Qt--在显示视频的Label上画矩形

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 <QLabel>
#include <QPoint>
#include <QPainter>
#include <QColor>
#include <QPaintEvent>
#include <QPen>
#include<QPainter>
#include <QWidget>

#include <QDebug>
#include <QMouseEvent>

#include <QPixmap>

class MyLabel : public QLabel
{
    //Q_OBJECT
public:
    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 <QApplication>

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 <QWidget>
#include <QTimer>
#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_OBJECT

public:
    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