[FFmpeg-devel] [PATCH] attachments support in matroska demuxer

[Uoti Urpala]

On Sun, 2008-01-20 at 16:05 +0000, M?ns Rullg?rd wrote:
> Based on what you say, I have to consider the entire concept of
> codec_tag flawed.  Since the decoder does not in general (and
> certainly should not) know what container the data came in, it cannot
> know how to interpret the value of codec_tag.  Similarly a muxer,
> whether transcoding or copying, does not know the source of the
> codec_tag value, and thus cannot interpret it one way or another.

You cannot always interpret the value unless you know which container it
is from. However you CAN know the container and in that case it's better
than nothing.

Decoders can only map container-specific codec tags to FFmpeg-specific
codec id values if the codec in question is known to FFmpeg (and not
always even in that case). Demuxers in libavformat can still be useful
even if FFmpeg does not recognize the specific codec, but that requires
exporting the container-specific codec type information somehow.