（转载）FFMpeg取一张图片

int writeJPEG(AVFrame* frame, int width, int height){
    const char* out_file = "hello_world.jpg";
    //新建一个输出的AVFormatContext 并分配内存
    AVFormatContext* output_cxt = avformat_alloc_context();
    avformat_alloc_output_context2(&output_cxt, NULL, "singlejpeg", out_file);

    //设置输出文件的格式
    // output_cxt->oformat = av_guess_format("mjpeg",NULL,NULL);

    //创建和初始化一个和该URL相关的AVIOContext
    if (avio_open(&output_cxt->pb, out_file, AVIO_FLAG_READ_WRITE) < 0){
        av_log(NULL, AV_LOG_ERROR, "不能打开文件  \n");
        return -1;
    }

    //构建新的Stream
    AVStream* stream = avformat_new_stream(output_cxt, NULL);
    if (stream == NULL){
        av_log(NULL, AV_LOG_ERROR, "创建AVStream失败  \n");
        return -1;
    }
    //初始化AVStream信息
    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;

    //打印输出文件信息
    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, "找不到编码器  \n");
        return -1;
    }

    if (avcodec_open2(codec_cxt, codec, NULL) < 0){
        av_log(NULL, AV_LOG_ERROR, "不能打开编码器  \n");
        return -1;
    }
    avcodec_parameters_from_context(stream->codecpar, codec_cxt);

    //写入文件头
    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, "编码失败  \n");
        return -1;
    }
    printf("got_picture %d \n", got_picture);
    if (got_picture == 1){
        //将packet中的数据写入本地文件
        result = av_write_frame(output_cxt, packet);
    }
    av_free_packet(packet);
    //将流尾写入输出媒体文件并释放文件数据
    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";
    //如果本地没有视频可以使用这个直播流地址
    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, "不能打开文件\n");
        return;
    }
    ret = avformat_find_stream_info(avf_cxt, NULL);
    if (ret < 0){
        av_log(NULL, AV_LOG_ERROR, "找不到流数据\n");
        goto _end;
    }
    //打印视频信息
    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, "没有找到视频流\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, "没有找到解码器\n");
        goto _end;
    }
    ret = avcodec_open2(avc_cxt, codec, NULL);
    if (ret < 0){
        av_log(NULL, AV_LOG_ERROR, "解码器无法打开\n");
        goto _end;
    }
    //为avpacket分配内存
    AVPacket* packet = av_packet_alloc();
    //为avFrame分配内存
    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;
}

来自：

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