[Libav-user] Is it possible to dynamically reconfigure a filter graph?

Michael Bradshaw mbradshaw at sorensonmedia.com
Tue Oct 23 22:45:53 CEST 2012

On Tue, Oct 23, 2012 at 2:38 PM, Stefano Sabatini <stefasab at gmail.com> wrote:
> On date Tuesday 2012-10-23 13:03:27 -0600, Michael Bradshaw encoded:
>> I'm using avfilter to rescale video (and convert it to the right pixel
>> format) and convert audio (to the right sample format, sample rate,
>> and channel_layout) (I'm also buffering audio using asetnsamples to
>> fill frames with a consistent number of samples). While reading the
>> source video and source audio, it's possible they change in the
>> stream.
>> If the source audio stream changes (i.e. sample rate, sample
>> format, channel_layout), or if the source video stream changes (width,
>> height, or pixel format), do I have to completely destroy and rebuild
>> the filter graph? Or is there a way to "update" the source filters to
>> the new formats and update (without destroying) the filter graph?
> Some filters do, others won't, so in general the best way is to
> "normalize" the input media before injecting it in the filtergraph, a
> consistent and robust way to deal with that will require some
> (re)design at the framework level.

How would you suggest to "normalize" the input media? Because that's
exactly what I'm (ab)using libavfilter for: given "unormalized" source
media, produce "normalized" output with consistent picture size/format
and audio format. If there's currently a preferred or better solution
to do this, I'd love to know.

Right now I'm thinking of abandoning libavfilter and just using
swscale and swresample directly, and dynamically reconfiguring the
swscale and swresample contexts as necessary, but I'm exploring

>> My primary concern is that asetnsamples may have some samples
>> buffered, and destroying and recreating the filter graph would lose
>> these buffered samples.
>> Also, if it's possible to update the filters without destroying the
>> graph and recreating it, would it be possible to change the number of
>> samples set for asetnsamples? I'm not seeing anything that makes it
>> look like this is possible, but it's worth double checking.
> This should be easily feasible by implementing a process_command()
> callback in asetnsamples, also have a look at the asendcmd filter,
> patches (or alternatively a feature request) are welcome.

Cool. I guess this can be added to the "On libavfilter: A summary of
issues" thread. I'll work on it if I have time, though I doubt I will
for a while.



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