[FFmpeg-devel] [PATCH 01/17] avcodec/avcodec: add side data to AVCodecContext

[Anton Khirnov]

Quoting James Almer (2023-09-05 13:26:22)
> On 9/5/2023 8:07 AM, Anton Khirnov wrote:
> > Quoting James Almer (2023-09-05 00:08:48)
> >> This will allow the propagation of global side data within the AVCodecContext
> >> instead of having to do it inside packets, and thus be available during init().
> >> Global and frame specific side data will therefore be distinct.
> > 
> > This commit message is misleading - there is already
> > AVCodecContext.coded_side_data for exactly this purpose. And after the
> > changes from the last iteration I see even less of a reason to replace
> > it with a new field.
> 
> I insist the new field in the form of a set is better, for the sake of
> unified helpers that can be used in avctx, codecpar, avstream, and
> potentially others in the future.

The big problem with this is that AVPacket is left as is. And since
changing it would be a huge break for very little gain, we'll have
different handling for packets and everything else for the foreseeable
future.

I think you'd get almost the same benefits with downsides by making the
helpers accept array+count as parameters. It's slightly less elegant,
but not hugely so IMO.

> It will also be the packet counterpart of Jan's frame side data set
> field. coded_side_data is currently used only to export CPB props, so
> the amount of users is probably very small (Maybe only lavf, even). I
> think the benefits in the long run outweigh the cons from the breakage
> that would mean replacing coded_side_data.
> 
> Also, my interpretation of coded is still that it refers to a coded 
> stream, much like we make a distinction between coded and raw for 
> bits_per_sample, and in decoding scenarios, side data entries would have 
> information that refer to the decoded raw stream (hdr, etc).

The intent was for it to be a direct counterpart to AVStream.side_data,
as is mentioned in the relevant commit message, so your interpretation
is objectively wrong.

> That said, I don't want to keep delaying this set much longer, so if 
> you're really against that change I'll try to remove it from the set and 
> keep the rest.

I'd appreciate more opinions on this, from whoever cares.

-- 
Anton Khirnov