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

[Gyan Doshi]

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.

Regards,
Gyan




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