Re: [FFmpeg-devel] [PATCH v3] ffmpeg: add option -isync

[Gyan Doshi <ffmpeg@gyani.pro>]

On 2022-07-14 12:16 pm, Anton Khirnov wrote:
> Quoting Gyan Doshi (2022-07-10 20:02:38)
>>
>> On 2022-07-10 10:46 pm, Anton Khirnov wrote:
>>> Quoting Gyan Doshi (2022-07-08 05:56:21)
>>>> On 2022-07-07 03:11 pm, Anton Khirnov wrote:
>>>>> Quoting Gyan Doshi (2022-07-04 18:29:12)
>>>>>> 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 not all inputs have timestamps, the wallclock times at the time of
>>>>>> reception of inputs shall be used. FFmpeg must have been compiled with
>>>>>> thread support for this last case.
>>>>> I'm wondering if simply using the other input's InputFile.ts_offset
>>>>> wouldn't achieve the same effect with much less complexity.
>>>> That's what I initially did. But since the code can also use two other
>>>> sources for start times (start_time_realtime, first_pkt_wallclock),
>>>> those intervals may not exactly match the difference between
>>>> fmctx->start_times so I use a generic calculation.
>>> In what cases is it better to use either of those two other sources?
>>>
>>> As per the commit message, the timestamps of both inputs are supposed to
>>> come from the same clock. Then it seems to me that offsetting each of
>>> those streams by different amounts would break synchronization rather
>>> than improve it.
>> The first preference, when available, stores the epoch time closest to
>> time of capture. That would eliminate some jitter.
>> The 2nd preference is the fmctx->start_time. The 3rd is the reception
>> wallclock. It is a fallback. It will likely lead to the worst sync.
> In which cases is this fallback useful? I would expect that all
> transports you'd want to use this with will have working timestamps.

Agreed. I just kept it as a last resort scenario. Can do without it.

Regards,
Gyan
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