1. Concept of double buffer

• Double buffered drawing is when drawing , First draw everything on a drawing device , And then the whole image is drawn on the part and displayed .
• Using double buffer drawing can avoid flicker when displaying .
• from Qt 4.0 Start ,QWidget All drawing of the part automatically uses double buffering , So generally there is no need to paintEvent() Function to avoid flicker .
• But to achieve some graphic effects , Or with the help of the concept of double buffer .

2. Application examples and code

• The following program realizes the function of drawing a rectangle of any size on the interface with the mouse .
  
• .h file

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>

namespace Ui {
    
class Widget;
}

class Widget : public QWidget
{
    
    Q_OBJECT

public:
    explicit Widget(QWidget *parent = 0);
    ~Widget();

private:
    Ui::Widget *ui;

    QPixmap pix;        // buffer 
    QPixmap tempPix;    // Temporary buffer 
    QPoint startPoint;
    QPoint endPoint;
    bool isDrawing;     // Are you drawing 

protected:
    void mousePressEvent(QMouseEvent * event);
    void mouseMoveEvent(QMouseEvent * event);
    void mouseReleaseEvent(QMouseEvent * event);
    void paintEvent(QPaintEvent * event);
};

#endif // WIDGET_H

• .cpp file

#include "widget.h"
#include "ui_widget.h"
#include <QMouseEvent>
#include <QPainter>

Widget::Widget(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Widget)
{
    
    ui->setupUi(this);

    pix = QPixmap(400,300);
    pix.fill(Qt::white);
    tempPix = pix;
    isDrawing = false;
}

Widget::~Widget()
{
    
    delete ui;
}

void Widget::mousePressEvent(QMouseEvent *event)
{
    
    if(event->button()==Qt::LeftButton)
    {
    
        // Get the starting point of the rectangle 
        startPoint = event->pos();
        isDrawing = true;
    }
}

void Widget::mouseMoveEvent(QMouseEvent *event)
{
    
    // For mouse movement events ,event->button() The return value is Qt::NoButton
    //event->buttons() The return value is all the buttons pressed 
    //if(event->buttons()&Qt::LeftButton) // As long as the return value contains the left mouse button press , Just meet the requirements , For example, press the left and right mouse buttons at the same time 
    if(event->buttons()==Qt::LeftButton)  // The return value can only contain one method of pressing the left mouse button , To meet the requirements 
    {
    
        // Get the end point of the rectangle 
        endPoint = event->pos();
        // Copy the contents of the buffer to the temporary buffer 
        tempPix=pix;
        update();
    }
}

void Widget::mouseReleaseEvent(QMouseEvent *event)
{
    
    if(event->button()==Qt::LeftButton)
    {
    
        endPoint=event->pos();
        isDrawing=false;
        update();
    }
}

void Widget::paintEvent(QPaintEvent *event)
{
    
    int x=startPoint.x();
    int y=startPoint.y();
    int width=endPoint.x()-x;
    int height=endPoint.y()-y;
    QPainter painter;
    painter.begin(&tempPix);
    painter.drawRect(x,y,width,height);
    painter.end();
    painter.begin(this);
    painter.drawPixmap(0,0,tempPix);
    if(!isDrawing)
        pix=tempPix;
}

3. elastic

• Qt Provided in QRubberBand Class to implement the rubber band .