[FFmpeg-devel] [PATCH] Implemented arrange feature to convert from low timebase formats to high timebase

[Nicolas George]

Le sextidi 26 vendémiaire, an CCXXI, Roman Arutyunyan a écrit :
> This patch rearranges frame timestamps in muxer.
> 
> The problem: When remuxing from low-timebase-resolution formats (like RTMP with 1KHz time base)
> to high-resolution formats (like MPEG-TS with 90KHz) frame timestamps are multiplied by
> resolution factor (*90 in this case). That significantly increases round-off error. If you
> have two audio frames next to one another that kind of remuxing may result in your second frame's 
> timestamp be too far from where it should be as a result of round-off error multiplication.

Have you discussed that particular problem somewhere (users mailing-list,
bug tracker)? It looks to me like a framerate detection failure, and I am
not sure that tweaking the timestamps randomly is the correct method of
fixing it.

> While I have not found a PC software sensitive to that kind of error Apple iPhone is very
> much sensitive producing crackles at audio frame boundaries after such conversion (I convert 
> RTMP to HLS).
> 
> The patch intoduces '-arrange[:stream_specifier] arrange_threshold' option, where arrange_threshold
> is max timestamp divergence in microseconds. When the argument is nonzero and current frame dts 
> is within arrange_threshold distance from muxer stream dts then packet dts is reset making
> current frame receive the currect auto-generated dts from muxer. If new packet dts differs
> too much then it's not changed in any way (that usually means a gap in audio stream).
> 
> The feature makes sence when applied to audio stream.
> 
> ---
>  ffmpeg.h             |    2 ++
>  ffmpeg_opt.c         |    4 ++++
>  libavcodec/avcodec.h |    7 +++++++
>  libavformat/utils.c  |    8 ++++++++

The flag is stored in the AVCodecContext, the work is done in libavformat
but the setting is controlled by the command-line tool. On the whole, it
does not look right.

If the problem is specific to the ffmpeg command-line tool, then, assuming
this is the right way of fixing it, then the flag should be in the
OutputStream structure and the work done by ffmpeg.c in write_frame().

>  4 files changed, 21 insertions(+), 0 deletions(-)
> 
> diff --git a/ffmpeg.h b/ffmpeg.h
> index 56f8dfc..cadbea4 100644
> --- a/ffmpeg.h
> +++ b/ffmpeg.h
> @@ -167,6 +167,8 @@ typedef struct OptionsContext {
>      int        nb_pass;
>      SpecifierOpt *passlogfiles;
>      int        nb_passlogfiles;
> +    SpecifierOpt *arrange;
> +    int        nb_arrange;
>  } OptionsContext;
>  
>  typedef struct InputFilter {
> diff --git a/ffmpeg_opt.c b/ffmpeg_opt.c
> index 2c5fbfb..a50a9cf 100644
> --- a/ffmpeg_opt.c
> +++ b/ffmpeg_opt.c
> @@ -994,6 +994,8 @@ static OutputStream *new_output_stream(OptionsContext *o, AVFormatContext *oc, e
>          st->codec->global_quality = FF_QP2LAMBDA * qscale;
>      }
>  
> +    MATCH_PER_STREAM_OPT(arrange, i64, st->codec->arrange_threshold, oc, st);
> +
>      if (oc->oformat->flags & AVFMT_GLOBALHEADER)
>          st->codec->flags |= CODEC_FLAG_GLOBAL_HEADER;
>  
> @@ -2343,6 +2345,8 @@ const OptionDef options[] = {
>          "extract an attachment into a file", "filename" },
>      { "debug_ts",       OPT_BOOL | OPT_EXPERT,                       { &debug_ts },
>          "print timestamp debugging info" },
> +    { "arrange",        HAS_ARG | OPT_INT64 | OPT_SPEC,              { .off = OFFSET(arrange) },
> +        "arrange closely spaced frames" },

Both the name of the option and its description seem way too vague. A more
specific name and an entry in the documentation would be in order.

Regards,

-- 
  Nicolas George
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