[Libav-user] Setting bitrate for x264 encoder?

Philip Schneider pjschneider at earthlink.net
Mon Apr 6 21:10:45 CEST 2015

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 <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)) {
		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 <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!


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