Time-shift buffering of a live video source

Thu Nov 9 19:13:01 UTC 2017

Hello,

Theoretically the queue2 element has what you are looking for and it has 
a ring buffer property.  However, at least from my own personal 
experimentation you can't seek on the data in the ring buffer, even if 
it is muxed video.  uridecodebin also has a ring buffer property, and it 
uses queue2, however it disables this functionality on streaming 
sources, so I am not exactly sure what the purpose of the ring buffer is 
actually for.  There is the timeshift element:

https://github.com/kkonopko/gst-fluendo-timeshift

But it may be deprecated for queue2.

You may be better served recording to a streamable mkv file and using 
another pipeline that reads from it.  You could jump to live playback by 
using the current time of the recording pipeline, because you wont know 
the duration otherwise.  In my experience this works fairly well, and 
may achieve the same effect you are trying to accomplish.

Hope that helps!

Cheers,
Michael.

On 11/6/2017 2:36 PM, Fritjof Büttner wrote:
> Hello,
>
> I am quite new to Gstreamer and it seems like an amazing tool to me.
> I would like to create a pipeline that does the following:
> - get a video stream from a live source (i.e., webcam)
> - encode the live stream as is into a file
> - at the same time, show the live stream in a user interface, however 
> the user is able to seek to abitrary positions in the entire stream.
> I know that I need a tee element for the two simultaneous tasks, but 
> currently I am having trouble realizing the time-shift buffering part 
> between the source and the videosink. I thought I could add a queue 
> element before the sink to act as a (ring-)buffer to enable seeking.
>
> This is my experimental pipeline so far (live videotestsrc instead of 
> a webcam):
>
> gst-launch-1.0 videotestsrc do-timestamp=true is-live=true name=src ! 
> video/x-raw, format=RGB, width=640, height=480, framerate=20/1 ! 
> videoconvert ! timeoverlay !  queue max-size-buffers=0 
> max-size-bytes=0 max-size-time=0 name=q ! autovideosink
>
> While this pipeline is running, I can send seek_simple signals to the 
> queue, which results in the frame of the requested timestamp being 
> shown (verified by the timeoverlay). But then the output pauses and 
> will not resume playing (pipeline state remains PLAYING). Weirdly, 
> when I query the current-level-* properties of the queue, it seems to 
> be empty. In that case I don't understand where the seeked frame comes 
> from. Does the pipeline itself also buffer data?
> At one point during my experiments, I saw the queue actually filling 
> up quickly (I don't recall the parameters I was using then), using 
> some GB or memory after only a few seconds.
> I found the time-shift buffering 
> <https://gstreamer.freedesktop.org/documentation/application-development/advanced/buffering.html#timeshift-buffering> 
> scenario in the documentation which I think describes best what I am 
> trying to do, but it's only a short paragraph and there is no 
> dedicated "file-ringbuffer" element, so I tried my luck with the queue.
>
> Is it possible to achieve my goal using Gstreamer? If so, which 
> elements should I look at? And is there a way to get the total 
> duration of a live pipeline? All query_duration calls return -1. 
> Lastly, is there a way to buffer minutes worth of live video (e.g. 
> 720p) in memory?
>
> I'm curious to hear your thoughts! Many thanks in advance,
> Fritjof
>
> --
>
> This is my experiment code in Python:
> GObject.threads_init()
> Gst.init(None)
>
> pipeline_string=  "videotestsrc do-timestamp=true is-live=true horizontal-speed=1 name=src "  \
>                    "! video/x-raw, format=RGB, width=640, height=480, framerate=20/1 "  \
>                    "! videoconvert "  \
>                    "! timeoverlay shaded-background=true "  \
>                    "! queue max-size-buffers=0 max-size-bytes=0 max-size-time=0 name=q "  \
>                    "! autovideosink"
>
> pipeline=  Gst.parse_launch(pipeline_string)
> source=  pipeline.get_by_name("src")
> queue=  pipeline.get_by_name("q")
>
> print(pipeline.set_state(Gst.State.PLAYING))
>
> time.sleep(5)
> print("Current source position:",  source.query_position(Gst.Format.TIME))
> print("Current queue position:",  queue.query_position(Gst.Format.TIME))
> print("Current pipeline position:",  pipeline.query_position(Gst.Format.TIME))
>
> seek=  pipeline.query_position(Gst.Format.TIME)[1]  -  3  *  Gst.SECOND
> print("seek to -3 sec:",  seek)
> if  queue.seek_simple(Gst.Format.TIME,  Gst.SeekFlags.FLUSH|  Gst.SeekFlags.KEY_UNIT,  seek):
>      print("Seek OK")
>
> print("Current source position:",  source.query_position(Gst.Format.TIME))
> print("Current queue position:",  queue.query_position(Gst.Format.TIME))
> print("Current pipeline position:",  pipeline.query_position(Gst.Format.TIME))
> print("Queue buffer level:",  queue.get_property("current-level-buffers"),
>        "- bytes:",  humanize.naturalsize(queue.get_property("current-level-bytes"),  binary=True),
>        "- time:",  queue.get_property("current-level-time")  /  Gst.SECOND)
>
> time.sleep(2)
> seek=  source.query_position(Gst.Format.TIME)[1]
> print("Current source position:",  source.query_position(Gst.Format.TIME))
> print("jump to front",  seek)
> queue.seek_simple(Gst.Format.TIME,  Gst.SeekFlags.FLUSH|  Gst.SeekFlags.KEY_UNIT,  seek)
>
> These are the print outputs:
>
> Current source position: (True, cur=5026269432)
> Current queue position: (True, cur=5026269432)
> Current pipeline position: (True, cur=4950176384)
> seek to -3 sec: 1950209076
> Seek OK
> Current source position: (True, cur=1950209076)
> Current queue position: (True, cur=1950209076)
> Current pipeline position: (False, cur=0)
> Queue buffer level: 0 - bytes: 0 Bytes - time: 0.0
> Current source position: (True, cur=2126269432)
> jump to front 2126269432
>
>
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