[FFmpeg-devel] [PATCH] ffmpeg: add option recast_media

[Anton Khirnov]

Quoting Gyan Doshi (2021-07-13 11:12:19)
> 
> 
> On 2021-07-13 13:14, Anton Khirnov wrote:
> > Quoting Gyan Doshi (2021-07-02 12:03:05)
> >> Allows forcing decoders of different media type.
> >> Needed to decode media data muxed as data streams.
> >> ---
> >>   doc/ffmpeg.texi      | 5 +++++
> >>   fftools/ffmpeg_opt.c | 7 ++++++-
> >>   2 files changed, 11 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/doc/ffmpeg.texi b/doc/ffmpeg.texi
> >> index 7827291755..c1065086e5 100644
> >> --- a/doc/ffmpeg.texi
> >> +++ b/doc/ffmpeg.texi
> >> @@ -449,6 +449,11 @@ output file already exists.
> >>   Set number of times input stream shall be looped. Loop 0 means no loop,
> >>   loop -1 means infinite loop.
> >>   
> >> + at item -recast_media (@emph{global})
> >> +Enable to allow forcing a decoder of a different media type than
> >> +the one detected or designated by the demuxer. Useful for decoding
> >> +media data muxed as data streams.
> >> +
> >>   @item -c[:@var{stream_specifier}] @var{codec} (@emph{input/output,per-stream})
> >>   @itemx -codec[:@var{stream_specifier}] @var{codec} (@emph{input/output,per-stream})
> >>   Select an encoder (when used before an output file) or a decoder (when used
> >> diff --git a/fftools/ffmpeg_opt.c b/fftools/ffmpeg_opt.c
> >> index a63bed54cf..76a220c21c 100644
> >> --- a/fftools/ffmpeg_opt.c
> >> +++ b/fftools/ffmpeg_opt.c
> >> @@ -186,6 +186,7 @@ static int input_sync;
> >>   static int input_stream_potentially_available = 0;
> >>   static int ignore_unknown_streams = 0;
> >>   static int copy_unknown_streams = 0;
> >> +static int recast_media = 0;
> >>   static int find_stream_info = 1;
> >>   
> >>   static void uninit_options(OptionsContext *o)
> >> @@ -759,7 +760,7 @@ static const AVCodec *find_codec_or_die(const char *name, enum AVMediaType type,
> >>           av_log(NULL, AV_LOG_FATAL, "Unknown %s '%s'\n", codec_string, name);
> >>           exit_program(1);
> >>       }
> >> -    if (codec->type != type) {
> >> +    if (codec->type != type && !recast_media) {
> >>           av_log(NULL, AV_LOG_FATAL, "Invalid %s type '%s'\n", codec_string, name);
> >>           exit_program(1);
> >>       }
> >> @@ -774,6 +775,8 @@ static const AVCodec *choose_decoder(OptionsContext *o, AVFormatContext *s, AVSt
> >>       if (codec_name) {
> >>           const AVCodec *codec = find_codec_or_die(codec_name, st->codecpar->codec_type, 0);
> >>           st->codecpar->codec_id = codec->id;
> >> +        if (recast_media && st->codecpar->codec_type != codec->type)
> >> +            st->codecpar->codec_type = codec->type;
> > The caller is not allowed to modify this struct for demuxing. This might
> > confuse demuxers that expect the values they put there to remain
> 
> choose_decoder() is called from within add_input_streams().
> 
> Near the end of this parent function, we have
> 
>      ret = avcodec_parameters_from_context(par, ist->dec_ctx);
> 
> where par is
> 
>      AVCodecParameters *par = st->codecpar;
> 
> 
> avcodec_parameters_from_context(), starts with
> {
>      codec_parameters_reset(par);   -->  sets codec_type to 
> AVMEDIA_TYPE_UNKNOWN
> 
>      par->codec_type = codec->codec_type;
>      ...
> }
> 
> So it's already being done. I did an immediate recast to avoid some 
> temporary variables as the media type is used in a few switch blocks 
> after the decoder is set.
> But that way also works for me, if you insist.

Okay, I still consider this API abuse, but since it was there before it
is not your responsibility to fix it. Guess I'll add it to my todo list.

-- 
Anton Khirnov