Calculate the real world time at which a frame was captured?

[Nicolas Dufresne]

Le dim. 4 août 2019 18 h 25, George Hawkins <gstreamer at beta-engine.net> a
écrit :

> On Mon, Jul 29, 2019 at 2:58 PM "pisymbol ." <pisymbol at gmail.com> wrote:
>
>> On Sun, Jul 28, 2019 at 2:10 PM George Hawkins <gstreamer at beta-engine.net>
>> wrote:
>>
>>> If I use the filesrc element to playback a video file I can retrieve
>>> data like this on a per frame basis:
>>>
>>>     index=0, timestamp=832000000, stream-time=0
>>>     index=1, timestamp=870000000, stream-time=38000000
>>>     ...
>>>
>>> But what is the first timestamp above relative to? How can I retrieve a
>>> real-world start time such that I can combine it with this timestamp in
>>> order to calculate the real-world time at which the frame was captured?
>>>
>>> I control the original file capture process as well as the playback but
>>> I haven't found how to capture and recover the start time that I need for
>>> combining with timestamps in this way.
>>>
>>> Currently, I capture the video file like so:
>>>
>>>     gst-launch-1.0 nvarguscamerasrc \
>>>         ! 'video/x-raw(memory:NVMM), width=3280, height=2464,
>>> framerate=21/1' \
>>>         ! nvjpegenc \
>>>         ! matroskamux \
>>>         ! filesink location=out.mkv
>>>
>>> I can change the container and video format if this makes it easier to
>>> encode and recover the start time later. I can obviously get an
>>> _approximate_ start time by recording the time at which the pipeline
>>> started - but I'd prefer something more precise (and _if possible_ I'd
>>> prefer that the value was encoded somewhere in the resulting video file
>>> rather than stored separately).
>>>
>>> I've used GST_DEBUG to see if I could see anything that looked like a
>>> start time when replaying the file but didn't spot anything.
>>>
>>> And if I look at the file with a tool like mediainfo the only date I see
>>> is:
>>>
>>>     Encoded date : UTC 2019-07-24 19:20:42
>>>
>>> TL;DR - when recording my video file how do I capture and later recover
>>> a value that can be combined with a relative timestamp value (like the one
>>> for index 0 up above) to give the real world time at which the frame was
>>> captured.
>>>
>>> Why can't you generate an epoch (UTC) on a per frame basis? That's what
>> I do with 'nvcamerasrc' at least. I read its timestamp it stores from the
>> ISP as well as generate my own epoch when the frame is received in the
>> pipeline (see identity's "handoff" signal).
>>
>> I mean you have to define "start time" precisely here too.
>>
>> -aps
>>
>
> Thanks for the reply Aps - and sorry for being slow in following-up. As it
> seems GStreamer is used in quite a lot of machine vision projects, I
> thought it would be quite a common thing to want to do, to record video and
> then, at a later stage, work with the video and time signals from another
> source.
>
> E.g. one might continuously record video of the night sky and later come
> back and want to view particular frames when some other information source
> tells you that there might have been something interesting to see in-frame
> at e.g. 3:24 AM.
>

TimeCode can be used for that, I believe there is an element to insert wall
time as TimeCode, but I'm no expert.

Last time I needed that, I simply encoded the start time in the filename,
and parse that in my app to offset the playback position.



> So I didn't want to reinvent the wheel. But in the end, I did as you
> suggested and coded up a new element, using identity as a template.
>
> The learning curve for getting started with coding GStreamer elements is
> fairly step - but ultimately, the actual code required to implement what I
> wanted was trivial.
>
> On the off-chance that it might be useful to someone else, the results can
> be found here: https://github.com/george-hawkins/gst-absolutetimestamps
>
> The README is far longer than the few lines of code required to print out
> the timestamps I wanted :)
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