[FFmpeg-devel] [PATCH 01/11] avcodec: add avcodec_get_supported_config()

[Niklas Haas]

On Mon, 08 Apr 2024 22:18:33 +0200 Michael Niedermayer <michael at niedermayer.cc> wrote:
> On Fri, Apr 05, 2024 at 08:57:11PM +0200, Niklas Haas wrote:
> > From: Niklas Haas <git at haasn.dev>
> > 
> > This replaces the myriad of existing lists in AVCodec by a unified API
> > call, allowing us to (ultimately) trim down the sizeof(AVCodec) quite
> > substantially, while also making this more trivially extensible.
> > 
> > In addition to the already covered lists, add two new entries for color
> > space and color range, mirroring the newly added negotiable fields in
> > libavfilter.
> > 
> > I decided to drop the explicit length field from the API proposed by
> > Andreas Rheinhardt, because having it in place ended up complicating
> > both the codec side and the client side implementations, while also
> > being strictly less flexible (it's trivial to recover a length given
> > a terminator, but requires allocation to add a terminator given
> > a length). Using a terminator also presents less of a porting challenge
> > for existing users of the current API.
> > 
> > Once the deprecation period passes for the existing public fields, the
> > rough plan is to move the commonly used fields (such as
> > pix_fmt/sample_fmt) into FFCodec, possibly as a union of audio and video
> > configuration types, and then implement the rarely used fields with
> > custom callbacks.
> > ---
> >  doc/APIchanges              |  5 ++++
> >  libavcodec/avcodec.c        | 51 +++++++++++++++++++++++++++++++++++++
> >  libavcodec/avcodec.h        | 27 ++++++++++++++++++++
> >  libavcodec/codec.h          | 19 +++++++++++---
> >  libavcodec/codec_internal.h | 21 +++++++++++++++
> >  libavcodec/version.h        |  4 +--
> >  6 files changed, 121 insertions(+), 6 deletions(-)
> 
> This patchset seems to overlap a bit with AVOptionRanges
> 
> I think ideally the user should at some point be able to query using some
> API on a AVCodecContext/AVCodecParameters/AVFormatContex/AVStream
> what for that specific instance are supported settings for each field
> 
> The API here seems to use a enum, which can make sense but it differs from
> how AVOption works which doesnt use enums to identify fields

I didn't know AVOptionRanges exists; indeed it can be seen as somewhat
overlapping here. That said, there is a vital distinction here: AVOptionRanges
represents *static* limits on what options can be set (e.g. via
`av_opt_set_int`), whereas this API represents *dynamic* limits on what can be
coded.

In particular, the list of supported colorspaces etc. can *depend* on the value
of other options. Currently, only strict_std_compliance, but I can easily see
this list growing in the future (e.g. for different profiles, dolbyvision,
...).

That aside, I personally find an API based on strings and doubles rather
cumbersome to use, especially when downstream clients expect an enum list.

> 
> 
> [...]
> -- 
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
> 
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