Simple AV pipeline stuck in prerolling state (mp4/h264/aac)

[Mandeep Sandhu]

Hi All,

I have pretty basic question. I'm trying to create a simple pipeline
for playing an mp4 (h264/aac) file as follows:

$ gst-launch-0.10 filesrc location=/path/to/video.mp4 ! qtdemux
name=qtd ! ffdec_h264 ! autovideosink async-handling=true qtd. !
ffdec_aac ! autoaudiosink

Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...

This opens up a window to show the first frame of the video and
remains paused there. If the audio sink is removed from the pipeline,
then the video plays fine.

I know I can use other bins like playbin/decodebin2 to play the file,
but I'm trying out individual elements for my own understanding's
sake.

I understand that PREROLLING means that the pipeline is 'preparing'
itself for processing the data. So what is stopping the audio sink
from getting it (I presume thats the element causing the pipeline
stall)?

I have read on some forums that setting "async-handling=true" in the
video sync will fix this problem. However that did not help (or maybe
I'm not setting it at the right place).

Also, this is slightly unrelated, the syntax for setting up a pipeline
(with individual elements) for audio AND video seems a little weird.
The way to do it, as I understand from other people's examples, is to
give a name to the demux element and then specify that name in the
video sink with a dot suffix and then construct the audio pipeline
from there. Is this how it's supposed to be done? Is there a doc which
explains this more clearly?

Thanks for your time.

Regards,
-mandeep
_______________________________________________
gstreamer-devel mailing list
[hidden email]
http://lists.freedesktop.org/mailman/listinfo/gstreamer-devel

[Tim-Philipp Müller-2]

On Mon, 2012-11-19 at 15:47 +0530, Mandeep Sandhu wrote:
Hi,

>
> I have pretty basic question. I'm trying to create a simple pipeline
> for playing an mp4 (h264/aac) file as follows:
>
> $ gst-launch-0.10 filesrc location=/path/to/video.mp4 ! qtdemux
> name=qtd ! ffdec_h264 ! autovideosink async-handling=true qtd. !
> ffdec_aac ! autoaudiosink
>
> Setting pipeline to PAUSED ...
> Pipeline is PREROLLING ...

You need queues for each branch after qtdemux (decodebin will plug a
multiqueue; note that the default size of 'queue' may be too small).

The reason for this is the preroll mechanism. Sinks will block once they
have received a buffer. The entire pipeline will only preroll / go to
PAUSED state once all sinks have a buffer prerolled. If you don't have
queues, the demuxer will push a packet out on one pad, the decoder will
output a packet, the sink will preroll and block. This will block the
demuxer thread, since there's no queue, so the demuxer can never push a
buffer to the other branch, so that sink will never preroll.

Cheers
 -Tim

> This opens up a window to show the first frame of the video and
> remains paused there. If the audio sink is removed from the pipeline,
> then the video plays fine.
>
> I know I can use other bins like playbin/decodebin2 to play the file,
> but I'm trying out individual elements for my own understanding's
> sake.
>
> I understand that PREROLLING means that the pipeline is 'preparing'
> itself for processing the data. So what is stopping the audio sink
> from getting it (I presume thats the element causing the pipeline
> stall)?
>
> I have read on some forums that setting "async-handling=true" in the
> video sync will fix this problem. However that did not help (or maybe
> I'm not setting it at the right place).
>
> Also, this is slightly unrelated, the syntax for setting up a pipeline
> (with individual elements) for audio AND video seems a little weird.
> The way to do it, as I understand from other people's examples, is to
> give a name to the demux element and then specify that name in the
> video sink with a dot suffix and then construct the audio pipeline
> from there. Is this how it's supposed to be done? Is there a doc which
> explains this more clearly?
>
> Thanks for your time.

_______________________________________________
gstreamer-devel mailing list
[hidden email]
http://lists.freedesktop.org/mailman/listinfo/gstreamer-devel

[Mandeep Sandhu]

>
> You need queues for each branch after qtdemux (decodebin will plug a
> multiqueue; note that the default size of 'queue' may be too small).
>
> The reason for this is the preroll mechanism. Sinks will block once they
> have received a buffer. The entire pipeline will only preroll / go to
> PAUSED state once all sinks have a buffer prerolled. If you don't have
> queues, the demuxer will push a packet out on one pad, the decoder will
> output a packet, the sink will preroll and block. This will block the
> demuxer thread, since there's no queue, so the demuxer can never push a
> buffer to the other branch, so that sink will never preroll.

Thanks for the explanation Tim. That indeed was the problem!

Adding queues after the demux fixed the issue:

$ gst-launch-0.10 filesrc location=/path/to/video.mp4 ! qtdemux
name=qtd ! queue2 ! ffdec_h264 ! autovideosink qtd. ! queue2 !
ffdec_aac ! autoaudiosink

Thanks again,
-mandeep
_______________________________________________
gstreamer-devel mailing list
[hidden email]
http://lists.freedesktop.org/mailman/listinfo/gstreamer-devel