[FFmpeg-devel] A few filter questions

Clément Bœsch u at pkh.me
Thu Jul 17 17:24:35 CEST 2014

On Thu, Jul 17, 2014 at 04:56:08PM +0200, Gerion Entrup wrote:
> > Also, you still have the string metadata possibility (git grep SET_META
> > libavfilter).
> Hmm, thank you, I will take a look at it. If I see it right, it is used to fill 
> a dictionary per frame with some kind of data?

Strings only, so you'll have to find a serialization somehow. Maybe simply
an ascii hex string or something. But yeah, it just allows you to map some
key → value string couples to the frames passing by in the filter.

How huge is the information to store per frame?

> > stdout/stderr really isn't a good thing. Using metadata is way better
> > because you can output them from ffprobe, and parse them according to
> > various outputs (XML, CSV, JSON, ...).
> Sounds good…

tools/normalize.py make use of such feature if you want examples (that's
the -of option of ffprobe)

> > Am I understanding right your wondering?
> No ;), but anyway thanks for your answer. In your 2nd method your filter is a 
> VV->V filter? Am I right, that this filter then also can take only one stream? 
> Said in another way: Can a VV->V filter also behave as a V->V filter?

Yes, fieldmatch is a (complex) example of this. But typically it's simply
a filter with dynamic inputs, based on the user input. The simplest
example would be the split filter. Look at it for an example of dynamic
allocation of the number of inputs based on the user input (-vf split=4 is
a V->VVVV filter)

> > Check tools/normalize.py, it's using ebur128 and the metadata system.
> Thats what I mean. Someone has to write an external script, which calls 
> ffmpeg/ffprobe two times, parse stdout of the first call and pass it to the 
> filteroptions of the second call. As I see, there is no direct way. Something 
> like:
> ffmpeg -i foo -f:a volume=mode=autodetect normalized.opus

We add a discussion several time for real time with that filter. If we do
a 2-pass, that's simply because it's "more efficient". Typically, doing
some live normalization can be done easily (we had patches for this):
ebur128 already attaches some metadata to frames, so a following filter
such as volume could reuse them, something like -filter_complex


Clément B.
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