutter has applications, but videos for users of low knowledge and high ambitions is not one of them.
  - nickparker 8y ago
    Could you expand on or link the bit about time of flight trickery in fog or rain? c * 1 s / 50,000 is still kilometers, so I’m not sure I understand how this shutter can do what sounds like using time of flight to selectively illuminate stuff at a specific depth
    - namibj 8y ago
      No, the part that can solve this is the fast time from insensitive to sensitive, which is shorter than the time form sensitive to no longer sensitive. You will need a pulse with a short enough duration, e.g. a Q-switched Nd:YAG with frequency doubling giving you up to about a dozen Joules at up to a few hundred Hz with pulse durations of under 50 ns, and, while limited in some ways by breakdown peak power in components, a lower limit of about .5 ns. Most of the shutter time is used to copy the data to the shadow pixel, but just releasing the dark pull won't take long.
  - Sharlin 8y ago
eigenloss 8y ago
A relevant point here is that video compression almost always destroys the signal relied upon in this research.

Sharlin 8y ago

And it is why mechanical shutters are still a thing in dedicated cameras. It is still much faster to move two pieces of metal or carbon fiber over the sensor than it is to read out a multi-megapixel CMOS sensor. When shooting video mechanical shutters cannot be used so the rolling shutter effect can cause artifacts such as fast-moving subjects appear slanted.

im3w1l 8y ago
Borrowing from rendering, it seems like a solution would be for every pixel to have "double buffers", one for the measurement in progress and one for the past measurement. Then you just need to make sure you can read all the past values in less than a frame and that all the buffer swaps happen in sync.
- zeta0134 8y ago
  I believe this is the premise behind image sensors which support global shutter (as opposed to rolling shutter) but other than being aware that this feature might exist, I'm no expert on the subject.
namibj 8y ago
Err, modern global shutters like those made by CMOSIS/ams are _much_ faster than that. Their fast one is an APS-C sensor with 4k*3k Px, a shutter closing time of 1s/~120000 and a minimum shutter open time of ~1s/50000. The shutter closing time might be even faster, just reconstruction from frame overhead time values I remember, and adjusting for the share the row skew had. Check the datasheet if you like to. If you know a mechanical shutter that can do such, I'd like to know. This, by chance, allows laser-flash illumination of objects that are behind a close wall of fog, as you can keep the shutter closed while the light travels to the further away object of interest. You will have the blur, but no longer the massive constrast loss due to light pollution. If it's not as bad as fog, and just e.g. normal rainfall or such, you lack the reflection artifacts that would be common, and only retain the refraction artifacts from the light passing through it by necessity. These sensors do lack a little dynamic range, but you can compensate with some slight trickery, see the datasheet, to get ~15 stops out of this particular device. Rolling shutter has applications, but videos for users of low knowledge and high ambitions is not one of them.
- nickparker 8y ago
  Could you expand on or link the bit about time of flight trickery in fog or rain? c * 1 s / 50,000 is still kilometers, so I’m not sure I understand how this shutter can do what sounds like using time of flight to selectively illuminate stuff at a specific depth
  - namibj 8y ago
    No, the part that can solve this is the fast time from insensitive to sensitive, which is shorter than the time form sensitive to no longer sensitive. You will need a pulse with a short enough duration, e.g. a Q-switched Nd:YAG with frequency doubling giving you up to about a dozen Joules at up to a few hundred Hz with pulse durations of under 50 ns, and, while limited in some ways by breakdown peak power in components, a lower limit of about .5 ns. Most of the shutter time is used to copy the data to the shadow pixel, but just releasing the dark pull won't take long.
- Sharlin 8y ago

eigenloss 8y ago

A relevant point here is that video compression almost always destroys the signal relied upon in this research.