Improve documentation and examples for AVCodecParameters related functions

[patacca]

Summary of the bug:
Bug previously opened on mpd bug tracker: ​https://bugs.musicpd.org/view.php?id=4564 (see cirrus's comment ​https://bugs.musicpd.org/view.php?id=4564#c10084)

When decoding a m4a file avcodec_send_packet fails if the AVCodecContext instance is created via avcodec_alloc_context3.

How to reproduce:
See mpd code:
​http://git.musicpd.org/cgit/master/mpd.git/commit/?id=8825393660b95664141ce818089359e1edd612a3
​http://git.musicpd.org/cgit/master/mpd.git/tree/src/decoder/plugins/FfmpegDecoderPlugin.cxx?h=v0.19.x#n497

[James]

Looks like they are not copying codec parameters from the AVStream.AVCodecParameters struct to the avcodec_alloc_context3 allocated context before calling avcodec_open2.
The function avcodec_parameters_to_context should be used for this purpose.

Can't really test but it could be the source of the problem.

[Pritam Baral]

I can confirm that jamrial's suggestion works.

[James]

Replying to pritambaral:

I can confirm that jamrial's suggestion works.

Could you report this to MPD's developers, then?

Thanks for testing.

[Max Kellermann]

jamrial, I can't find documentation on this requirement in the FFmpeg API documentation. Quite contrary, none of the code samples has this call.

I'm not quite happy with the situation. I closed the bug report on the MPD bug tracker as an apparent FFmpeg bug, because I couldn't find anything that's wrong with MPD.

This looks like a secret trick, but it shouldn't be secret. Did I miss something?

[James]

Replying to cirrus:

jamrial, I can't find documentation on this requirement in the FFmpeg API documentation. Quite contrary, none of the code samples has this call.

If you check ffplay.c and ffprobe.c you'll see this in action (ffmpeg.c has yet to be updated to this API).

Similarly, i recently pushed a patch for the demuxing_decoding example as commit bba6a03b2816d805d44bce4f9701a71f7d3f8dad.

I'm not quite happy with the situation. I closed the bug report on the MPD bug tracker as an apparent FFmpeg bug, because I couldn't find anything that's wrong with MPD.

This looks like a secret trick, but it shouldn't be secret. Did I miss something?

No, you didn't. The doxygen documentation in avcodec.h for avcodec_open2() doesn't mention it because an input AVStream is not required unless you're demuxing with libavformat.
I agree it should be mentioned somewhere, though.

Basically, since AVStream.AVCodecContext (which has all the stream's parameters already filled) shouldn't be used anymore, the manually allocated AVCodecContext needs to be filled with said parameters in some way, and as its doxy states that's what the above function does.

[Stefano Sabatini]

Fixed in ​dcf963f4906fa59.