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

Ronald S. Bultje rsbultje at gmail.com
Tue May 17 18:26:18 CEST 2016


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

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...



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