On Mon, Nov 29, 2010 at 04:07, Abe Bachrach <[email protected]> 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 <[email protected]> wrote: >> >> On Sun, Nov 28, 2010 at 7:30 PM, Abe Bachrach <[email protected]> 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 >> > > _______________________________________________ Support-list mailing list [email protected] http://support.elphel.com/mailman/listinfo/support-list_support.elphel.com
