webrtcbin fake-live filesrc and seeking

[Marianna Smidth Buschle]

On 19.04.2022 08.20, gstreamer-devel-request at lists.freedesktop.org wrote:
> Hello mailing list members,
>
> I am still working on a "media (file) player over WebRTC": A file is
> decoded into audio/video and transmitted over webrtcbin.
> As WebRTCBin requires (AFAIK) live input, I use the usual tricks to
> convert multifilesrc into a live source.
>
> My pipeline roughly looks like:
>
> multifilesrc do-timestamp=1 ! (..queues, decoders, encoders) !
> queue ! tee ! queue ! fakesink
>                \- ! queue ! webrtcbin
>
> (not sure if the ASCII art survives the mail encoding, I basically
> have a fakesink as default sink linked to a tee, so that the media is played
> even when no consumers are connected)
>
> I am experimenting with "identity sync=true" at various positions,
> currently I plan to have it in the audio leg only, but all that will be
> finalized later.

You can make your fakesink with sync=true which should give you a 
similar behavior than to "identity sync=true", at least on that branch 
of the 'tee'.

Otherwise you should also be able to use 'clocksync' instead of 
"identity sync=true", which should also do the same.
This is at least how I handle 'filesrc's.

|filesrc name=replay location=/home/msb/test.ts ! decodebin ! identity 
sync=true ! videoconvert ! ximagesink sync=0|

Note that both "identity sync=true" and 'clocksync' only seem to work on 
demuxed streams (which makes good sense).

> My problem is, that I cannot seek while playback happens. Obviously,
> a real live source does not support seek, so its not too surprising.
> Anyways, I like to mimic the behaviour you would get when feeding
> a VCR/DVD player into video in which is grabbed as a live source then.
>
> I have no clue why seek actually fails. I could imagine, its because of
> "identity sync=true" elements, but I tested without them, and its still
> not working. Maybe webrtcbin does not support seeking at all?

In my experience stream duration and seek don't seem to work with 
|multifilesrc|.
They seem to work with |filesrc| as long as the data is demuxed.

  * Seek and duration/position queries work (also with |sync=0|)

|filesrc name=replay location=/home/msb/test.ts ! decodebin ! identity 
sync=true ! videoconvert ! ximagesink sync=1|

  * duration/position queries DON'T work

|filesrc name=replay location=/home/msb/test.ts ! fakesink|

  * duration/position queries work

|filesrc name=replay location=/home/msb/test.ts ! tsdemux ! fakesink|

  * duration/position queries DON'T work

|multifilesrc name=replay location=/home/msb/test.ts ! tsdemux ! fakesink|

  * Seek and duration query DON'T work (position query works, but
    position increases for each loop)

|multifilesrc name=replay location=/home/msb/test.ts ! decodebin ! 
identity sync=true ! videoconvert ! ximagesink sync=0 |

> If I send the seek to the pipe (or, in most other cases) it returns false.
> (Which is somewhat confusing for me, as the documentation states
> "In a completely prerolled PAUSED or PLAYING pipeline, seeking is always
> guaranteed to return TRUE on a seekable media")
>
> In case I send the seek to the filesrc directly (which I assume to be wrong),
> it returns true, but the pipeline simply stops.
>
> I tried to read about pipeline states, tried diffferent seek modes,
> but I couldnt find anything that would explain my issue or change behaviour.
>
> Any help appreciated! Is there any specific issue or underlying concept
> I need to understand?
>
> Thanks!
> Philipp

-- 
Best regards / Med venlig hilsen
“Marianna Smidth Buschle”
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