学步园

代码截取自FFMPEG官方example，添加注释以FFMPEG的编码流程就变得很清楚了

static void video_encode_example(const char *filename, int codec_id)
{
                                                                    //1.定义如下数据结构
    AVCodec *codec;
    AVCodecContext *c= NULL;
    int i, ret, x, y, got_output;
    FILE *f;
    AVFrame *frame;
    AVPacket pkt;
    uint8_t endcode[] = { 0, 0, 1, 0xb7 };//MPEG文件尾						  

    printf("Encode video file %s\n", filename);
    
    /* find the mpeg1 video encoder */
    codec = avcodec_find_encoder(codec_id);                         //2.查找编码器
    if (!codec) {
        fprintf(stderr, "Codec not found\n");
        exit(1);
    }
    
    c = avcodec_alloc_context3(codec);                              //3.分配编码器结构体
    if (!c) {
        fprintf(stderr, "Could not allocate video codec context\n");
        exit(1);
    }
    
    /* put sample parameters */                                     //4.设置编码器参数
    c->bit_rate = 400000;
    /* resolution must be a multiple of two */
    c->width = 352;
    c->height = 288;
    /* frames per second */
    c->time_base= (AVRational){1,25};
    c->gop_size = 10; /* emit one intra frame every ten frames */
    c->max_b_frames=1;
    c->pix_fmt = AV_PIX_FMT_YUV420P;

    if(codec_id == AV_CODEC_ID_H264)
        av_opt_set(c->priv_data, "preset", "slow", 0);

    
    /* open it */
    if (avcodec_open2(c, codec, NULL) < 0) {                        //5.打开编码器
        fprintf(stderr, "Could not open codec\n");
        exit(1);
    }
    
    f = fopen(filename, "wb");                                      //6.打开文件
    if (!f) {
        fprintf(stderr, "Could not open %s\n", filename);
        exit(1);
    }
    
    frame = avcodec_alloc_frame();                                  //7.分配帧结构体
    if (!frame) {
        fprintf(stderr, "Could not allocate video frame\n");
        exit(1);
    }
    frame->format = c->pix_fmt;
    frame->width  = c->width;
    frame->height = c->height;

    /* the image can be allocated by any means and av_image_alloc() is
     * just the most convenient way if av_malloc() is to be used */
                                                                    //8.分配帧数据空间frame->data， 
    ret = av_image_alloc(frame->data, frame->linesize, c->width, c->height,
                         c->pix_fmt, 32);
    if (ret < 0) {
        fprintf(stderr, "Could not allocate raw picture buffer\n");
        exit(1);
    }

    /* encode 1 second of video */
    for(i=0;i<25;i++) {       
        av_init_packet(&pkt);                                        //9.初始化 packet 结构体，packet数据由编码器分配
        pkt.data = NULL;    // packet data will be allocated by the encoder
        pkt.size = 0;

        fflush(stdout);
        /* prepare a dummy image */
        /* Y */
                                                                    //10.设定帧数据
        for(y=0;y<c->height;y++) {
            for(x=0;x<c->width;x++) {
                frame->data[0][y * frame->linesize[0] + x] = x + y + i * 3;
            }
        }

        /* Cb and Cr */
        for(y=0;y<c->height/2;y++) {
            for(x=0;x<c->width/2;x++) {
                frame->data[1][y * frame->linesize[1] + x] = 128 + y + i * 2;
                frame->data[2][y * frame->linesize[2] + x] = 64 + x + i * 5;
            }
        }

        frame->pts = i;

        /* encode the image */                                      //11.编码一帧数据到 packet 中        
        ret = avcodec_encode_video2(c, &pkt, frame, &got_output);
        if (ret < 0) {
            fprintf(stderr, "Error encoding frame\n");
            exit(1);
        }

        if (got_output) {
            printf("Write frame %3d (size=%5d)\n", i, pkt.size);
            fwrite(pkt.data, 1, pkt.size, f);
            av_free_packet(&pkt);
        }
    }

    /* get the delayed frames */                                    //12.编码延迟帧，avcodec_encode_video2()中 frame参数置为NULL
    for (got_output = 1; got_output; i++) {
        fflush(stdout);

        ret = avcodec_encode_video2(c, &pkt, NULL, &got_output);    
        if (ret < 0) {
            fprintf(stderr, "Error encoding frame\n");
            exit(1);
        }

        if (got_output) {
            printf("Write frame %3d (size=%5d)\n", i, pkt.size);
            fwrite(pkt.data, 1, pkt.size, f);
            av_free_packet(&pkt);                                   //13.释放 packet
        }
    }

    /* add sequence end code to have a real mpeg file */
    
    fwrite(endcode, 1, sizeof(endcode), f);                         //14.写入MPEG文件尾，close文件和结构体，释放指针数据
    fclose(f);

    avcodec_close(c);
    av_free(c);
    av_freep(&frame->data[0]);
    avcodec_free_frame(&frame);
    printf("\n");
}