[FFmpeg-devel] [RFC] Dynamic filtergraph reconfiguration

Paul B Mahol onemda at gmail.com
Mon Sep 16 13:25:53 CEST 2013

On 10/24/12, Stefano Sabatini <stefasab at gmail.com> wrote:
> Hi,
> maybe it's time to finally tackle this, since it seems to be a
> recurring issue.
> Sometimes the input of the filterchain changes configuration
> parameters. For audio it means it could change channel layout/sample
> rate/sample format/timebase, for video it may change frame rate, size,
> pixel format, sample aspect ratio and timebase.
> Basically most filters assume these values to be invariant during the
> whole life of the filtergraph, others (e.g. the scale filter) monitors
> *some* of the input values and reconfigure themselves accordingly,
> sometimes calling configure on the following filters (which may or may
> not be correct). We need to make reconfiguration consistently applied
> at the framework level, and properly design it, in order to avoid
> brittle code and glitches.
> Problems to tackle are:
> - not all filters may support reconfigurations, for example some
>   filters rely on some specific values defined during configuration,
>   so in general we need a mechanism which allows to reconfigure only
>   the filters which can do that
> - some filters may support only *one* parameter change (e.g. timebase
>   may easily change).
> - since we don't mark codecs/containers which support variable
>   configuration parameters (do we?), we may need to "normalize" the
>   output before the terminal sink (depending on the application).
> Related questions: where should we check the changes (possibly in the
> framework code)? Where should we add the "buffer" filters which
> normalize input towards a "non-reconfigurable" filter (possibly we
> should auto-insert it only when required).
> Filter supporting reconfiguration should be marked as such (or
> alternatively we may mark filters *not* supporting it). Also I wonder
> if we should also support filters where the *number* of inputs or
> outputs change dinamically (e.g. a movie source which adds a new
> stream).
> I'll try to come with a more detailed design and proof of concept once
> we'll clear some of the design issues.
> --
> FFmpeg = Frightening and Faithful Murdering Pacific Explosive Guru
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Any updates?

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