[Elphel-support] Global Shutter

[Sebastian Pichelhofer]

On Tue, Nov 30, 2010 at 20:01, Abe Bachrach <abachrac at mit.edu> wrote:
> Sebastian, that sounds like something to try, but wouldn't that
> significantly reduce the field of view?

Yes the field of view will be much smaller, you would have to select a
wider angle lens to compensate.


> Also, do you know if the WOI settings avoid the "horizontal blanking" issue
> which is what would cause the binning/skipping solution to not provide that
> great of an improvement? I don't see anything about how the windowing works
> in the datasheet.

Not sure, but in general the maximum possible fps is higher with
reduced WOI compared to using binning to achieve the same resolution
For example:

1280x720 (2x2 binning) JP4 RAW max. 46.2 fps
1280x720 (WOI setting) JP4 RAW max 60 fps

Regards Sebastian

>
> thanks,
> -=Abe
>
> On Mon, Nov 29, 2010 at 5:38 AM, Sebastian Pichelhofer
> <sebastian.pichelhofer at gmail.com> wrote:
>>
>> On Mon, Nov 29, 2010 at 04:07, Abe Bachrach <abachrac at mit.edu> wrote:
>> > 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.
>>
>> You mentioned you need only a small fraction of the 5 megapixels
>> resolution the sensor offers. If you reduce the WOI (Window of
>> Interest) you can increase the framerate and therefore the readout
>> time (more than with binning):
>>
>> For example at 640x480 you can reach a max. of 126fps which would
>> reduce ERS artefacts to just a little more than 10% of the 33cm you
>> mentioned at 1/15s.
>>
>> Regards Sebastian
>>
>>
>> > 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
>> > 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
>> >>
>> >
>> >
>
>