[FFmpeg-devel] [PATCH] avcodec: Add AVClass to AVCodecParameters
h.leppkes at gmail.com
Tue May 17 19:24:37 CEST 2016
On Tue, May 17, 2016 at 7:22 PM, Ronald S. Bultje <rsbultje at gmail.com> wrote:
> 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>
>> > 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,
>> > 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
>> > 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 .
>> > 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
>> > not.
>> > So how do you write generic user-facing code where the demuxer and
>> > 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
>> > required to be set and which weren't.
>> > I was hoping that AVCodecParameters would solve this. Ideally, it would
>> > 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
>> > 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
>> > what. Then I don't think AVOptions or AVClass in AVCodecParameters helps
>> > all. It just creates a minimized AVCodecContext from 2003 that will
>> > become the current AVCodecContext, which is not all that useful for
>> > 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).
Maybe the ports to those other languages should devise an interface
appropriate for that language, and we should keep a clean C interface?
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