The rolling shutter on a digital camera can give interesting effects(globalnerdy.com)
globalnerdy.com
The rolling shutter on a digital camera can give interesting effects
http://www.globalnerdy.com/2009/01/13/cheap-camera-interesting-shot/
7 comments
I found this MatLab visualization of what happens inside the camera very helpful in understanding how it works:
http://scalarmotion.wordpress.com/2009/03/15/propeller-image...
The article mentions that this is true of cheaper digital cameras, but in fact SLRs have a similar effect. At high shutter speeds, the back curtain closes shortly after the front curtain opens, causing the exposure to happen as a slit of light that passes across the sensor or film.
As a sidenote this only happens with faster shutter speeds than about 1/250 sec, which also happens to explain why "high speed synchronization" of flashes is needed and visualized here:
http://webs.lanset.com/rcochran/flash/hss.html
Actually I am not sure you can get this effect on cheap camera. Which cheap camera uses curtain shutter? And I would be extremely surprised to see that kind of shutter on the cameraphone.
There are two different effects here.
One, as in the iPhone and all similar devices, is a scanning phenomenon caused by the fact that cheap or very small sensors are usually CCDs. Since such cameras do not actually have real shutters, the equivalent effect of a shutter is achieved by delaying the time that each line is exposed and read. Because cheap CCDs do not have multiple channels, the lines are read one at a time in sequence. The maximum speed that the lines may be read sequentially is the maximum "shutter speed" of the camera.
The other is the traditional curtain shutter of an SLR. Even a high-end CMOS multi-channel sensor will still see a slit of light on any speed upwards of 250, even though the data is not necessarily read a line at a time.
Thus, whereas the sensor in an SLR is exposed via a slit of light at higher shutter speeds, cheaper non-SLR cameras are always exposed one line at a time. Thus this effect is more likely to be seen on a camera phone than on an SLR.
One, as in the iPhone and all similar devices, is a scanning phenomenon caused by the fact that cheap or very small sensors are usually CCDs. Since such cameras do not actually have real shutters, the equivalent effect of a shutter is achieved by delaying the time that each line is exposed and read. Because cheap CCDs do not have multiple channels, the lines are read one at a time in sequence. The maximum speed that the lines may be read sequentially is the maximum "shutter speed" of the camera.
The other is the traditional curtain shutter of an SLR. Even a high-end CMOS multi-channel sensor will still see a slit of light on any speed upwards of 250, even though the data is not necessarily read a line at a time.
Thus, whereas the sensor in an SLR is exposed via a slit of light at higher shutter speeds, cheaper non-SLR cameras are always exposed one line at a time. Thus this effect is more likely to be seen on a camera phone than on an SLR.
That was taken on an iPhone. It happens in camera phones not because of a physical shutter, but because the entire frame isn't captured at once.
Two examples of deliberately abusing this effect on a static subject (house and church) by pivoting the iphone while taking the shot:
http://farm4.static.flickr.com/3158/2421813332_3c36b3b808.jp...
http://farm4.static.flickr.com/3187/2421813434_dbdc309901_b....
http://farm4.static.flickr.com/3158/2421813332_3c36b3b808.jp...
http://farm4.static.flickr.com/3187/2421813434_dbdc309901_b....
True, I think it has something to do with the burst mode used in most sensors.
The sensor will transmit the picture, but the iPhone is not ready to process all of it. So the next burst is processed later but is used for the same image.
The sensor will transmit the picture, but the iPhone is not ready to process all of it. So the next burst is processed later but is used for the same image.
Here is a video of a plane propeller filmed with an iPhone 3GS, showing pretty cool effects due to the rolling shutter: http://vimeo.com/5934808
There's some exciting work being done in our lab with a "coded" rolling shutter, where you change the readout times on individual rows to give you better pictures or videos -- high dynamic range, super-fast capture, etc.
There's no project page up yet, but this video explains most of the key parts:
http://www.cs.columbia.edu/~jwgu/crsp_video.mp4
If you want full details, here's the paper:
http://www.cs.columbia.edu/~jwgu/crsp_final.pdf
There's no project page up yet, but this video explains most of the key parts:
http://www.cs.columbia.edu/~jwgu/crsp_video.mp4
If you want full details, here's the paper:
http://www.cs.columbia.edu/~jwgu/crsp_final.pdf
Could this be used to send 'secret' messages only visible to camera-shots of certain specs/orientation?
Yes. This could be an example of steganography, the art of concealing that you are sending a message: http://en.wikipedia.org/wiki/Steganography
On the other extreme, there are cameras that take 70 thousand frames per second. Yikes.