[Libav-user] Bitrate parameter ignored when writing videos

[Paul B Mahol]

On 8/10/16, Masneri, Stefano <stefano.masneri at brain.mpg.de> wrote:
> I am trying to write an .mp4 video with a specified bitrate and gopsize set
> to 1.
> Apparently libavcodec ignores the settings I provide and creates a video
> with a much higher bitrate
>
> Here's the code I use to setup the output video:
>
>   m_pcVideoSt->codec->gop_size = m_iGopSize; /* emit one intra frame every
> m_iGopSize frames at most */
>   m_pcVideoSt->codec->pix_fmt = AV_PIX_FMT_YUV420P; /* default pix_fmt */
>   m_pcVideoSt->codec->max_b_frames = 1;
>   AVDictionary* pcOpts = nullptr;
>   int res1 = av_dict_set(&pcOpts, "b", "20k", 0);
>   int res = avcodec_open2(m_pcVideoSt->codec, codec, &pcOpts);
>
> m_pcVideoSt is of type AVStream*
>
> This is the output of ffprobe ran on the video I wrote:
>
> Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'testGray.mp4':
>   Metadata:
>     major_brand     : isom
>     minor_version   : 512
>     compatible_brands: isomiso2avc1mp41
>     encoder         : Lavf56.40.101
>   Duration: 00:00:20.00, start: 0.000000, bitrate: 8905 kb/s
>     Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p,
> 720x576, 8904 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
>     Metadata:
>       handler_name    : VideoHandler
>
> When watching the video, it seems that the quality is low for the first few
> frames, but then the quality gets much better (explaining the 8905Kb/s
> bitrate).
> Any idea on what I am missing?

Above I do not see relevant code that calls encoder.