Re: [FFmpeg-devel] [PATCH v2 2/2] ffmpeg: add option -isync

[Anton Khirnov <anton@khirnov.net>]

Quoting Gyan Doshi (2022-07-05 19:10:33)
> 
> 
> On 2022-07-05 09:45 pm, Anton Khirnov wrote:
> > Quoting Gyan Doshi (2022-07-04 10:20:22)
> >>
> >> On 2022-07-04 11:51 am, Anton Khirnov wrote:
> >>> Quoting Gyan Doshi (2022-07-02 11:51:53)
> >>>> On 2022-07-02 02:12 pm, Anton Khirnov wrote:
> >>>>> Quoting Gyan Doshi (2022-07-01 13:03:04)
> >>>>>> On 2022-07-01 03:33 pm, Anton Khirnov wrote:
> >>>>>>> Quoting Gyan Doshi (2022-06-25 10:29:51)
> >>>>>>>> This is a per-file input option that adjusts an input's timestamps
> >>>>>>>> with reference to another input, so that emitted packet timestamps
> >>>>>>>> account for the difference between the start times of the two inputs.
> >>>>>>>>
> >>>>>>>> Typical use case is to sync two or more live inputs such as from capture
> >>>>>>>> devices. Both the target and reference input source timestamps should be
> >>>>>>>> based on the same clock source.
> >>>>>>> If both streams are using the same clock, then why is any extra
> >>>>>>> synchronization needed?
> >>>>>> Because ffmpeg.c normalizes timestamps by default. We can keep
> >>>>>> timestamps using -copyts, but these inputs are usually preprocessed
> >>>>>> using single-input filters which won't have access to the reference
> >>>>>> inputs,
> >>>>> No idea what you mean by "reference inputs" here.
> >>>> The reference input is the one the target is being synced against. e.g.
> >>>> in a karaoke session -  the music track from a DAW would be ref and the
> >>>> user's voice via mic is the target.
> >>>>
> >>>>>> or the merge filters like e.g. amix don't sync by timestamp.
> >>>>> amix does seem to look at timestamps.
> >>>> amix does not *sync* by timestamp. If one input starts at 4 and the
> >>>> other at 7, the 2nd isn't aligned by timestamp.
> >>> So maybe it should?
> >>>
> >>> My concern generally with this patchset is that it seems like you're
> >>> changing things where it's easier to do rather than where it's correct.
> >> There are many multi=input filters which may be used. amix is just one
> >> example.
> >>
> >> The basic 'deficiency' here is that filters operate upon frames and only
> >> look at single frames for the most part, even though frames are part of
> >> streams. These streams may have companion streams (which may be part of
> >> programs) which are part of a single input. These inputs may have
> >> companion inputs.  Anything in this tree may be relevant for a
> >> particular operation as a reference, e.g. we have a bespoke filter
> >> scale2ref so that we can look at another stream's frames. But we don't
> >> have pad2ref, crop2ref ..etc.  So, the absolutely correct thing to do
> >> would be to supply a global context to processing modules like
> >> filtergraphs , maybe an array of dicts, containing attributes of all
> >> inputs like starting time stamps, resolution, string metadata..etc. That
> >> would obviate need for these bespoke fields and even filters.
> > I don't see how the second paragraph relates to the first one. scale,
> > pad, or crop are not multi-input filters, so why are you comparing them
> 
> scale is a singe-input filter but scale2ref is a multi-input filter 
> which is needed solely because there is no means at present to convey 
> info about other streams to a single input filter.
> Similarly, we would need a crop2ref, pad2ref..etc to achieve the same 
> attribute transfer.  If we had a global context, these counterpart 
> filters wouldn't be necessary.

In my experience, global *anything* is almost always a sign of bad
design and only leads to pain and suffering. The proper solution in this
case would be making the filtergraph construction API more flexible.
Then the code that actually has all the necessary information (i.e.
ffmpeg.c or other library caller) would set the filter parameters
however you want. Then none of these whatever2ref hacks would be needed.

-- 
Anton Khirnov
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