[FFmpeg-devel] [PATCH]Reset rotation information when using the transpose filter

[Clément Bœsch]

On Thu, Nov 15, 2012 at 09:28:20AM +0000, Carl Eugen Hoyos wrote:
> Clément Bœsch <ubitux <at> gmail.com> writes:
> 
> > > +    av_dict_set(&picref->metadata, "rotate", NULL, 0);
> > > +
> > >      outlink->out_buf->pts = picref->pts;
> > >  
> > 
> > I can think of several problems with this:
> >  - the metadata is per frame while AFAIK the rotate 
> >    metadata is at stream level in the MOV
> 
> This is true and it means the patch does not work.
> Is there a way to access the AVStream (metadata) 
> from a filter?

I don't think so

> (I just realized that my other work-around, setting 
> the rotate metadata from the command line, does not 
> work either.)
> 
> >  - transpose is a filter to physically change the 
> >    video: the correct way is likely to kept the 
> >    metadata after the transcode (isn't it the case?)
> 
> I believe that this is the bug:
> The metadata should not be kept if the transcode 
> filter is used (it should be reset).
> 

Why? Imagine an input with a meta rotate=90. Video players honoring that
metadata will rotate the video correctly at playback. Now two scenarios:

 - the first one is your situation: the user has a player not honoring
   that metadata (ex: ffplay), so he wants to physically rotate the video
   so it is playable everywhere. He will use something like:
       ffmpeg -i ... -metadata:s:v rotate=0 -vf transpose=...
   to reset the soft rotate, and do the hard rotate.
   You might think the rotate=0 should be done automatically, but it's a
   problem for the second case:

 - the user has a player honoring correctly the metadata, but he wants to
   rotate the video even more (without even knowing there is already a
   soft rotation done). So he will just add naively a vf transpose: this
   will still work for him, except if you decided to drop the rotate
   metadata from the filter: the transmux/transcode needs to keep a
   maximum of information from the input.

-- 
Clément B.
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