[FFmpeg-devel] [PATCH] avcodec: Add AVClass to AVCodecParameters

Ronald S. Bultje rsbultje at gmail.com
Tue May 17 19:22:07 CEST 2016


On Tue, May 17, 2016 at 12:34 PM, Hendrik Leppkes <h.leppkes at gmail.com>

> On Tue, May 17, 2016 at 6:26 PM, Ronald S. Bultje <rsbultje at gmail.com>
> wrote:
> > Hi,
> >
> > On Tue, May 17, 2016 at 11:18 AM, Michael Niedermayer <
> > michael at niedermayer.cc> wrote:
> >
> >> AVCodecContext contains all the fields that AVCodecParameters has.
> >> Examples are width, height, bitrate, profile, sample_rate, channels
> >>
> >> In AVCodecContext these fields are accessible through AVOptions
> >> in AVCodecParameters these fields are not accessible through AVOptions
> >> and your code will crash mysteriously if you try to access them that
> >> way
> >>
> >> This is a inconsistency that was not there before
> >
> >
> > So, at the risk of going semi-offtopic, let me take a different angle at
> > this problem.
> >
> > AVCodecParameter is supposed to be an intermediate layer that removes the
> > need for lavf to access lavc structs like AVCodecContext, right?
> >
> > So, I worked (a long long time ago) on this software called GStreamer,
> and
> > specifically, one of the plugins I worked on early on was gst-ffmpeg. I
> > loved working on it because it practically added so many codecs to the
> > player which were previously unsupported. Yay! Now, the API back then was
> > horrible. You had an AVCodecContext (no AVOptions) and there were
> > approximately 100 million fields that could be set. Most were
> undocumented,
> > and it was totally unclear which needed to be set. For example, to play a
> > WMA file, you might have to set block_align, but for other audio codecs,
> > you do not [1].
> >
> > For video, sometimes you would have to set extradata (e.g. for SVQ3), but
> > other times, you would not. Huffyuv required bits_per_sample, but h264
> did
> > not.
> >
> > So how do you write generic user-facing code where the demuxer and
> decoder
> > actually are separate entities and merely interact over a serialized
> > communication channel? You can't serialize AVCodecContext. I mean, you
> > could, but that's a terrible idea. And nothing documented which fields
> were
> > required to be set and which weren't.
> >
> > I was hoping that AVCodecParameters would solve this. Ideally, it would
> be
> > a dictionary of fields that are to be shared between demuxer and decoder
> > (or codec and muxer). But I think it has sort of strayed off of that
> intent
> > and is just a flat minimized AVCodecContext as it existed in 2003.
> >
> > Can we fix this? I think AVOptions would be ideally suited to fix this.
> > AVClass maybe helps also. But if introspection suggests I can set
> > block_align on the h264 decoder, or width on an audio decoder, or who
> knows
> > what. Then I don't think AVOptions or AVClass in AVCodecParameters helps
> at
> > all. It just creates a minimized AVCodecContext from 2003 that will
> slowly
> > become the current AVCodecContext, which is not all that useful for
> people
> > that don't use ffmpeg.exe...
> >
> >
> This is C, I don't want to have to run introspection on an object to
> find out what I can set.
> A clean and doumented struct is a far more effective way to get this
> information across, which is also why I don't want AVOptions for such
> a struct, if its not needed to set private options, it would just
> double the need for docs (both in doxy and in the AVOption
> definition), and to figure out which fields actually exist, you still
> have to read the struct or a comment in the header file - so why not
> use the struct in the first place?
> Why use something "generic" with a dict or some weirdness, which loses
> the strong typing and self-documentation of such a struct? Sounds like
> you want to be working in another language, honestly. :)
> AVOption will never "know" which fields a particular codec will need,
> so that point is not going to get solved either way.

It's not for us (ffmpeg.c developers), it's for people using the ffmpeg
libraries on other language platforms (e.g. python).


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