[Elphel-support] Global Shutter

[Abe Bachrach]

thanks for the quick response.

In talking with one of the other people in lab, we realized that most of the
experience that we had working with cameras with rolling shutters was with
cheap webcams. We hoped that the readout on the elphel sensor *might* be
fast enough that the rolling shutter effect would be insignificant.
Unfortunately it doesn't sound like that is the case.

- if the time gap is ~1/15 sec that means that if the camera is moving at
5m/s (~11mph) then camera would move 33cm during the readout period. 4X
binning would make it a bit better, however the sensor would move ~8cm,
which is still quite significant.

For more info on the rolling shutter distortion that I'm referring to, see:
http://en.wikipedia.org/wiki/Rolling_shutter

There are algorithms out there, that try to compensate for the rolling
shutter distortions:
http://mpac.ee.ntu.edu.tw/Exhibition/rolling-shutter.php
http://www.cvl.isy.liu.se/research/rs-dataset/0382.pdf
http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=05459408

<http://www.cvl.isy.liu.se/research/rs-dataset/0382.pdf>However it is much
better to avoid them altogether by using a sensor with a global shutter.
This is the approach taken by most people in the robotics and machine vision
communities.

As I said on the phone, I believe the Elphel platform would be VERY
attractive for many people in the machine vision/robotics community, however
a global shutter is a must for any of the applications that involve fast
motion which many of them do.
thanks!
-=Abe


On Sun, Nov 28, 2010 at 9:45 PM, Andrey Filippov <andrey at elphel.com> wrote:

>
> On Sun, Nov 28, 2010 at 7:30 PM, Abe Bachrach <abachrac at mit.edu> wrote:
>
>> One other question for Andrey/someone else is:
>>
>> - How much time elapses between when the first and last row are read-out?
>>
>> from looking at the datasheet, it says that the maximum datarate is
>> 96Mp/s, which would mean that for full resolution, the time gap would be at
>> least 0.052488 seconds.
>>
>
> Abe,
>
> It is somewhat longer than that because of the large "margins", the average
> data rate of the sensor running at 96MHz is ~75MPix/sec. There are formulae
> in the datasheet that allow to calculate line readout time for different ROI
> and decimation
>
>>
>> - is the sensor being run at the full 96MHz clock rate? Is that time gap
>> number correct?
>>
>
> 96MHz - yes, correct, but the "gap" is wrong - at full resolution readout
> time (and so the delay between the first and last line exposure) is ~1/15
> sec
>
>>
>> also,
>> - How does the subsampling mode effect this. If we put the sensor in
>> binning/skipping mode, and downsample by 4x, ideally, this would mean that
>> there it takes 0.0032 seconds to read out a frame.
>>
>
> Yes, that is correct. Just keep in mind that there is a large "dead" time
> (horizontal blanking)  added to each scan line, but small on top and bottom
> (vertical blanking)
>
> Andrey
>
>
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