int writeJPEG(AVFrame* frame, int width, int height){
    const char* out_file = "hello_world.jpg";
    //新建一個輸出的AVFormatContext 並分配記憶體
    AVFormatContext* output_cxt = avformat_alloc_context();
    avformat_alloc_output_context2(&output_cxt, NULL, "singlejpeg", out_file);

    //設定輸出檔案的格式
    // output_cxt->oformat = av_guess_format("mjpeg",NULL,NULL);

    
 
//建立和初始化一個和該URL相關的AVIOContext
    if (avio_open(&output_cxt->pb, out_file, AVIO_FLAG_READ_WRITE) < 0){
        av_log(NULL, AV_LOG_ERROR, "不能開啟檔案  \n");
        return -1;
    }

    //構建新的Stream
    AVStream* stream = avformat_new_stream(output_cxt, NULL);
    if (stream == NULL){
        av_log(NULL, AV_LOG_ERROR, 
 
"建立AVStream失敗  \n");
        return -1;
    }
    //初始化AVStream資訊
    AVCodecContext* codec_cxt = stream->codec;

    codec_cxt->codec_id = output_cxt->oformat->video_codec;
    codec_cxt->codec_type = AVMEDIA_TYPE_VIDEO;
    codec_cxt->pix_fmt = AV_PIX_FMT_YUVJ420P;
    codec_cxt->height = height;
    codec_cxt
 
->width = width;
    codec_cxt->time_base.num = 1;
    codec_cxt->time_base.den = 25;

    //列印輸出檔案資訊
    av_dump_format(output_cxt, 0, out_file, 1);

    AVCodec* codec = avcodec_find_encoder(codec_cxt->codec_id);
    if (!codec){
        av_log(NULL, AV_LOG_ERROR, "找不到編碼器  \n");
        return -1;
    }

    if (avcodec_open2(codec_cxt, codec, NULL) < 0){
        av_log(NULL, AV_LOG_ERROR, "不能開啟編碼器  \n");
        return -1;
    }
    avcodec_parameters_from_context(stream->codecpar, codec_cxt);

    //寫入檔案頭
    avformat_write_header(output_cxt, NULL);
    int size = codec_cxt->width * codec_cxt->height;

    AVPacket* packet;
    //av_new_packet(packet, size * 3);
    av_new_packet(packet, size);

    int got_picture = 0;
    int result = avcodec_encode_video2(codec_cxt, packet, frame, &got_picture);
    if (result < 0){
        av_log(NULL, AV_LOG_ERROR, "編碼失敗  \n");
        return -1;
    }
    printf("got_picture %d \n", got_picture);
    if (got_picture == 1){
        //將packet中的資料寫入本地檔案
        result = av_write_frame(output_cxt, packet);
    }
    av_free_packet(packet);
    //將流尾寫入輸出媒體檔案並釋放檔案資料
    av_write_trailer(output_cxt);
    if (frame){
        av_frame_unref(frame);
    }
    avio_close(output_cxt->pb);
    avformat_free_context(output_cxt);
    return 0;
}

void FFmpegDemo::on_pictureBtn1_clicked()
{
    // char* filename = "./jichi.mp4";
    //如果本地沒有視訊可以使用這個直播流地址
    char* filename = "./2.mp4";
    AVFormatContext* avf_cxt = avformat_alloc_context();
    int ret = avformat_open_input(&avf_cxt, filename, NULL, NULL);
    if (ret < 0){
        av_log(NULL, AV_LOG_ERROR, "不能開啟檔案\n");
        return;
    }
    ret = avformat_find_stream_info(avf_cxt, NULL);
    if (ret < 0){
        av_log(NULL, AV_LOG_ERROR, "找不到流資料\n");
        goto _end;
    }
    //列印視訊資訊
    av_dump_format(avf_cxt, 0, filename, 0);
    int video_index = -1;
    for (int i = 0; i < avf_cxt->nb_streams; i++){
        if (avf_cxt->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO){
            video_index = i;
            break;
        }
    }
    if (video_index == -1){
        av_log(NULL, AV_LOG_ERROR, "沒有找到視訊流\n");
        goto _end;
    }
    AVCodecContext* avc_cxt = avf_cxt->streams[video_index]->codec;
    enum AVCodecID codecId = avc_cxt->codec_id;
    AVCodec* codec = avcodec_find_decoder(codecId);
    if (!codec){
        av_log(NULL, AV_LOG_ERROR, "沒有找到解碼器\n");
        goto _end;
    }
    ret = avcodec_open2(avc_cxt, codec, NULL);
    if (ret < 0){
        av_log(NULL, AV_LOG_ERROR, "解碼器無法開啟\n");
        goto _end;
    }
    //為avpacket分配記憶體
    AVPacket* packet = av_packet_alloc();
    //為avFrame分配記憶體
    AVFrame* frame = av_frame_alloc();
    while (av_read_frame(avf_cxt, packet) >= 0){
        if (packet && packet->stream_index == video_index){
            int gotFrame = 0;
            ret = avcodec_decode_video2(avc_cxt, frame, &gotFrame, packet);
            if (gotFrame){
                ret = writeJPEG(frame, avc_cxt->width, avc_cxt->height);
                if (ret == 0){
                    break;
                }
            }
        }
    }

_end:
    av_frame_free(&frame);
    avcodec_close(avc_cxt);
    avformat_free_context(avf_cxt);
//    return 0;
}

來自：

https://www.iteye.com/blog/cmzx3444-965792
https://zhuanlan.zhihu.com/p/37697332
https://blog.csdn.net/qqqq245425070/article/details/87529209