[Libav-user] New api and deprecation warning
jodonk at gmail.com
Thu Mar 15 22:15:17 EET 2018
On Thu, Mar 15, 2018 at 4:54 AM, Michael IV <explomaster at gmail.com> wrote:
> *AVCodecParameters is for information that codecs need to do their workbut
> is stored in a structured way in the container format rather thanencoded in
> the bitstream. Codecs vary in what they need and what theyput in the
> bitstream, and formats vary in what they are able to encode,but there is
> enough common ground for quite a few fields
> inAVCodecParameters.AVCodecContext is for actual encoding en decoding. It
> needs theinformation from AVCodecParameters, but it also needs settings
> that arespecific to the process at hand.Complaining that AVCodecParameters
> does not have a flags field is likecomplaining that the TV magazine tells
> you how to turn the volumebutton on your TV.*
> Thanks you for this even more confusing explanation, Nicolas ))
> It is as clear as the FFMPEG API ;) I guess,I am not working enough with
> the av libs to completely understand those subtle ideas.
> Libav-user mailing list
> Libav-user at ffmpeg.org
Hello. I tried posting a very long version of questions along these lines
but it never made it to this list. This thread is so similar it's been
revealing and I hopped on IRC to get a few answers confirmed. Here is my
understanding, please feel free to correct me if I am wrong:
AVCodecContext is everything a codec needs to encode video (including
settings specific to the codec you're using).
AVCodecParameters is more like the "header info" (like a bitmap header)
shared for all codecs. Muxers and Demuxers will pretty much store this
header info somewhere.
The AVStream::codec deprecated threw me for a long time. But here is my
new understanding (confirmed on IRC):
1. Use avcodec_find_encoder_by_name result is AVCodec.
2. Pass that AVCodec to avformat_new_stream. The newly created AVStream
will take the information from AVCodec and store it in its
3. Create your own AVCodecContext.
4. Copy the parameters out of AVStream::codecpar into your own
AVCodecContext using avcodec_parameters_to_context.
5. Configure your own AVCodecContext (in my case, using AVOptions).
6. Copy the newly configured parameters from AVCodecContext back to your
AVStream::codecpar using avcodec_parameters_from_context.
7. Do all of your encoding using your AVCodecContext and forget that
AVStream::codec ever existed.
Given this workflow is the new blessed one, AVStream::codec must be
unused. Poking at the source it looks like internally, it checks to see if
AVStream::codecpar is mismatched with AVStream::codec, and if so, it
prefers AVStream::codec by copying it again into AVStream::codecpar.
However, all AVStream needs is AVStream::codecpar.
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