[vlc-devel] Problem in VLC using V4L2 with analog camera.

Luis aproxs at gmail.com
Tue Mar 31 17:15:12 CEST 2009


In my company we faced the same problem with v4l2 a couple of years ago.
Antoine, if you need further code review you might want to check if the
structure containing the image format is properly set to what the user
needs, cause as you know we need to bargain with the driver to find
something suitable for both.

I never checked this, but in my opinion the driver could be returning
interlaced frames with bad sequence numbers. This is: let A an B be complete
progressive images that when decomposed in interlaced images give you Aodd,
Aeven, Bodd, Beven. maybe the driver is labeling with the sequence number of
B the subimages Aeven and Bodd, and when composing the image you see both
frames at the same time interlaced.

It's just a theory, but maybe it's worth to dump a couple of interlaced
sequences to check.

For us the driver doing the funny thing was bttv too, and I've seen this
effect in Multi-Media Hard Drives directly attached to TV, so my guess is
that is more likely to be a confusion in the subimage labeling than a
problem in the v4l2 code for VLC.

I would try with a webcam attached to USB and same image format (when
possible) to see if the problem can be reproduced in different

Best of luck.


On Tue, Mar 31, 2009 at 3:34 AM, Antoine Cellerier <dionoea at videolan.org>wrote:

> Hello,
> On Mon, Mar 30, 2009, Beyonlo wrote:
> >     I have a interlaced analog SD camera connected a capture card
> > (bttv) and capturing via composite.
> >     I'm using v4l2 and I have problems with flicker (interlaced
> > combing problem) with vlc 0.9.x when capturing in 320x240. But not is
> > possible has flicker when capturing 240 lines, because 240 lines get
> > only the even lines. If I getting 480 lines for example, flicker is
> > correct, because getting even and odd lines, with 16 ms delay, because
> > is needed two Fields to compose a 480 lines.
> >     I tested too with v4l2 using mplayer and not have problem. I
> > tested too with vlc 0.8.6 with v4l and not have problem. Problem
> > happen only with VLC using v4l2.
> Could you compile VLC with --disable-libv4l2 and test again? This will
> disable a wrapper lib around the kernel v4l2 API which has been known to
> cause issues. (Compiling 0.9.9 with or without that flag should also work,
> since libv4l2 use is now default disabled at runtime)
> If that doesn't fix anything, I'll try to investigate further.
> Cheers,
> --
> Antoine Cellerier
> dionoea
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