[Libav-user] AVStream::codec deprecated, but example uses it?

[Steve Myers]

On Tue, Oct 25, 2016 at 10:32 AM, Jason C <jason.cipriani at gmail.com> wrote:

> In 3.1.5 it appears that AVStream::codec is deprecated:
>
>     /**
>      * @deprecated use the codecpar struct instead
>      */
>     attribute_deprecated
>     AVCodecContext *codec;
>
> But the examples linked to from the 3.1.x documentation
> <https://ffmpeg.org/doxygen/3.1/examples.html> all use it, for example
> from the demuxing/decoding example
> <https://ffmpeg.org/doxygen/3.1/demuxing_decoding_8c-example.html>:
>
>     /* find decoder for the stream */
>     dec_ctx = st->codec;
>     dec = avcodec_find_decoder(dec_ctx->codec_id);
>
> So:
>
>   - Either the example needs to be updated, or
>   - AVStream::codec needs to not be deprecated any more, with the API
> returned to the previously already-working-totally-fine-for-everything
> form, or
>   - avcodec_open2 needs to take an AVCodecParameters not an
> AVCodecContext, or
>   - The deprecated warning needs to be updated to make sense, because "use
> the codecpar struct instead" isn't really actual information, it's mostly a
> tantalizing breadcrumb to use when going back to scour all the disjoint
> documentation.
>
> As for what to actually do now... going from the mystery deprecation
> message:
>
> 1. The docs for codecpar
> <https://ffmpeg.org/doxygen/3.1/structAVStream.html#a12826d21779289356722971d362c583c> are
> completely blank, so that's a dead end.
> 2. The docs for AVCodecParameters
> <https://ffmpeg.org/doxygen/3.1/structAVCodecParameters.html#details>
> don't really contain any useful info, although they do mention some
> avcodec_parameters_alloc() function, so let's follow that trail...
> 3. The docs for avcodec_parameters_alloc()
> <https://ffmpeg.org/doxygen/3.1/group__lavc__core.html#ga647755ab2252e93221bb345f3d5e414f>
> just state the obvious, so that's a dead end.
> 4. Uh... let's maybe try googling for AVStream codecpar?
>   - Looked promising
> <https://ffmpeg.zeranoe.com/forum/viewtopic.php?t=3678>, but nothing.
>   - Everything else is pretty much just like that.
> 5. So... let's look at the avformat docs index
> <https://ffmpeg.org/doxygen/3.1/avformat_8h.html> for something that
> takes an AVCodecParameters: Nope, bad guess. Next guess:
> 6. The avcodec docs index <https://ffmpeg.org/doxygen/3.1/avcodec_8h.html>:
> Well, there's this avcodec_parameters_to_context
> <https://ffmpeg.org/doxygen/3.1/group__lavc__core.html#gac7b282f51540ca7a99416a3ba6ee0d16>
>  thing. The docs say what it does but not why you'd want to use it. So in
> the absence of any actual high level information about how the parts of
> these libraries fit together to accomplish goals, I'm just going to guess
> that I should take the AVStream::codecpar, convert it (or whatever it is
> that function does) to an AVCodecContext (instead of just using
> AVStream::codec to begin with), then pass that off to avcodec_open2 (at
> least, until that's deprecated in favor of avcodec_open456 the next time
> the perpetually unstable API gets a facelift), all the while wondering what
> I really just gained by using codecpar instead of codec.
>
> So:
>
> - Is that what it meant by use codecpar instead? I'm really just guessing,
> because none of it is *actually *documented.
>
> - What did I just gain by switching over to codecpar from codec?
>
> - That was way too much of a wild goose chase. Is there actual organized
> documentation anywhere so I don't have to keep doing this?
>
> Jason
>
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>
>
Here's something I found about why it's deprecated:
https://lists.libav.org/pipermail/libav-commits/2016-February/018031.html

And I did this to use codecpar (without checking for errors for brevity):

pCodec=avcodec_find_decoder
(pFormatCtx->streams[audioStream]->codecpar->codec_id);
pCodecCtx=avcodec_alloc_context3(pCodec);
avcodec_parameters_to_context(pCodecCtx,
pFormatCtx->streams[audioStream]->codecpar);
avcodec_open2(pCodecCtx, pCodec,NULL);
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