Re: [FFmpeg-devel] [PATCH] swr/swresample: avoid reapplication of firstpts

[Gyan Doshi <ffmpeg@gyani.pro>]

On 2023-12-15 12:43 am, Michael Niedermayer wrote:
> On Tue, Dec 12, 2023 at 04:50:08PM +0530, Gyan Doshi wrote:
>> During a resampling operation where
>>
>> 1) user has specified first_pts
>> 2) SWR_FLAG_RESAMPLE is not set initially (directly or otherwise)
>> 3) first_pts has been fulfilled (always using hard compensation)
>>
>> then upon first encountering a delay where a soft compensation is
>> required, swr_set_compensation will lead to another init of swr which
>> will reset outpts to the specified firstpts thus leading to an output
>> frame having its pts = firstpts. When the next input frame is received,
>> swr will see a large delay and inject silence from firstpts to the
>> current frame's pts. This can lead to severe desync and in worst case,
>> loss of audio playback.
>>
>> Parameter initcount added to keep track of swr_init calls of the swrcontext
>> and to ultimately avoid enforcing firstpts again.
>>
>> Fixes #4131.
>> ---
>>   libswresample/swresample.c          | 7 +++++--
>>   libswresample/swresample_internal.h | 1 +
>>   libswresample/version.h             | 2 +-
>>   3 files changed, 7 insertions(+), 3 deletions(-)
> Would it work to instead set firstpts = AV_NOPTS_VALUE on allocation
> and then use firstpts == AV_NOPTS_VALUE to detect if it has been set
> already ?
> If that workes, that seems simpler than counting init

That does work.  I avoided it since I didn't know if I needed to make 
swr_alloc2() or change in place.

v2 sent.

Regards,
Gyan

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