[Libav-user] Problem Generating B-Frames Using libavcodec/libx264

Robert Krüger krueger at lesspain.de
Wed Nov 6 15:30:44 CET 2013

On Wed, Nov 6, 2013 at 3:23 PM, Robert Schmidt
<rob.f.schmidt86 at gmail.com> wrote:
> I finally tracked down the source of my issue and thought I should
> post the answer in case someone else has similar problems.
> The problem ended up not being my stream and encoder set up, as I
> originally thought, but how I was passing pictures to the encoder.
> Since my test utility simply transcodes video data, I was passing
> pictures emitted by the H.264 encoder to the libx264 encoder.
> However, the decoder sets the pict_type field of AVFrame structures it
> populates to AV_PICTURE_TYPE_P.  In addition, the libx264 encoder uses
> the pict_type field of incoming pictures to determine what kind of
> frame to output.  As a result, the encoder only produced P-frames
> since all of the incoming pictures specified that picture type.
> I updated the code to reset the pict_type field to
> AV_PICTURE_TYPE_NONE prior to passing the picture to the encoder.
> This allows the encoder to select the frame type it feels is best.
> Once I made this change, the encoder produced both B-frames and
> P-frames and my output was identical to what ffmpeg produced.
> This step is not done explicitly in any of the code samples since they
> normally allocate a new AVFrame structure for each frame they encode
> causing the pict_type field to already be set to AV_PICTURE_TYPE_NONE.
>  I also missed the line that was performing this step in the ffmpeg
> code when I was comparing it to my own.
> Hopefully this will save some grief for someone else should they
> encounter the same issue in the future.

thanks for the effort. I am sure it will.


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