When the button is clicked, I create 16 threads in Qt, and then pass the rtsp data address and the label to be rendered to the process, and then the process does this:
run:

void rtspthread::run()
{

    while(!shouldStop){
        openRtspStream(rtspUrl.toUtf8().constData(),index);
    }

    qDebug() << "RTSP stream stopped.";
    emit finished();  
}

open input stream:

void rtspthread::openRtspStream(const char* rtspUrl,int index)

{

    AVDictionary *options = nullptr;
    AVFrame *pFrameRGB = nullptr;
    uint8_t *pOutBuffer = nullptr;
    struct SwsContext *swsContext;
    AVFormatContext *pFormatCtx = nullptr;
    pFormatCtx = avformat_alloc_context();
    av_dict_set(&options, "rtsp_transport", "tcp", 0);
    av_dict_set(&options, "maxrate", "4000k", 0);
    if (avformat_open_input(&pFormatCtx, rtspUrl, nullptr, &options) != 0) {
        printf("Couldn't open stream file.n");
        return;
    }

    if (avformat_find_stream_info(pFormatCtx, NULL)<0)
    {
        printf("Couldn't find stream information.n");
        return;
    }
    int videoStreamIndex = -1;
    for (int i = 0; i < pFormatCtx->nb_streams; i++) {
        if (pFormatCtx->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {
            videoStreamIndex = i;
            break;
        }
    }
    if (videoStreamIndex!=-1){
        AVStream* videoStream = pFormatCtx->streams[videoStreamIndex];
        
        AVCodecParameters* codecpar = videoStream->codecpar;
        const AVCodec* videoCodec = avcodec_find_decoder(codecpar->codec_id);

        AVCodecContext* videoCodecContext = avcodec_alloc_context3(videoCodec);

        avcodec_parameters_to_context(videoCodecContext,codecpar);

        avcodec_open2(videoCodecContext,videoCodec,nullptr);

        AVPixelFormat srcPixFmt = videoCodecContext->pix_fmt;
        QLabel* label = this->parentWidget->findChild<QLabel*>("videoLabel");
        int targetWidth = label->width();
        int targetHeight = label->height();
        
        pOutBuffer = (uint8_t*)av_malloc(av_image_get_buffer_size(AV_PIX_FMT_RGB32,
                                                                    videoCodecContext->width,
                                                                    videoCodecContext->height, 1));

        
        pFrameRGB = av_frame_alloc();
        av_image_fill_arrays(pFrameRGB->data, pFrameRGB->linesize, pOutBuffer,
                             AV_PIX_FMT_RGB32, videoCodecContext->width, videoCodecContext->height, 1);


        swsContext= sws_getContext(
            videoCodecContext->width,videoCodecContext->height,srcPixFmt,
            targetWidth, targetHeight,AV_PIX_FMT_RGB32,
            SWS_BICUBIC,nullptr,nullptr,nullptr
            );
        
        AVPacket packet;
        AVFrame* frame = av_frame_alloc();
        int frameCounter = 0;
        while (av_read_frame(pFormatCtx, &packet) >= 0) {
            if (shouldStop) {
                break;
            }
            if (packet.stream_index == videoStreamIndex) {
                
                int ret = avcodec_send_packet(videoCodecContext,&packet);
                int rets = avcodec_receive_frame(videoCodecContext, frame);
                if (rets < 0) {
                    qDebug() << "Error receiving frame from codec context";
                }
                
                sws_scale(swsContext, frame->data, frame->linesize, 0, videoCodecContext->height,
                          pFrameRGB->data, pFrameRGB->linesize);

                
                QImage img(pFrameRGB->data[0], targetWidth, targetHeight,
                           pFrameRGB->linesize[0], QImage::Format_RGB32);
                
                qDebug() << index;

                emit frameReady(img.copy(),index);


                QThread::msleep(30);  // 控制帧率
            }
            av_packet_unref(&packet);

        }
        av_frame_free(&frame);
        av_frame_free(&pFrameRGB);
        sws_freeContext(swsContext);
        avcodec_free_context(&videoCodecContext);
        avformat_close_input(&pFormatCtx);
        avformat_free_context(pFormatCtx);

    }


}

The video is stuck and has snow screen. I want to lower the resolution and reduce the snow screen. The server cannot change the resolution.