ffmpeg bit_rate 设置后，没有作用（libx264

1、码率

• 码率与视频大小关系：size = x / 8 * times,其中size为视频大小，x为码率单位为kbit，times为视频时长。
• 码率与视频的清晰度有关，相同分辨率下，码率越高视频的清晰度越好，但是码率高到一定程度后，清晰度反而没有太大的改变，带来的问题是文件越来越大，可以根据公式得到结论。

• 如何设置视频的码率呢？这里给出了一个表格，直接从网上copy下来的。当然这只是建议码率，实际还得看需求。

  
  建议码率

2、码率控制

• 这里主要推荐一篇文章，这哥们儿写的很好，分析了x264，x265，vpx的码率控制，以及各种控制方法的实用场景，这是个英文版本：https://slhck.info/video/2017/03/01/rate-control.html。 （当然也有中文版本，翻译的不全 https://www.jianshu.com/p/3488bca94d97。）

2.1 CRF码率控制方法

• 针对libx264，crf的建议值为18~23，越大码率越底，视频越小。这里有张图，也是前面所提到的哥们儿画的，图中横坐标为crf的取值，纵坐标为码率，不同颜色的线代表了不同分辨率的视频。

  
  crf与码率的关系

• 通过上一点的说明，可以直接使用crf进行码率控制，命令行如下，同时也给出了在ffmepg api中crf的设置，需要设置AVCodecContext，没有直接的参数，需要通过av_opt_set_int设置。

ffmpeg -i input.mp4 -c:v libx264 -crf 23 output.mp4
//对应的ffmepg api
av_opt_set_int(AVCodecContext->priv_data, "crf", 23, 0);

2.2 AVCodecContext中的bit_rate参数

• 问题：设置了bit_rate参数，为何码率很大，会达到几十兆bps。该问题针对h264编码。在使用h264_nvenc进行编码时没有这个问题，其他的编码方式没有试过。
• 从example中的muxing.c得到的灵感，在这个demo中，发现h264编码，设置的码率有效果。找到了一个解决方案，在编码之前将AVFrame的pts从0开始计算，依次累加，在写入文件时，调整pts，正如muxing.c中的代码。列出关键性代码（*a、c->bit_rate = 400000; b、ost->frame->pts = ost->next_pts++; c、av_packet_rescale_ts(pkt, time_base, st->time_base);），摘取了大部分代码：

static int write_frame(AVFormatContext *fmt_ctx, const AVRational *time_base, AVStream *st, AVPacket *pkt)
{
    /* rescale output packet timestamp values from codec to stream timebase */
    //转换时间
    av_packet_rescale_ts(pkt, *time_base, st->time_base);
    pkt->stream_index = st->index;

    /* Write the compressed frame to the media file. */
    log_packet(fmt_ctx, pkt);
    return av_interleaved_write_frame(fmt_ctx, pkt);
}

/* Add an output stream. */
static void add_stream(OutputStream *ost, AVFormatContext *oc,
                       AVCodec **codec,
                       enum AVCodecID codec_id)
{
    AVCodecContext *c;
    
    ...
    
    case AVMEDIA_TYPE_VIDEO:
        c->codec_id = codec_id;

        c->bit_rate = 400000;  //设置码率
        /* Resolution must be a multiple of two. */
        c->width    = 352;
        c->height   = 288;
        /* timebase: This is the fundamental unit of time (in seconds) in terms
         * of which frame timestamps are represented. For fixed-fps content,
         * timebase should be 1/framerate and timestamp increments should be
         * identical to 1. */
        ost->st->time_base = (AVRational){ 1, STREAM_FRAME_RATE };
        c->time_base       = ost->st->time_base;

        c->gop_size      = 12; /* emit one intra frame every twelve frames at most */
        c->pix_fmt       = STREAM_PIX_FMT;
        if (c->codec_id == AV_CODEC_ID_MPEG2VIDEO) {
            /* just for testing, we also add B-frames */
            c->max_b_frames = 2;
        }
        if (c->codec_id == AV_CODEC_ID_MPEG1VIDEO) {
            /* Needed to avoid using macroblocks in which some coeffs overflow.
             * This does not happen with normal video, it just happens here as
             * the motion of the chroma plane does not match the luma plane. */
            c->mb_decision = 2;
        }
    break;

    default:
        break;
    }

    /* Some formats want stream headers to be separate. */
    if (oc->oformat->flags & AVFMT_GLOBALHEADER)
        c->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
}

static AVFrame *get_video_frame(OutputStream *ost)
{
    AVCodecContext *c = ost->enc;

    /* check if we want to generate more frames */
    if (av_compare_ts(ost->next_pts, c->time_base,
                      STREAM_DURATION, (AVRational){ 1, 1 }) >= 0)
        return NULL;

    /* when we pass a frame to the encoder, it may keep a reference to it
     * internally; make sure we do not overwrite it here */
    if (av_frame_make_writable(ost->frame) < 0)
        exit(1);

    if (c->pix_fmt != AV_PIX_FMT_YUV420P) {
        /* as we only generate a YUV420P picture, we must convert it
         * to the codec pixel format if needed */
        if (!ost->sws_ctx) {
            ost->sws_ctx = sws_getContext(c->width, c->height,
                                          AV_PIX_FMT_YUV420P,
                                          c->width, c->height,
                                          c->pix_fmt,
                                          SCALE_FLAGS, NULL, NULL, NULL);
            if (!ost->sws_ctx) {
                fprintf(stderr,
                        "Could not initialize the conversion context\n");
                exit(1);
            }
        }
        fill_yuv_image(ost->tmp_frame, ost->next_pts, c->width, c->height);
        sws_scale(ost->sws_ctx, (const uint8_t * const *) ost->tmp_frame->data,
                  ost->tmp_frame->linesize, 0, c->height, ost->frame->data,
                  ost->frame->linesize);
    } else {
        fill_yuv_image(ost->frame, ost->next_pts, c->width, c->height);
    }

    ost->frame->pts = ost->next_pts++; //pts累加

    return ost->frame;
}


/**************************************************************/
/* media file output */

int main(int argc, char **argv)
{
    OutputStream video_st = { 0 }, audio_st = { 0 };
    const char *filename;
    AVOutputFormat *fmt;
    AVFormatContext *oc;
    AVCodec *audio_codec, *video_codec;
    int ret;
    int have_video = 0, have_audio = 0;
    int encode_video = 0, encode_audio = 0;
    AVDictionary *opt = NULL;
    int i;

    if (argc < 2) {
        printf("usage: %s output_file\n"
               "API example program to output a media file with libavformat.\n"
               "This program generates a synthetic audio and video stream, encodes and\n"
               "muxes them into a file named output_file.\n"
               "The output format is automatically guessed according to the file extension.\n"
               "Raw images can also be output by using '%%d' in the filename.\n"
               "\n", argv[0]);
        return 1;
    }

    filename = argv[1];
    for (i = 2; i+1 < argc; i+=2) {
        if (!strcmp(argv[i], "-flags") || !strcmp(argv[i], "-fflags"))
            av_dict_set(&opt, argv[i]+1, argv[i+1], 0);
    }

    /* allocate the output media context */
    avformat_alloc_output_context2(&oc, NULL, NULL, filename);
    if (!oc) {
        printf("Could not deduce output format from file extension: using MPEG.\n");
        avformat_alloc_output_context2(&oc, NULL, "mpeg", filename);
    }
    if (!oc)
        return 1;

    fmt = oc->oformat;

    /* Add the audio and video streams using the default format codecs
     * and initialize the codecs. */
    if (fmt->video_codec != AV_CODEC_ID_NONE) {
        add_stream(&video_st, oc, &video_codec, fmt->video_codec);
        have_video = 1;
        encode_video = 1;
    }
    if (fmt->audio_codec != AV_CODEC_ID_NONE) {
        add_stream(&audio_st, oc, &audio_codec, fmt->audio_codec);
        have_audio = 1;
        encode_audio = 1;
    }

    /* Now that all the parameters are set, we can open the audio and
     * video codecs and allocate the necessary encode buffers. */
    if (have_video)
        open_video(oc, video_codec, &video_st, opt);

    if (have_audio)
        open_audio(oc, audio_codec, &audio_st, opt);

    av_dump_format(oc, 0, filename, 1);

    /* open the output file, if needed */
    if (!(fmt->flags & AVFMT_NOFILE)) {
        ret = avio_open(&oc->pb, filename, AVIO_FLAG_WRITE);
        if (ret < 0) {
            fprintf(stderr, "Could not open '%s': %s\n", filename,
                    av_err2str(ret));
            return 1;
        }
    }

    /* Write the stream header, if any. */
    ret = avformat_write_header(oc, &opt);
    if (ret < 0) {
        fprintf(stderr, "Error occurred when opening output file: %s\n",
                av_err2str(ret));
        return 1;
    }

    while (encode_video || encode_audio) {
        /* select the stream to encode */
        if (encode_video &&
            (!encode_audio || av_compare_ts(video_st.next_pts, video_st.enc->time_base,
                                            audio_st.next_pts, audio_st.enc->time_base) <= 0)) {
            encode_video = !write_video_frame(oc, &video_st);
        } else {
            encode_audio = !write_audio_frame(oc, &audio_st);
        }
    }

    ...

    return 0;
}

• 关于这个问题，这里也有个回答：https://stackoverflow.com/questions/11466184/setting-video-bit-rate-through-ffmpeg-api-is-ignored-for-libx264-codec

参考

• http://blog.sina.com.cn/s/blog_5f7e17640100f1og.html
• https://www.zhihu.com/question/49460691
• https://slhck.info/video/2017/03/01/rate-control.html