Monday, 25 July 2011

Some notes - opencv

Just quickly throwing a recording app together yesterday I found that the video size wasn't being changed - a little digging suggests that the reliance on icv_SetPropertyCAM_V4L is always going to fail if the change in resolution between the width and height calls result in an unsupported resolution on the first call. Why isn't a simple call to set the video size with both height and width  parameters by exposing icvSetVideoSize supported?

Its not my aim to patch opencv though, so for my purposes I've updated the values for DEFAULT_V4L_WIDTH and DEFAULT_V4L_HEIGHT in highgui/src/cap_v4l.cpp for 1280x720 and rebuilt. Yes its a fudge, and if I remember I'll have to bug it.

But with that fixed I have a little recorder application ready to go with the only issue left to solved being, well, the usual fun games and open source politics. I get the following error when recording :
Corrupt JPEG data: 1 extraneous bytes before marker 0xd0
Reading this thread suggests that ffmpegs MJPEG support is broken. There's a patch to fix it, but it will need manually applying. I want to avoid to many custom changes to 3rd party libs hence I'm going to ignore that for now and try and get openCV to use YUYV rather than MJPEG but my initial attempt at that failed. Hoping to get some video recorded while there's some sunshine about hence that's one for later.

Oh an extra note - calls to :

cv.SetCaptureProperty( capture, cv.CV_CAP_PROP_FPS, fps )

appear to work but if I try :

cv.GetCaptureProperty( capture, cv.CV_CAP_PROP_FPS )

I get -1 returned. In addition although the initial set returns without error, if I try to set it too high, because I'm using the same FPS as in the initial set, to set the video FPS, this can result in some funky high speed video recordings.


Of course I could have left well enough alone and just used the defaults which did work fine.

UPDATE: A little more playing reveals that setting WITH_JPEG=OFF in CMakeCache.txt switches off JPEG support (command line switch changed or broken?) and finally we can access the YUYV stream. Sadly though the performance is about 1/2 of that of the MJPEG stream :/

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