[FFmpeg-devel] [PATCH] avformat/movenc: Fix writing a sample size of 0 for PCM in MP4

["zhilizhao(赵志立)"]

> On Jul 26, 2023, at 00:28, Raphaël Zumer <raphael.zumer at vimeo.com> wrote:
> 
> Encoding PCM in MP4 currently causes subsequent decoding to fail due to a sample size of 0.

This doesn’t give a context on which case the sample size is 0.

> Use bits per coded sample instead, which are set correctly based on my tests and allow muxed files to be decoded as expected.

Since neither bits_per_raw_sample or bits_per_coded_sample has strong guarantee,
I decided to fall back to av_get_exact_bits_per_sample().

http://ffmpeg.org/pipermail/ffmpeg-devel/2023-July/312653.html

> 
> Note: PCM in MP4 muxed with versions of FFmpeg 6.0 and prior (before implementation of the pcmC box) will continue to fail decoding due to the sample size not being available. I see that it was assumed to be 16-bit before commit d4ee17.

The assumption is incorrect. We didn’t recognize those samples are
ISO/IEC 23003-5 before.

> 
> Signed-off-by: Raphaël Zumer <rzumer at tebako.net>
> ---
> libavformat/movenc.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/libavformat/movenc.c b/libavformat/movenc.c
> index f1cc80b1b3..3c44ace5b0 100644
> --- a/libavformat/movenc.c
> +++ b/libavformat/movenc.c
> @@ -1237,7 +1237,7 @@ static int mov_write_pcmc_tag(AVFormatContext *s, AVIOContext *pb, MOVTrack *tra
>                     track->par->codec_id == AV_CODEC_ID_PCM_S24LE ||
>                     track->par->codec_id == AV_CODEC_ID_PCM_S32LE);
>     avio_w8(pb, format_flags);
> -    avio_w8(pb, track->par->bits_per_raw_sample);
> +    avio_w8(pb, track->par->bits_per_coded_sample);
>      return update_size(pb, pos);
> }
> -- 
> 2.41.0
> 
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