Re: [FFmpeg-devel] [PATCH v3] swr/swresample: avoid reapplication of firstpts

[Gyan Doshi <ffmpeg@gyani.pro>]

On 2023-12-16 03:44 pm, 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 firstpts initialized to AV_NOPTS_VALUE in swr_alloc and then
> checked in swr_init to avoid resetting outpts, thus avoiding reapplication
> of firstpts.
>
> Fixes #4131.
> ---
> Added fate test

Plan to push soon.

Regards,
Gyan

>
>   libswresample/options.c           |  1 +
>   libswresample/swresample.c        |  5 +++--
>   tests/fate/libswresample.mak      |  3 +++
>   tests/ref/fate/swr-async-firstpts | 24 ++++++++++++++++++++++++
>   4 files changed, 31 insertions(+), 2 deletions(-)
>   create mode 100644 tests/ref/fate/swr-async-firstpts
>
> diff --git a/libswresample/options.c b/libswresample/options.c
> index fb109fdbab..d8cf85c053 100644
> --- a/libswresample/options.c
> +++ b/libswresample/options.c
> @@ -171,6 +171,7 @@ av_cold struct SwrContext *swr_alloc(void){
>       if(s){
>           s->av_class= &av_class;
>           av_opt_set_defaults(s);
> +        s->firstpts = AV_NOPTS_VALUE;
>       }
>       return s;
>   }
> diff --git a/libswresample/swresample.c b/libswresample/swresample.c
> index f2a9b40474..1cf83a803f 100644
> --- a/libswresample/swresample.c
> +++ b/libswresample/swresample.c
> @@ -375,8 +375,9 @@ av_cold int swr_init(struct SwrContext *s){
>       if (s->firstpts_in_samples != AV_NOPTS_VALUE) {
>           if (!s->async && s->min_compensation >= FLT_MAX/2)
>               s->async = 1;
> -        s->firstpts =
> -        s->outpts   = s->firstpts_in_samples * s->out_sample_rate;
> +        if (s->firstpts == AV_NOPTS_VALUE)
> +            s->firstpts =
> +            s->outpts   = s->firstpts_in_samples * s->out_sample_rate;
>       } else
>           s->firstpts = AV_NOPTS_VALUE;
>   
> diff --git a/tests/fate/libswresample.mak b/tests/fate/libswresample.mak
> index f2108016af..0d29f76024 100644
> --- a/tests/fate/libswresample.mak
> +++ b/tests/fate/libswresample.mak
> @@ -1082,6 +1082,9 @@ $(call CROSS_TEST,$(SAMPLERATES_LITE),ARESAMPLE_EXACT_LIN_ASYNC,s32p,s32le,s16)
>   $(call CROSS_TEST,$(SAMPLERATES_LITE),ARESAMPLE_EXACT_LIN_ASYNC,fltp,f32le,s16)
>   $(call CROSS_TEST,$(SAMPLERATES_LITE),ARESAMPLE_EXACT_LIN_ASYNC,dblp,f64le,s16)
>   
> +FATE_SWR_RESAMPLE-$(call FILTERDEMDEC, ARESAMPLE ASETPTS ATRIM SINE, , PCM_S16LE, LAVFI_INDEV) += fate-swr-async-firstpts
> +fate-swr-async-firstpts: CMD = framecrc -auto_conversion_filters -copyts -f lavfi -i "sine=r=1000:samples_per_frame=100,asetpts=PTS+S+S*floor(ld(1)/4)+st(1\,ld(1)+1)*0,atrim=end=2" -filter:a aresample=async=300:first_pts=0
> +
>   FATE_SWR_RESAMPLE-$(call FILTERDEMDECENCMUX, ARESAMPLE, WAV, PCM_S16LE, PCM_S16LE, WAV) += $(FATE_SWR_RESAMPLE)
>   fate-swr-resample: $(FATE_SWR_RESAMPLE-yes)
>   FATE_SWR += $(FATE_SWR_RESAMPLE-yes)
> diff --git a/tests/ref/fate/swr-async-firstpts b/tests/ref/fate/swr-async-firstpts
> new file mode 100644
> index 0000000000..3f6b290bab
> --- /dev/null
> +++ b/tests/ref/fate/swr-async-firstpts
> @@ -0,0 +1,24 @@
> +#tb 0: 1/1000
> +#media_type 0: audio
> +#codec_id 0: pcm_s16le
> +#sample_rate 0: 1000
> +#channel_layout_name 0: mono
> +0,          0,          0,      132,      264, 0xc2981f45
> +0,        132,        132,       68,      136, 0xe78e468d
> +0,        200,        200,      100,      200, 0xd55c67d0
> +0,        300,        300,      100,      200, 0xd55c67d0
> +0,        400,        400,      100,      200, 0xd55c67d0
> +0,        500,        500,       93,      186, 0x85ca5db4
> +0,        593,        593,      110,      220, 0xa2655d0b
> +0,        703,        703,      108,      216, 0x95cb6f01
> +0,        811,        811,      108,      216, 0xf35668b8
> +0,        919,        919,      149,      298, 0xc273245f
> +0,       1068,       1068,      136,      272, 0xedeb6e0a
> +0,       1204,       1204,       98,      196, 0xea18668e
> +0,       1302,       1302,       98,      196, 0x412861e7
> +0,       1400,       1400,       98,      196, 0x7ec361b2
> +0,       1498,       1498,      110,      220, 0xf3ae6a6a
> +0,       1608,       1608,      108,      216, 0xab2f6c93
> +0,       1716,       1716,      107,      214, 0x50de6eb9
> +0,       1823,       1823,      106,      212, 0x67b8656d
> +0,       1929,       1929,       18,       36, 0x2b7911c6

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