[FFmpeg-devel] [PATCH]lavf/mov: Support alac extradata in wave atom

Michael Niedermayer michael at niedermayer.cc
Mon Aug 3 21:57:50 CEST 2015

On Mon, Aug 03, 2015 at 10:32:02AM +0000, Carl Eugen Hoyos wrote:
> Paul B Mahol <onemda <at> gmail.com> writes:
> > > Attached patch fixes ticket #4747 for me, 
> > > I don't know how to detect that the wave 
> > > atom contains no frma / alac atom...
> > 
> > And that is mentioned because?
> The file from ticket #4747 does not contain an 
> alac atom which is required for alac decoding. 
> Instead it contains the necessary data in the 
> wave atom (where you normally would find the 
> alac atom). To distinguish, it would be 
> necessary to look "into" the wave atom to know 
> if it contains an alac atom or not.
> I meanwhile found that the mpeg demuxer 
> sometimes seeks eight bytes back which I will 
> probably copy. The alternative, iiuc, would be 
> to copy the whole wave atom into a buffer and 
> call mov_read_default() from the wave_read() 
> function with that buffer.
> But this should not be necessary if somebody 
> can confirm that it is ok to seek back x bytes 
> in a demuxer.

it is safe if ffio_ensure_seekback() is used with appropriate
arguments before these x bytes are read
(or the stream is seekable or ...)


Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

No snowflake in an avalanche ever feels responsible. -- Voltaire
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