How can the bit_rate in AVFormatContext be altered?

Hello, I’m seeking guidance on adjusting the bit_rate within AVFormatContext.

Could you provide detailed instructions on how to change the bit_rate in AVFormatContext? I’m currently working with a basic setup for pushing RTMP video streams, as shown in the provided minimal reproducible example. My objective is to gain control over the video’s bitrate during this process. How can I achieve this?

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Hey Sahil!
To alter the bit_rate in AVFormatContext, you’ll primarily focus on changing the bit_rate attribute in the AVCodecContext associated with your AVFormatContext. This adjustment should be made before the codec is opened with avcodec_open2(). Here’s a brief code example:

avformat_alloc_output_context2(&oc, NULL, "flv", output_url);

AVStream *video_st = avformat_new_stream(oc, NULL);

AVCodecContext *c = video_st->codec;

c->bit_rate = your_target_bitrate;

avcodec_open2(c, codec, NULL);

Replace “your_target_bitrate” with the bitrate you’re aiming for.

Okay, I set the bitrate before opening the codec. But how do I maintain this bitrate throughout the streaming?

That’s an important aspect. Bitrate consistency is maintained by the codec’s rate control mechanism. After setting the bit_rate, you should also configure the rate control settings. For example, with H.264 codec, you could use:

c->rc_min_rate = c->bit_rate;

c->rc_max_rate = c->bit_rate;

c->rc_buffer_size = c->bit_rate;

This configures the stream for a constant bitrate. Remember, different codecs and settings can impact the output video’s quality and file size.

What if I encounter quality issues or the actual bitrate differs significantly from what I set?

In case of quality issues or bitrate variations, you might need to fine-tune other encoding parameters. Adjusting settings like gop_size, max_b_frames, or trying different rate control modes, such as CRF for x264, can help. Each adjustment can significantly influence the video’s quality and bitrate consistency.

Here’s how to set them in your AVCodecContext:

c->gop_size = 12; // defines the number of frames in a GOP

c->max_b_frames = 2; // sets the max number of B-frames

// For CRF mode in x264

av_opt_set(c->priv_data, "crf", "23", 0); // 23 is a CRF value, change as needed

The best settings will depend on your specific requirements for video quality and bitrate, and also on the encoder’s capabilities.

After setting your parameters, they should be automatically utilized by the encoder during the streaming. However, it’s wise to test your stream. Use tools like FFmpeg’s ffprobe to monitor the output video’s bitrate and quality. If the results aren’t as expected, you might need to tweak your settings to achieve the desired balance for your project.