[FFmpeg-devel] Global variables in FFmpeg command expressions?

[Paul B Mahol]

On Mon, May 15, 2023 at 8:45 PM Joan Lluch <joan.lluch at icloud.com> wrote:

> Hi all,
>
> I recently made a feature request to one of the community members. I was
> ready to pay a reasonable amount for it, but as it was a relatively quick
> fix, he was kind enough to implement it on his own, and I made a donation
> to the community instead. The feature I refer to is the “keep” parameter on
> the mpdecimate filter, which was implemented by Thilo Borgmann.
>
> I am a retired software developer and although I embraced some really
> complex projects in the past (including occasional code contributions to
> the LLVM compiler project) I am not currently in the disposition to do it
> anymore due to several reasons mostly related to age.
>
> So, I just joined this mailing list because I want to suggest a further
> improvement to FFmpeg, or otherwise ask for workarounds.
>
> The problem I want to solve is to trim duplicated frames of a video clip,
> in the same exact way that the mpdecimate does, but preserving the
> corresponding audio sections of the preserved video frames in correct sync
> on the output video. Or in other words, trim duplicated frames from a video
> input, while also trimming the corresponding audio, in sync.
>
> To accomplish this, I attempted commands such as this one:
>
> ffmpeg -i $INPUT -vf "select='if( gt(scene,0.00001), (st(1,t);st(2,1)),
> if(lte(t-ld(1),${KEEP}),st(2,1),st(2,0)) )', setpts=N/FRAME_RATE/TB" -af
> "aselect='ld(2)', asetpts=N/SR/TB”  $OUTPUT
>
> The trick here is (or would be) to use variable number 2 from the video
> section to tell the audio section which frames to select (based on what the
> video section is selecting).  However, this won't work because ‘variable’
> number 2 from the first ‘select’ expression is out of scope on the
> ‘aselect’ expression. Therefore it always evaluates to 0 in the ‘aselect’
> expression and the output goes with no audio.
>
> I think that having a way to specify ‘global’ variables, with valid scope
> through the entire ffmpeg command, featuring 'store global' and 'load
> global’ semantics, would be very helpful and would enormously increase the
> use cases for this library. So it would be nice if somebody would look at
> implementing it. I understand this is far from straightforward, but It
> would be game changing if done in my opinion.
>
> As said, my current use case is trimming the audio stream in sync with the
> video stream as described above. So alternatively, I would appreciate any
> hints on how to accomplish this if this is /already/ possible.
>

That approach is prone to many errors and makes it useless if frame
threading is added to libavfilter.
Instead filter that takes both A and V stream and outputs A and V stream is
more solid approach.


>
> Thanks
>
> John LLuch
>
>
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