Qt5 Tutorial QPainter - 2020

In this tutorial, we will learn QPainter.

The common use of QPainter is inside a widget's paint event: Construct and customize (e.g. set the pen or the brush) the painter. QPainter provides highly optimized functions to do most of the drawing GUI programs require. It can draw everything from simple lines to complex shapes like pies and chords. It can also draw aligned text and pixmaps.

Qt->Qt Gui Application:

By reimplementing QWidget::paintEvent(), we can create customized widgets and have complete control over their appearance.

Here is the modified painterdialog.h:

// painterdialog.h

#ifndef PAINTERDIALOG_H
#define PAINTERDIALOG_H

#include <QDialog>
#include <QPainter>

namespace Ui {
class PainterDialog;
}

class PainterDialog : public QDialog
{
    Q_OBJECT
    
public:
    explicit PainterDialog(QWidget *parent = 0);
    ~PainterDialog();
    
private:
    Ui::PainterDialog *ui;

protected:
    void paintEvent(QPaintEvent *e);
};

#endif // PAINTERDIALOG_H

Here, we're going to reimplement paintEvent().

Here is the implementation file, painterdialog.cpp:

#include "painterdialog.h"
#include "ui_painterdialog.h"

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

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

void PainterDialog:: paintEvent(QPaintEvent *e)
{
    QPainter painter(this);
    painter.setPen(Qt::blue);
    painter.setFont(QFont("Arial", 80));
    painter.drawText(rect(), Qt::AlignCenter, "Qt");
}

In the paintEvent(), we created QPainter object. Then, we the pen color, font type, font size, and the text itself with alignment.

If we run the code, we get:

We can draws a line as well as points. Here we reimplemented the paintEvent() again:

void PainterDialog:: paintEvent(QPaintEvent *e)
{
    QPainter MyPainter(this);

    QPen PointPen(Qt::red);
    PointPen.setWidth(5);

    QPen LinePen(Qt::green);
    LinePen.setWidth(2);

    QPoint p1;
    p1.setX(100);
    p1.setY(100);

    QPoint p2;
    p2.setX(300);
    p2.setY(200);

    MyPainter.setPen(PointPen);
    MyPainter.drawPoint(p1);
    MyPainter.drawPoint(p2);

    MyPainter.setPen(LinePen);
    MyPainter.drawLine(p1, p2);

    QPen LinePen2(Qt::black);
    LinePen2.setStyle( Qt::DashDotLine );
    LinePen2.setWidth(3);

    MyPainter.setPen(LinePen2);
    MyPainter.drawLine(QPoint(300,100), QPoint(100,200));
}

Run the code, then we will have: