ffmpeg超详细综合教程——摄像头直播

本文的示例将实现：读取PC摄像头视频数据并以RTMP协议发送为直播流。示例包含了
1、ffmpeg的libavdevice的使用
2、视频解码、编码、推流的基本流程
具有较强的综合性。
要使用libavdevice的相关函数，首先需要注册相关组件

avdevice_register_all();

接下来我们要列出电脑中可用的dshow设备

AVFormatContext *pFmtCtx = avformat_alloc_context();
    AVDeviceInfoList *device_info = NULL;
    AVDictionary* options = NULL;
    av_dict_set(&options, "list_devices", "true", 0);
    AVInputFormat *iformat = av_find_input_format("dshow");
    printf("Device Info=============
");
    avformat_open_input(&pFmtCtx, "video=dummy", iformat, &options);
    printf("========================
");

可以看到这里打开设备的步骤基本与打开文件的步骤相同，上面的代码中设置了AVDictionary，这样与在命令行中输入下列命令有相同的效果

ffmpeg -list_devices true -f dshow -i dummy

以上语句得到的结果如下

这里我的电脑上只有一个虚拟摄像头软件虚拟出来的几个dshow设备，没有音频设备，所以有如上的结果。
需要说明的是，avdevice有一个avdevice_list_devices函数可以枚举系统的采集设备，包括设备名和设备描述，非常适合用于让用户选择要使用的设备，但是不支持dshow设备，所以这里没有使用它。
下一步就可以像打开普通文件一样将上面的具体设备名作为输入打开，并进行相应的初始化设置，如下

av_register_all();
    //Register Device
    avdevice_register_all();
    avformat_network_init();

    //Show Dshow Device
    show_dshow_device();

    printf("
Choose capture device: ");
    if (gets(capture_name) == 0)
    {
        printf("Error in gets()
");
        return -1;
    }
    sprintf(device_name, "video=%s", capture_name);

    ifmt=av_find_input_format("dshow");

    //Set own video device's name
    if (avformat_open_input(&ifmt_ctx, device_name, ifmt, NULL) != 0){
        printf("Couldn't open input stream.（无法打开输入流）
");
        return -1;
    }
    //input initialize
    if (avformat_find_stream_info(ifmt_ctx, NULL)<0)
    {
        printf("Couldn't find stream information.（无法获取流信息）
");
        return -1;
    }
    videoindex = -1;
    for (i = 0; i<ifmt_ctx->nb_streams; i++)
        if (ifmt_ctx->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO)
        {
            videoindex = i;
            break;
        }
    if (videoindex == -1)
    {
        printf("Couldn't find a video stream.（没有找到视频流）
");
        return -1;
    }
    if (avcodec_open2(ifmt_ctx->streams[videoindex]->codec, avcodec_find_decoder(ifmt_ctx->streams[videoindex]->codec->codec_id), NULL)<0)
    {
        printf("Could not open codec.（无法打开解码器）
");
        return -1;
    }

在选择了输入设备并进行相关初始化之后，需要对输出做相应的初始化。ffmpeg将网络协议和文件同等看待，同时因为使用RTMP协议进行传输，这里我们指定输出为flv格式，编码器使用H.264

//output initialize
    avformat_alloc_output_context2(&ofmt_ctx, NULL, "flv", out_path);
    //output encoder initialize
    pCodec = avcodec_find_encoder(AV_CODEC_ID_H264);
    if (!pCodec){
        printf("Can not find encoder! (没有找到合适的编码器！)
");
        return -1;
    }
    pCodecCtx=avcodec_alloc_context3(pCodec);
    pCodecCtx->pix_fmt = PIX_FMT_YUV420P;
    pCodecCtx->width = ifmt_ctx->streams[videoindex]->codec->width;
    pCodecCtx->height = ifmt_ctx->streams[videoindex]->codec->height;
    pCodecCtx->time_base.num = 1;
    pCodecCtx->time_base.den = 25;
    pCodecCtx->bit_rate = 400000;
    pCodecCtx->gop_size = 250;
    /* Some formats,for example,flv, want stream headers to be separate. */
    if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER)
        pCodecCtx->flags |= CODEC_FLAG_GLOBAL_HEADER;

    //H264 codec param
    //pCodecCtx->me_range = 16;
    //pCodecCtx->max_qdiff = 4;
    //pCodecCtx->qcompress = 0.6;
    pCodecCtx->qmin = 10;
    pCodecCtx->qmax = 51;
    //Optional Param
    pCodecCtx->max_b_frames = 3;
    // Set H264 preset and tune
    AVDictionary *param = 0;
    av_dict_set(¶m, "preset", "fast", 0);
    av_dict_set(¶m, "tune", "zerolatency", 0);

    if (avcodec_open2(pCodecCtx, pCodec,¶m) < 0){
        printf("Failed to open encoder! (编码器打开失败！)
");
        return -1;
    }

    //Add a new stream to output,should be called by the user before avformat_write_header() for muxing
    video_st = avformat_new_stream(ofmt_ctx, pCodec);
    if (video_st == NULL){
        return -1;
    }
    video_st->time_base.num = 1;
    video_st->time_base.den = 25;
    video_st->codec = pCodecCtx;

    //Open output URL,set before avformat_write_header() for muxing
    if (avio_open(&ofmt_ctx->pb,out_path, AVIO_FLAG_READ_WRITE) < 0){
    printf("Failed to open output file! (输出文件打开失败！)
");
    return -1;
    }

    //Show some Information
    av_dump_format(ofmt_ctx, 0, out_path, 1);

    //Write File Header
    avformat_write_header(ofmt_ctx,NULL);

完成输入和输出的初始化之后，就可以正式开始解码和编码并推流的流程了，这里要注意，摄像头数据往往是RGB格式的，需要将其转换为YUV420P格式，所以要先做如下的准备工作

//prepare before decode and encode
    dec_pkt = (AVPacket *)av_malloc(sizeof(AVPacket));
    //enc_pkt = (AVPacket *)av_malloc(sizeof(AVPacket));
    //camera data has a pix fmt of RGB,convert it to YUV420
    img_convert_ctx = sws_getContext(ifmt_ctx->streams[videoindex]->codec->width, ifmt_ctx->streams[videoindex]->codec->height,
        ifmt_ctx->streams[videoindex]->codec->pix_fmt, pCodecCtx->width, pCodecCtx->height, PIX_FMT_YUV420P, SWS_BICUBIC, NULL, NULL, NULL);
    pFrameYUV = avcodec_alloc_frame();
    uint8_t *out_buffer = (uint8_t *)av_malloc(avpicture_get_size(PIX_FMT_YUV420P, pCodecCtx->width, pCodecCtx->height));
    avpicture_fill((AVPicture *)pFrameYUV, out_buffer, PIX_FMT_YUV420P, pCodecCtx->width, pCodecCtx->height);

下面就可以正式开始解码、编码和推流了

//start decode and encode
    int64_t start_time=av_gettime();
    while (av_read_frame(ifmt_ctx, dec_pkt) >= 0){
        if (exit_thread)
            break;
        av_log(NULL, AV_LOG_DEBUG, "Going to reencode the frame
");
        pframe = av_frame_alloc();
        if (!pframe) {
            ret = AVERROR(ENOMEM);
            return -1;
        }
        //av_packet_rescale_ts(dec_pkt, ifmt_ctx->streams[dec_pkt->stream_index]->time_base,
        //  ifmt_ctx->streams[dec_pkt->stream_index]->codec->time_base);
        ret = avcodec_decode_video2(ifmt_ctx->streams[dec_pkt->stream_index]->codec, pframe,
            &dec_got_frame, dec_pkt);
        if (ret < 0) {
            av_frame_free(&pframe);
            av_log(NULL, AV_LOG_ERROR, "Decoding failed
");
            break;
        }
        if (dec_got_frame){
            sws_scale(img_convert_ctx, (const uint8_t* const*)pframe->data, pframe->linesize, 0, pCodecCtx->height, pFrameYUV->data, pFrameYUV->linesize);

            enc_pkt.data = NULL;
            enc_pkt.size = 0;
            av_init_packet(&enc_pkt);
            ret = avcodec_encode_video2(pCodecCtx, &enc_pkt, pFrameYUV, &enc_got_frame);
            av_frame_free(&pframe);
            if (enc_got_frame == 1){
                //printf("Succeed to encode frame: %5d	size:%5d
", framecnt, enc_pkt.size);
                framecnt++;
                enc_pkt.stream_index = video_st->index;

                //Write PTS
                AVRational time_base = ofmt_ctx->streams[videoindex]->time_base;//{ 1, 1000 };
                AVRational r_framerate1 = ifmt_ctx->streams[videoindex]->r_frame_rate;// { 50, 2 };
                AVRational time_base_q = { 1, AV_TIME_BASE };
                //Duration between 2 frames (us)
                int64_t calc_duration = (double)(AV_TIME_BASE)*(1 / av_q2d(r_framerate1));  //内部时间戳
                //Parameters
                //enc_pkt.pts = (double)(framecnt*calc_duration)*(double)(av_q2d(time_base_q)) / (double)(av_q2d(time_base));
                enc_pkt.pts = av_rescale_q(framecnt*calc_duration, time_base_q, time_base);
                enc_pkt.dts = enc_pkt.pts;
                enc_pkt.duration = av_rescale_q(calc_duration, time_base_q, time_base); //(double)(calc_duration)*(double)(av_q2d(time_base_q)) / (double)(av_q2d(time_base));
                enc_pkt.pos = -1;

                //Delay
                int64_t pts_time = av_rescale_q(enc_pkt.dts, time_base, time_base_q);
                int64_t now_time = av_gettime() - start_time;
                if (pts_time > now_time)
                    av_usleep(pts_time - now_time);

                ret = av_interleaved_write_frame(ofmt_ctx, &enc_pkt);
                av_free_packet(&enc_pkt);
            }
        }
        else {
            av_frame_free(&pframe);
        }
        av_free_packet(dec_pkt);
    }

解码部分比较简单，编码部分需要自己计算PTS、DTS，比较复杂。这里通过帧率计算PTS和DTS
首先通过帧率计算每两帧之间的时间间隔，但是要换算