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1、实现方法：需重写Label的鼠标事件和绘图事件

2、工程代码如下：

mylabel.h

#ifndef MyLabel_H#define MyLabel_H#include 
   
    #include 
    
     #include 
     
      #include 
      
       #include 
       
        #include 
        
         #include
         
          #include 
          
           #include 
           
            #include 
            
             #include 
             
              class MyLabel : public QLabel{ //Q_OBJECTpublic: explicit MyLabel(QWidget *parent = 0); ~MyLabel(); protected: void paintEvent(QPaintEvent *event); //绘制矩形 void mousePressEvent(QMouseEvent *e); //鼠标按下 void mouseMoveEvent(QMouseEvent *e); //鼠标移动 void mouseReleaseEvent(QMouseEvent *e); //鼠标抬起private: QPoint m_rectStartPoint; QPoint m_rectEndPoint;};#endif // MyLabel_H
             
            
           
          
         
        
       
      
     
    
   

mylabe.cpp

#include "mylabel.h"MyLabel::MyLabel(QWidget *parent /*= 0*/ ) : QLabel(parent){   }//鼠标按下void MyLabel::mousePressEvent(QMouseEvent *e){       m_rectStartPoint = e->pos();}//鼠标移动void MyLabel::mouseMoveEvent(QMouseEvent *e){       m_rectEndPoint = e->pos();}//鼠标抬起void MyLabel::mouseReleaseEvent(QMouseEvent *e){   }//绘图void MyLabel::paintEvent(QPaintEvent *event){       QLabel::paintEvent(event);      //先调用父类的paintEvent为了显示'背景'!!!    QPainter painter(this);    painter.setPen(QPen(Qt::green, 2));    if (m_rectStartPoint != m_rectEndPoint)    {           painter.drawRect(QRect(m_rectStartPoint, m_rectEndPoint));    }}//MyLabel::~MyLabel(){   }

main.cpp

#include "mywidget.h"#include 
   
    int main(int argc, char *argv[]){       QApplication a(argc, argv);    MyWidget w;    w.show();    return a.exec();}
   

mywidget.h

#ifndef MYWIDGET_H#define MYWIDGET_H#include 
   
    #include 
    
     #include "mylabel1.h"#include "opencv2/opencv.hpp"#include "opencv2/imgproc/imgproc.hpp"#include "opencv2/highgui/highgui.hpp"#include "opencv2/videoio/videoio.hpp"using namespace cv;namespace Ui {   class MyWidget;}class MyWidget : public QWidget{       Q_OBJECTpublic:    explicit MyWidget(QWidget *parent = 0);    ~MyWidget();    QImage mat2QImage(cv::Mat cvImg);    QImage ScaleImage2Label(QImage qImage, QLabel* qLabel);private slots:    void readFrame();private:    Ui::MyWidget *ui;    QTimer *timer;    Mat frame;    VideoCapture cap;    MyLabel *label;};#endif // MYWIDGET_H
    
   

mywidget.cpp

#include "mywidget.h"#include "ui_mywidget.h"MyWidget::MyWidget(QWidget *parent) :    QWidget(parent),    ui(new Ui::MyWidget){       ui->setupUi(this);    label = new MyLabel;    label->setStyleSheet("border:2px solid;border-color:blue");    ui->horizontalLayout->addWidget(label);    timer = new QTimer(this);    timer->start(10);    connect(timer, SIGNAL(timeout()), this, SLOT(readFrame()));    cap.open(1);}void MyWidget::readFrame(){      cap >> frame;   if(frame.empty())   {           qDebug()<<"frame is empty";        cap.release();   }   QImage img = ScaleImage2Label( mat2QImage(frame), label );   label->setAlignment(Qt::AlignCenter);   label->setPixmap(QPixmap::fromImage(img));}QImage MyWidget::ScaleImage2Label(QImage qImage, QLabel* qLabel){       QImage qScaledImage;    QSize qImageSize = qImage.size();    QSize qLabelSize = qLabel->size();    double dWidthRatio = 1.0*qImageSize.width() / qLabelSize.width();    double dHeightRatio = 1.0*qImageSize.height() / qLabelSize.height();    if (dWidthRatio > dHeightRatio)    {           qScaledImage = qImage.scaledToWidth(qLabelSize.width());    } else {           qScaledImage = qImage.scaledToHeight(qLabelSize.height());    }    return qScaledImage;}QImage MyWidget::mat2QImage(cv::Mat cvImg){       QImage qImg;    if (cvImg.channels() == 3)                             //3 channels color image    {           cv::cvtColor(cvImg, cvImg, CV_BGR2RGB);        qImg = QImage((const unsigned char*)(cvImg.data),                      cvImg.cols, cvImg.rows,                      cvImg.cols*cvImg.channels(),                      QImage::Format_RGB888);    }    else if (cvImg.channels() == 1)                    //grayscale image    {           qImg = QImage((const unsigned char*)(cvImg.data),                      cvImg.cols, cvImg.rows,                      cvImg.cols*cvImg.channels(),                      QImage::Format_Indexed8);    }    else    {           qImg = QImage((const unsigned char*)(cvImg.data),                      cvImg.cols, cvImg.rows,                      cvImg.cols*cvImg.channels(),                      QImage::Format_RGB888);    }    return qImg;}MyWidget::~MyWidget(){       delete ui;}

3、效果

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