[FFmpeg-devel] [PATCH 1/2] avformat/movenc: fix sample size being zero in pcmC

[Zhao Zhili]

> From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of Raphaël Zumer
> Sent: 2023年7月26日 21:38
> To: ffmpeg-devel at ffmpeg.org
> Subject: Re: [FFmpeg-devel] [PATCH 1/2] avformat/movenc: fix sample size being zero in pcmC
> 
> On 7/25/23 23:45, Zhao Zhili wrote:
> > From: Zhao Zhili <zhilizhao at tencent.com>
> >
> > -    avio_w8(pb, track->par->bits_per_raw_sample);
> > +    sample_size = track->par->bits_per_raw_sample;
> > +    if (!sample_size)
> > +        sample_size = av_get_exact_bits_per_sample(track->par->codec_id);
> > +    av_assert0(sample_size);
> > +    avio_w8(pb, sample_size);
> 
> Why not just replace bits_per_raw_sample completely with av_get_exact_bits_per_sample()?
> 
> Are there edge cases, like incorrect codec ID, or sample sizes smaller than specified by the codec ID (and if there is ever a mismatch,
> which is expected in pcmC?)
> 
> If not, there should be no need to prioritize bits_per_raw_sample in the first place and add a fallback - going straight to
> av_get_exact_bits_per_sample() should suffice and keep the code simpler.

Check bits_per_raw_sample first for a little bit of performance.
Don't use bits_per_coded_sample because it make less sense in this case and to keep the code simpler.

> 
> As noted in another thread, bits_per_raw_sample seems to always be 0 when muxing PCM to MP4 with FFmpeg, whereas
> bits_per_coded_sample has the correct value from the samples that I tested.

That's only one usecase. bits_per_coded_sample has been set while bit_per_raw_sample wasn't only
because the implementation of wav demux and there's no encoding process. (You can try -c:a pcm_s16le)

There are more usecases to support, e.g., only use mp4 muxer without any demuxer or encoder.
User can create and set a AVCodecParameters manually, and let bits_per_coded_sample be zero.

> 
> Raphaël Zumer
> 
> _______________________________________________
> ffmpeg-devel mailing list
> ffmpeg-devel at ffmpeg.org
> https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
> 
> To unsubscribe, visit link above, or email
> ffmpeg-devel-request at ffmpeg.org with subject "unsubscribe".