[FFmpeg-devel] [PATCH v6 1/3] avcodec: add flags for packets with top/bottom field

[wm4]

On Wed, 23 May 2018 14:29:38 -0400 (EDT)
Patrick Keroulas <patrick.keroulas at savoirfairelinux.com> wrote:

> ----- Original Message -----
> > From: "wm4" <nfxjfg at googlemail.com>
> > To: ffmpeg-devel at ffmpeg.org
> > Sent: Wednesday, May 23, 2018 12:02:45 PM
> > Subject: Re: [FFmpeg-devel] [PATCH v6 1/3] avcodec: add flags for packets with top/bottom field  
> 
> > On Wed, 23 May 2018 16:46:17 +0100
> > Rostislav Pehlivanov <atomnuker at gmail.com> wrote:
> >   
> >> On 23 May 2018 at 16:18, wm4 <nfxjfg at googlemail.com> wrote:
> >>   
> >> > On Tue, 22 May 2018 17:19:35 -0400 (EDT)
> >> > Patrick Keroulas <patrick.keroulas at savoirfairelinux.com> wrote:
> >> >   
> >> > > ----- Original Message -----  
> >> > > > From: "Rostislav Pehlivanov" <atomnuker at gmail.com>
> >> > > > To: "FFmpeg development discussions and patches" <  
> >> > ffmpeg-devel at ffmpeg.org>  
> >> > > > Sent: Friday, May 18, 2018 5:28:42 PM
> >> > > > Subject: Re: [FFmpeg-devel] [PATCH v6 1/3] avcodec: add flags for  
> >> > packets with top/bottom field  
> >> > >   
> >> > > > On 18 May 2018 at 22:17, wm4 <nfxjfg at googlemail.com> wrote:  
> >> > > 
> >> > > 
> >> > >   
> >> > > > > But I think a new side data type would be much saner. We could even
> >> > > > > just make it something generic, like AV_PKT_DATA_ANCILLARY or
> >> > > > > something. It's apparently just packet data which somehow couldn't go
> >> > > > > into the packet data.  
> >> > > 
> >> > >   
> >> > > > I agree, a generic ancillary side data type sounds better. It would  
> >> > have to  
> >> > > > be handled the same way as mastering metadata (e.g. to allocate it  
> >> > you'd  
> >> > > > need to use a separate function), since the size of the data struct  
> >> > can't  
> >> > > > be part of the API if we intend to add fields later.
> >> > > > Patrick, if you're okay with that you should submit a patch which bases
> >> > > > such side data on libavutil/mastering_display_metadata.h/.c  
> >> > > 
> >> > > No problem for transmitting field flags through side data. But the given
> >> > > example (libavutil/mastering_display_metadata.h/.c) attaches data to
> >> > > AVFrame, not AVPacket, so I'm not sure where to place this separate
> >> > > allocator function. Do you recommend to create a new
> >> > > libavcodec/ancillary.c/h utility?  
> >> > 
> >> > The example you mentioned exists for AVPacket too (it's just not easy
> >> > to see how it can end up in AVPacket, because no demuxer does that
> >> > directly).
> >> > 
> >> > Anyway, ancillary side data would just be an untyped byte array, so I
> >> > don't think it needs any helpers. Just an addition to the packet side
> >> > data enum (I think it's somewhere in avcodec.h).
> >> > _______________________________________________
> >> > ffmpeg-devel mailing list
> >> > ffmpeg-devel at ffmpeg.org
> >> > http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
> >> >   
> >> 
> >> I'd rather have it as a well defined typed array rather than a bunch of
> >> bytes. Otherwise we'd start sending unknown side data info and users
> >> wouldn't know what to do with it.  
> > 
> > Unless you're adding some meta object system for describing arbitrary
> > types at runtime I don't know how you'd do that.  
> 
> Is that ok if I simply define a basic struct to hold the field?
> 
> Any suggestion on where to insert the definition of this data and the 
> accessors in lavc? In a new source file?

If you make it a struct, then make a new file in libavutil, with
at least a helper to get the struct size (this is for ABI reasons, so
we can extend the struct later). But then this side data would need a
specific name, not a generic one like "ancillary".

The display mastering thing is valid for both packets and frames, which
might be confusing. The thing you add is needed for packets only.

I'd prefer the "ancillary" name and making it just a flat byte array
instead of a struct and something specific. The former would be like
extradata, just per packet.