h264 synchronization

[Andressio]

Dear allI have a camera which emits a h264 stream via rtsp.This stream
follows two paths: the firts stores the stream as raw h264 (this in handled
by an external application) and the second one performs some real-time video
analysis operations. The second path generates some events (like moving
objecs) which should be stored in a database and later displayed in onverlay
on the recorded videos coming from path one.As the recording and the video
analysis might be sligthly shifted in time (let's say that a random number
of frames are dropped before the external application starts recording) I
cannot rely on the frame number. Also I would like to avoid to rely on a 
NTP to sync the timestamps because even a difference of a bounch of
milliseconds might lead to a bad user experience in terms of ovelayed info.
I am not that expert in h264 and since now I've used opencv to decode
frames, but I've read that h264 streams are packed into NAL unit. Are these
units numbered in some way? and if so, can  these info be extracted using
gstreamer? If a counter was naturally embedded in the h264 stream and it
could be reached I think that would be the best synchronization
option.Before I start editing opencv's source code or change the video
analysis application to use pure gstreamer I need to know if this can be
achieved in some way.Thanks in advance for your help!



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