[FFmpeg-devel] [RFC] libavfilter-soc and variable-frame-size video streams

Stefano Sabatini stefano.sabatini-lala
Sat Dec 13 16:26:54 CET 2008

On date Saturday 2008-12-13 14:23:40 +0100, Michael Niedermayer encoded:
> On Fri, Dec 12, 2008 at 04:54:26PM -0800, Jason Garrett-Glaser wrote:
> > > But maybe I'm just confused and there is a far simpler solution, or
> > > maybe we could just decide that supporting variable-frame-size video
> > > streams is not worth that hassle.
> > 
> > This sounds reasonable to me: thinking through what every filter would
> > have to do to support such a thing, it sounds completely
> > unmaintainable, especially for filters that act temporally: how will
> > they deal with the fact that cached temporal data from previous frames
> > (such as for temporal denoisers) is no longer the correct size
> > matching the current frame?!
> > 
> > Avisynth gets away just fine without variable frame size support: I
> > have actually never seen such a thing as a feature request either.  I
> > don't think libavfilter needs it either.
> I think variable size is pretty much a requirement for libavfilter.
> I dont remember exactly how/if this was discussed during lavfi development
> but it surly is not ok to drop it. Also lack of support of it in Avisynth
> is not an argument not to support it in lavfi.
> About individual filters, variable size support is surely optional and
> that way its no burden on a per filter level.
> Filters that are "stateless" should be able to very easily support it,
> the scale filter has to support it for the obvious reason to remove
> size changes at the begin of a filter chain when that is wanted.
> and temporal filters may or may not be able to easily support it, after
> all they dont have to do much more than free their stuff and pretend its
> the start of a stream.

I'm thinking about something like avfilter_config_output_links(),
which should work in a way very similar to avfilter_config_links, and
maybe will share some code with it.

It may work like this: when the request_frame callback of a filter
recognize that the frame to output changed its size, it will call
avfilter_config_output_links(), which will run all the config_props of
the *input* pads of the following filters in the chain.

FFmpeg = Foolish and Furious Magnificient Pitiful Egregious Guru

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