[FFmpeg-devel] [RFC] lavf/tee per-output stream selection

[Stefano Sabatini]

On date Sunday 2013-08-04 12:07:14 +0200, Nicolas George encoded:
> Le sextidi 16 thermidor, an CCXXI, Stefano Sabatini a écrit :
> > >From 7b08890d825abc7df66bd49e8cbb1f4ae5c888b1 Mon Sep 17 00:00:00 2001
> > From: Stefano Sabatini <stefasab at gmail.com>
> > Date: Sat, 3 Aug 2013 11:23:59 +0200
> > Subject: [PATCH] lavf/tee: add special select option
> > 
> > TODO: bump micro
> > ---
> >  doc/muxers.texi   |  5 +++++
> >  libavformat/tee.c | 48 ++++++++++++++++++++++++++++++++++++++++--------
> >  2 files changed, 45 insertions(+), 8 deletions(-)
> > 
> > diff --git a/doc/muxers.texi b/doc/muxers.texi
> > index 7c5a1c8..37a1734 100644
> > --- a/doc/muxers.texi
> > +++ b/doc/muxers.texi
> > @@ -844,6 +844,11 @@ The BNF description of the bitstream filters specification is given by:
> >  @var{BSF} ::= @var{BSF_NAME}[+ at var{STREAM_SPECIFIER}]
> >  @var{BSFS} ::= @var{BSF}[, at var{BSFS}]
> >  @end example
> > +
> > + at item select
> > +Select the streams that should be mapped to the slave output,
> > +specified by a stream specifier. If not specified, this defaults to
> > +all the input streams.
> 

> Examples may be welcome. Is it possible to have several stream specifiers,
> i.e. map "video + English audio + French audio" to "video + English audio"
> and "video + French audio"?

This is something I considered, but requires either an extension of
the specifier syntax or a local hack.

For example we may extend the specifier to take into account
compositions like a+v, and consider the language like in a[lang=en]+v.

Composition is probably a good idea since it would extend other parts
of the codebase for free (e.g. ffprobe -select_streams a0+v1). As for
the language thing I'm open to suggestions.

> 
> >  @end table
> >  
> >  Example: encode something and both archive it in a WebM file and stream it
> > diff --git a/libavformat/tee.c b/libavformat/tee.c
> > index 90947c7..4604ab4 100644
> > --- a/libavformat/tee.c
> > +++ b/libavformat/tee.c
> > @@ -30,6 +30,7 @@
> >  typedef struct {
> >      AVFormatContext *fmt_ctx;
> >      AVBitStreamFilterContext **bsf_ctxs; ///< bitstream filters per stream
> 
> > +    int *stream_map; ///< map between input and output streams, disabled streams are set to -1
> 
> Nit: please try to keep lines below 80 chars.
> 
> Also, the comment is unclear about the direction of the map, it could be
> either:
> 
> 	stream_num_in_slave  = stream_map[stream_num_in_global]
> 	stream_num_in_global = stream_map[stream_num_in_slave]
> 
> >  } TeeSlave;
> >  
> >  typedef struct TeeContext {
> > @@ -179,9 +180,10 @@ static int open_slave(AVFormatContext *avf, char *slave, TeeSlave *tee_slave)
> >      AVDictionary *options = NULL;
> >      AVDictionaryEntry *entry;
> >      char *filename;
> > -    char *format = NULL, *bsfs = NULL;
> > +    char *format = NULL, *bsfs = NULL, *select = NULL;
> >      AVFormatContext *avf2 = NULL;
> >      AVStream *st, *st2;
> > +    int stream_count;
> >  
> >      if ((ret = parse_slave_options(avf, slave, &options, &filename)) < 0)
> >          return ret;
> > @@ -195,14 +197,42 @@ static int open_slave(AVFormatContext *avf, char *slave, TeeSlave *tee_slave)
> >          entry->value = NULL; /* prevent it from being freed */
> >          av_dict_set(&options, "bsfs", NULL, 0);
> >      }
> > +    if ((entry = av_dict_get(options, "select", NULL, 0))) {
> > +        select = entry->value;
> > +        entry->value = NULL; /* prevent it from being freed */
> > +        av_dict_set(&options, "select", NULL, 0);
> > +    }
> 
> This hunk really needs factoring.
> 
> >  
> >      ret = avformat_alloc_output_context2(&avf2, NULL, format, filename);
> >      if (ret < 0)
> >          goto fail;
> >      av_free(format);
> >  
> > +    tee_slave->stream_map = av_calloc(avf->nb_streams, sizeof(int));
> 
> sizeof(*tee_slave->stream_map)
> 
> > +    if (!tee_slave->stream_map) {
> > +        ret = AVERROR(ENOMEM);
> > +        goto fail;
> > +    }
> > +
> > +    stream_count = 0;
> >      for (i = 0; i < avf->nb_streams; i++) {
> >          st = avf->streams[i];
> 
> > +        if (select && select[0]) {
> 
> Why select[0]?

The empty string is parsed as "all", I can drop the check though.

[...]
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