widget.h

#ifndef WIDGET_H
#define WIDGET_H
extern "C"{
    #include "libavcodec/avcodec.h"
    #include "libavformat/avformat.h"
    #include "libswscale/swscale.h"
    #include "libavdevice/avdevice.h"
    #include "libavformat/avio.h"
    #include "libavutil/imgutils.h"
}

#include <QWidget>

class Widget : public QWidget
{
    Q_OBJECT
public:
    Widget(QWidget *parent = nullptr);
    ~Widget();
private:
    QImage image;
    int lineSize[4];
    AVFormatContext *formatContext;
    AVCodecParameters *codecParameters;
    int videoIndex;
    AVCodecContext* codecContext;
    AVPacket *packet;
    AVFrame *frame;
    SwsContext *imgConvertContext;

protected:
    virtual void paintEvent(QPaintEvent *event) override;
};
#endif // WIDGET_H

Widget.cpp

#include "widget.h"
#include <QDebug>
#include <QPainter>
#include <QTimer>

Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
    avdevice_register_all();                                    //初始化所有设备
    formatContext=avformat_alloc_context();                     //分配format上下文
    AVInputFormat *inputFormat=av_find_input_format("gdigrab"); //寻找输入设备【gdigrab】
    AVDictionary* options = NULL;
    av_dict_set(&options,"framerate","60",0);                   //设置帧数为60
    if(avformat_open_input(&formatContext,"desktop",inputFormat,&options)){ //开启输入设备
        qDebug()<<"cant`t open input stream.";
        return ;
    }   
    if(avformat_find_stream_info(formatContext,nullptr)){       //加载流中存储的信息
        qDebug()<<"can`t find stream information.";
        return ;
    }

    videoIndex=-1;                                              //寻找视频流
    for(uint i=0;i<formatContext->nb_streams;i++){
        if(formatContext->streams[i]->codecpar->codec_type==AVMEDIA_TYPE_VIDEO){
            videoIndex=i;
            break;
        }
    }
    if(videoIndex==-1){
        qDebug()<<"can`t find video stream.";
        return ;
    }
    codecParameters=formatContext->streams[videoIndex]->codecpar;
    codecContext=avcodec_alloc_context3(nullptr);
    avcodec_parameters_to_context(codecContext,codecParameters);
    AVCodec* codec=avcodec_find_decoder(codecParameters->codec_id);
    
    if(codec==nullptr){
        qDebug()<<"can`t find codec";
        return ;
    }
    if(avcodec_open2(codecContext,codec,nullptr)){
        qDebug()<<"can`t open codec";
        return ;
    }

    packet=av_packet_alloc();
    frame=av_frame_alloc();
   
    imgConvertContext=sws_getContext(codecParameters->width,codecParameters->height,codecContext->pix_fmt,codecParameters->width,codecParameters->height,AV_PIX_FMT_RGB24,SWS_BICUBIC,nullptr,nullptr,nullptr);
    image=QImage(codecParameters->width,codecParameters->height,QImage::Format_RGB888);
    av_image_fill_linesizes(lineSize,AV_PIX_FMT_RGB24,codecParameters->width);
    
    QTimer *timer=new QTimer;           //定时刷新
    connect(timer,&QTimer::timeout,this,static_cast<void (QWidget::*)()>(&QWidget::repaint));
    timer->setInterval(20);
    timer->start();

    resize(codecParameters->width*0.6,codecParameters->height*0.6);
}

Widget::~Widget()
{
}

void Widget::paintEvent(QPaintEvent *event)
{
    if(av_read_frame(formatContext,packet)){
        return ;
    }
    if(packet->stream_index==videoIndex){
        if(avcodec_send_packet(codecContext,packet))
            return;
        if(avcodec_receive_frame(codecContext,frame))
            return;
        uint8_t* dst[]={image.bits()};
        sws_scale(imgConvertContext,frame->data,frame->linesize,0,codecParameters->height,dst,lineSize);
        av_packet_unref(packet);                //清空数据包

        QPainter painter(this);
        painter.fillRect(rect(),image.scaled(width(),height(),Qt::IgnoreAspectRatio,Qt::SmoothTransformation));
    }
}

main.cpp

#include "widget.h"
#include <QDebug>
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication app(argc,argv);
    Widget w;
    w.show();
    return app.exec();
}