[Elphel-support] issues with OpenCV

[Alexandre Poltorak]

Jeff, Thanks a lot for your feedback !

On Wed, Mar 16, 2011 at 10:24 AM, jeff witz <witzjean at gmail.com> wrote:

> Hello Sebastian,
>
> OpenCV hasn't good methods to handle rtp and http flux for now.
> We already try all the backend and we finally succeed in getting picture in
> compiling the gstreamer form scratch without ffmpeg support.
> The problem is that the only way to deal with url, is the module uriscr,
> which didn't allow any set-up. So once we get a picture it was with 3 sec.
> of delay, which was unacceptable for our realtime purposes.
>

Yes, uriscr have no parameters, try to auto-guess the pipeline to launch..
and is unreliable if used with OpenCV.

I had some e-mail exchange with Nils Hasler <hasler at mpi-inf.mpg.de> where he
wrote that at some point they was considering adding a constructor video
source that allows passing a string like this:

"rtspsrc location=rtsp://192.168.0.9:554 latency=100 ! rtpjpegdepay !
jpegdec ! OpenCVsink" or similar.

You may want to ask for this feature on the opencv-devel list.

Best regards,
Alexandre


> Furthermore, I see that the connection to the camera is quite long and it
> can be a problem for our purpose. So we finally choose to convert the camera
> stream to a virtual device using v4l2loopBack plugin. As we had some
> stability issues with gstreamer, we decide to use the mencoder backend,
> which is much more stable.
> Depending of the kernel version One can have simultaneous access the
> virtual device. So in order to be sure that we can treat the datas and save
> the movies we have avticated the multicast option, then we can now work with
> OpenCV, with the classical example given for a webcam.
> The other issue we have to solve came from the fact that OpenCV hadn't been
> designed for big picture at an high framerate. So we had to remove every
> windows that come from OpenCV to use Qt which have better performances.
> Finally we achieve our realtime process of picture at any framerate. We
> succeed in the treatment of 700 FPS.
> If you are interested in such methods, you can send me private mail and I
> can guide you. But I don't give any help on the windows platform.
> Everything work well with an ubuntu LTS, so there is no difficulties to use
> it in this particular context.
>
> Best regards
>
>
>
>
> >Message: 2
> >Date: Tue, 15 Mar 2011 16:23:46 +0100
> >From: "Schuster, Sebastian" <Sebastian.Schuster at kmweg.de>
> >To: <support-list at support.elphel.com>
> >Subject: [Elphel-support] issues with OpenCV
> >Message-ID:
> >     <A54007A6D3ADA94CBD674D9FE82B13CF05C35A59 at MU00EXV01.kmweg.wtlsh.de>
> >Content-Type: text/plain; charset="iso-8859-1"
>
> >
> >Hello,
> >
> >I try to open a stream to the camera in OpenCV (Windows OS). OpenCV throws
> these error messages:
> >>>>>>>>>>>>
> [>rtsp @ 0xbefd80]Estimating duration from bitrate, this may be inaccurate
> >picture size invalid (0x0)
> >Last message repeated 1 times
> >[rtsp @ 0xbf2480]Estimating duration from bitrate, this may be inaccurate
> ><<<<<<<<<<<
> >So I have experienced the same issues with the FFMPEG like in this example
> <http://wiki.elphel.com/index.php?title=Talk:OpenCV>
> >Unfortunately it was not possible for us to rebuild the Windows version of
> OpenCV with GStreamer instead of FFMPEG.
> >Could you give us some advice for establishing the stream?
> >
> >Thanks,
> >Sebastian
>
>
>
>
> --
> Jean-François WITZ
>
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>
>
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