Re: [FFmpeg-devel] [PATCH 000/279 v2] New channel layout API

[Michael Niedermayer <michael@niedermayer.cc>]

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On Fri, Dec 17, 2021 at 01:04:19AM +0100, Marton Balint wrote:
> 
> 
> On Thu, 16 Dec 2021, James Almer wrote:
> 
> > Resending the first two patches only, since this is meant to
> > show the implementation of one of the several suggestions made
> > in the previous set that need to be discussed and hopefully
> > resolved in a call.
> 
> Can you push the full branch somewhere?
> 
> > 
> > The proposals so far to extend the API to support either custom
> > labels for channels are, or some form of extra user information.
> > 
> > - Fixed array of bytes to hold a label. Simple solution, but
> >  the labels will have a hard limit that can only be extended
> >  with a major bump. This is what i implemented in this version.
> > - "char *name" per channel that the user may allocate and the
> >  API will manage, duplicate and free. Simple solution, and the
> >  name can be arbitrarily long, but inefficient (av_strdup() per
> >  channel with a custom label on layout copy).
> > - "const char *name" per channel for compile time constants, or
> >  that the user may allocate and free. Very efficient, but for
> >  non compile time strings ensuring they outlive the layout can
> >  be tricky.
> > - Refcounted AVChannelCustom with a dictionary. This can't be
> >  done with AVBufferRef, so it would require some other form
> >  of reference counting. And a dictionary may add quite a bit of
> >  complexity to the API, as you can set anything on them.
> 
> Until we have proper refcounting API we can make the AVBufferRef in
> AVChannelLayout a void *, and only allow channel_layout functions to
> dereference it as an AVBufferRef. This would mean adding some extra helper
> functions to channel layout, but overall it is not unsolvable.
> 
> The real question is that if you want to use refcounting and add helpers to
> query / replace per-channel metadata, or you find the idea too heavy weight
> and would like to stick to flat structs.

what is the advantage of refcounting for channel metadata ?
is it about the used memory, about the reduced need to copy ?

what kind of metadata and what size do you expect ?
bytes, kilobytes, megabytes, gigabytes per channel ?

what is the overhead for dynamic allocation and ref counting?
that is at which point does it even make sense ?

thx

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Nations do behave wisely once they have exhausted all other alternatives. 
-- Abba Eban

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