[FFmpeg-devel] [PATCH 4/8] metadata: add callbacks to conversion system.

[Anton Khirnov]

On Fri, Jun 04, 2010 at 01:35:45AM +0200, Michael Niedermayer wrote:
> On Wed, Jun 02, 2010 at 03:16:01PM +0200, Anton Khirnov wrote:
> [...]
> > diff --git a/libavformat/metadata.h b/libavformat/metadata.h
> > index 309147d..24296c4 100644
> > --- a/libavformat/metadata.h
> > +++ b/libavformat/metadata.h
> > @@ -42,6 +42,10 @@ typedef struct AVMetadataConvTable {
> >  
> >  struct AVMetadataConv {
> >      const AVMetadataConvTable *conv_table;
> > +    /** Convert tags from src to generic/native format either a) in place, so they'll pass
> > +     *  through the table next or b) remove them from src and store results in dst. */
> > +    void (*conv2generic)(AVMetadata **src, AVMetadata **dst);
> > +    void (*conv2native)( AVMetadata **src, AVMetadata **dst);
> >  };
> 
> this does not seem very practical, each (de)muxer would just have a blackbox
> convert to/from native and no 2 would be able to work with the same callback.
> The system i suggested where AVMetadataConvTable entries have a convert
> callback could share individual convertion functions like year<->date
> between several (de)muxers. Also it would be less blackboxish as one would
> know which tags are passed unchanged and which are changed
> 

I don't think such an approach would solve the problem. First, we need
to be able to merge tags (done by the id3v2 patch) and conversely split
them (e.g. if we wanted to encode id3v2.3). Your concept assumes one
output tag for one input tag and anything else would look like an ugly
hack in it i think. Second, as shown by the matroska patch (convert tags
to uppercase), we want to apply arbitrary transformation to ALL tags,
not just the few ones we know about.

OTOH my system doesn't prevent us from factorizing common code into a
function and calling it from several (de)muxers if it's needed.

Anton Khirnov
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