[gst-devel] Re: [patch] gstreamer v4l2 source frame rate detection

Edgard Lima edgard.lima at indt.org.br
Thu May 4 13:42:06 CEST 2006

BTW: in the meantime, to use your webcam with v4l2src use 
use-fixed-fps=false, I hope it works.


ext Edgard Lima wrote:

> Hi Martin,
> Thanks. I'm coding v4l2src right now. I ll put credits for you when 
> commiting it.
> Ive have a tv-card here, but a webcam, so we could test together :)
> BR,
> Edgard - alima
> edgard.lima at indt.org.br
> ext Martin_Rubli at logitech.com wrote:
>> Hi Edgard,
>> I've been working with the author of the Linux UVC driver
>> (http://linux-uvc.berlios.de/) on a few issues and when we did some 
>> tests
>> with the GStreamer v4l2 source we discovered a problem with the frame 
>> rate
>> detection.
>> The gst_v4l2src_get_fps function relies on the driver's video standard
>> announcements, something that's not usually available in webcams (and it
>> certainly isn't in the UVC driver). If the norm cannot be detected, it
>> bails out and capturing fails. Instead, VIDIOC_G_PARM should be used in
>> such cases.
>> Something else that stroke me as a little weird was the fact that the 
>> frame
>> rate gets queried before every frame (actually twice, once in 
>> 'buffer_new'
>> and once in 'create'). In the original case where the frame rate came 
>> from
>> a cached list of video standards, the overhead is minimal, yet present.
>> When the frame rate comes from a G_PARM ioctl request, the overhead is
>> somewhat higher. I'm not aware of any devices that dynamically change 
>> the
>> frame rate, so I think there's no point. If such devices do exist, 
>> please
>> correct me! :-)
>> The first version of the patch only adds G_PARM support to the get_fps
>> function. The second one is a superset of the first one and caches 
>> the fps
>> numerator and denominator in the _GstV4l2Src structure after querying 
>> it in
>> the capture_init method to avoid repeated ioctls. The changes allow UVC
>> devices to work with GStreamer.
>> Unfortunately, I don't have a TV card here that I could test it with, 
>> but I
>> think the patch shouldn't break anything. Please let me know, if you 
>> have
>> any questions/suggestions.
>> Cheers,
>> Martin
>> (See attached file: 01-webcam_framerate.patch)(See attached file:
>> 02-webcam_framerate.patch)
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