[gst-devel] V4L2 real timestamps

[Wim Taymans]

On Wed, 2010-02-17 at 10:07 -0500, Daniel Crews wrote:
> Short version: I need some advice on the best way to timestamp frames
> of video coming from a v4l2 web cam.
> 
> Long Version: I'm trying to gather real time data from web cams on
> multiple computers, and gstreamer seems the best way to do it. Get the
> image stream (v4l2src), process it with a custom plug-in for feature
> detection, and then dump the info out through a gobject bus to a gtk
> app for collection. However, I need to have time information for each
> frame on something like the order of millisecond precision so that the
> data for each frame can be fit into a time line and be compared from
> computer to computer. As far as I can tell from the documentation, the
> buffer timestamps in gstreamer seem to be meant for syncing video and
> audio rather than true time stamping, and I'm not sure if the v4l2src
> makes use of even that....

If you distribute the same clock to multiple machines (or have the
systemclock synched with ntp or something) and you distribute the same
start_time (gst_element_set_start_time()) on the pipelines, you can
compare the timestamps of v4l2src between machines.

wim

> 
> The Theory: So far as I can figure my best bet would be to write a
> custom v4l2src that time stamps the frames with a precision clock of
> some kind in C, and then pass that meta-data out the gobject bus with
> the blob data. The precision clocks would be difficult to truly sync
> to the same time, but I could just store offsets from a known common
> event to sync the time data from different computers at the central
> collector instead. But I've no idea what I'm doing here and may well
> be horribly mistaken. I would appreciate any guidance I can get if
> anyone would like to help me.
> --
> 
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