[FFmpeg-devel] [PATCH] avcodec: Add AVClass to AVCodecParameters
michael at niedermayer.cc
Tue May 10 16:40:39 CEST 2016
On Tue, May 10, 2016 at 03:39:09PM +0200, Hendrik Leppkes wrote:
> On Tue, May 10, 2016 at 2:35 PM, Michael Niedermayer
> <michael at niedermayer.cc> wrote:
> > On Tue, May 10, 2016 at 02:02:52PM +0200, Hendrik Leppkes wrote:
> >> On Tue, May 10, 2016 at 1:43 PM, Michael Niedermayer
> >> <michael at niedermayer.cc> wrote:
> >> > On Tue, May 10, 2016 at 11:17:12AM +0100, Derek Buitenhuis wrote:
> >> >> On 5/10/2016 7:24 AM, Hendrik Leppkes wrote:
> >> >> > I don't like this, the struct is pretty cleanly defined and unlikely
> >> >> > to be extended much over time.
> >> >> > Most other structs have AVOptions so the CLI can interact with it, but
> >> >> > this struct is not meant to be modified by users, its just a direct
> >> >> > line of communication between demuxer->decoder or encoder->muxer.
> >> >>
> >> >> To me this mostly still feels like trying to make a demuxing library
> >> >> something it isn't. It demuxes and muxes.
> >> >>
> >> >> You can argue that it should be easier than decoding a frame to get this
> >> >> info (which I don't personally think is much trouble at all). However,
> >> >> jamming it into libavformat for reasons like "we've always done this" and
> >> >> "there's no better place" is not good design IMO.
> >> >>
> >> >> It sounds like what you really want is a higher level "easy" API. Don't
> >> >> jam it in to existing decoupled APIs because it is convenient, IMO. That's
> >> >> how horrible things like having ffmpeg.c functionality in libavformat came
> >> >> to exist, and a decade of misery followed.
> >> >
> >> > This patch adds an AVClass field to AVCodecParameters like we use in
> >> > other public structures.
> >> > i fail to see how this patch is related to your reply
> >> >
> >> > What the patch does, is it makes the API more consistent and easy to
> >> > use. Users call the AVOption functions to set fields in the main
> >> > public structures, if they do that for AVCodecParameters it would
> >> > crash.
> >> > Even an empty AVClass that doesnt list options would reduce that
> >> > misery by at least not crashing but giving a clear error message.
> >> >
> >> We should not design our API around people not caring to read our API,
> >> because thats not a fight we can ever win.
> > APIs should be designed with the "Principle of least astonishment"
> > its quite surprising that AVOption APIs work with most public API
> > structures but crash with some
> > (AVPacket is another and i am of the oppinon and stated that previously
> > IIRC that it too should have a AVClass as its first element, this is
> > just hard to add as it requires ABI bumping everything which is why
> > i didnt push much for that being done, it needs to be done at a
> > time we bump all stuff for more important reasons)
> The number of structs that don't have it probably greatly outnumber
> those that do.
> From the top of my head:
> AVFrame and AVSubtitle
the struct dont have the class as their first field because when we
wanted to add it we could not due to ABI, it would have required a
ABI major bump
> AVCodecParameters, of course
> Any and all structs used for side-data (AVCPBProperties, AVPanScan, etc)
> In fact the only that do have it are those where it was originally
> used to access private options of the underlying components.
> AVFilterContext / AVFilterGraph
> So I'm not sure I can follow your argument of "everything else has it".
Most of the major user interface structs have it.
Probably/maybe because we hit problems over many years extending the
structs without it. New structs dont have it because people again
belive their initial API will not be extended much and it wont be
needed. We will see if that will work out this time
Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
Those who are too smart to engage in politics are punished by being
governed by those who are dumber. -- Plato
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