[Libav-user] Setting bitrate for x264 encoder?

[Max Vlasov]

I think you should set frame pts before avcodec_encode_video2. After
avcodec_encode_video2 this value is transferred to the packet by the
encoder where it should be converted into the stream time base.

On Mon, Apr 6, 2015 at 10:10 PM, Philip Schneider <pjschneider at earthlink.net
> wrote:

> Greetings -
>
> I’m attempting to encode a sequence of frames with libx264. For testing,
> I’m using the sample code from
> http://www.imc-store.com.au/Articles.asp?ID=276. This is all pretty
> vanilla FFmpeg API usage, and is similar to what one might write oneself.
> It produces correct output with libx264, however the bitrate I specify is
> not honored at all…
>
> I can set the bitrate (m_AVIMOV_BPS) to whatever number I choose, and the
> actual bitrate used is some other value:
>  AVCodec *m_video_codec = avcodec_find_encoder(m_fmt->video_codec);
> if (!(m_video_codec)) {
>     return;
> }
> AVStream       *st  = avformat_new_stream(m_oc, m_video_codec);
> AVCodecContext *m_c = st->codec;
>
>
> m_c->codec_id = m_fmt->video_codec;
> m_c->bit_rate = m_AVIMOV_BPS;
>
>
> Googling this issue, I find that it’s the subject of a lot of discussion.
> Evidently, simply setting the bit rate in the context is not sufficient — I
> can set m_AVIMOV_BPS to 400 million, but the bitrate used in encoding
> ends up being something like 176 Kbps (using the info inspector in one of
> any number of players/utilities). As well, visually I see significant
> undersampling artifacts. I assume the effective bitrate used is computed by
> ffmpeg or the encoder itself, based on some other (default) parameters. But
> in any case, my value is being ignored, making the output useless to me.
>
> One forum I post I found (
> http://libav-users.943685.n4.nabble.com/Setting-libx264-bitrate-via-API-td4655453.html )
> suggests that the reason the encoder (or ffmpeg itself?) is ignoring the
> bit rate is because of the use of “pts” and “dts” in the encoder and output
> stream writer. Specifically, the encoder’s input should use, say, integer
> frame numbers, while the stream writer should use values in its own time
> base:
>
> My code was sending pictures into the encoder using a pts in the stream's
>
> time_base of 1/90000 (e.g. 3003, 6006, 9009).  The solution was to first
>
> rescale the AVFrame's pts from the stream's time_base to the codec
> time_base
>
> to get a simple frame number (e.g. 1, 2, 3).
>
>
> pic->pts = av_rescale_q(pic->pts, ost->time_base, enc->time_base);
>
> avcodec_encode_video2(enc, &newpkt, pic, &got_packet_ptr);
>
>
> Then when a packet is received from the encoder, you need to rescale pts
> and
>
> dts back to the stream time_base.
>
>
> newpkt.pts = av_rescale_q(newpkt.pts, enc->time_base, ost->time_base);
>
> newpkt.dts = av_rescale_q(newpkt.dts, enc->time_base, ost->time_base);
>
> av_interleaved_write_frame(out, &newpkt);
>
>
> However, this is not working for me. I suspect it may be due to other
> differences between my code and theirs.. :-(
>
> In any case, surely someone out there has the understanding (and code
> snippets?) of how to get the libx264 encoder to honor the specified
> bitrate? Any help/pointers/advice/code would be greatly appreciated!
>
> Thanks!
>
>
>
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>
>
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