[FFmpeg-devel] [RFC] v4l2 frame rates

Stefano Sabatini stefano.sabatini-lala
Tue Jan 6 23:16:14 CET 2009

On date Tuesday 2009-01-06 12:21:35 +0000, Luca Abeni encoded:
> Stefano Sabatini wrote:
> [...]
> >> I am still unsure if this is the best solution (I think it is an
> >> improvement respect to the current behaviour, but maybe we can do
> >> something better...), hence the RFC (yes, I already considered the idea
> >> of querying the video standard to obtain the correct frame rate... But
> >> this does not work for some webcams).
> > 
> > Can you elaborate on this, what does it mean that it doesn't work
> Basically, with some webcams the ioctl() for getting the list of 
> supported video standards fails... And this (AFAIK) makes it impossible 
> to know the video frame rate (the only way I know to get this 
> information is to read it in the structure describing the video standard).
> > is it a bug of the module/hardware?
> I do not know... Maybe v4l2 allows the driver to fail on some ioctl()s 
> if they are not supported? Anyway, I did not check.
> > Is it possible to detect somehow when they don't work correctly?
> Well, detecting this problem is easy, as the ioctl() fails. But the v4l2 
> input must work even if we cannot get the list of supported video 
> standards...

Yes, it could try to set some meaningful default value.
> Anyway, support for automatically detecting the frame rate should come 
> in a different patch.

OK, and thanks for working on this.

FFmpeg = Faboulous and Fierce Mystic Puristic Extroverse Gymnast

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