Qt5 Tutorial QBrush and QRect - 2020

In this tutorial, we will learn QBrush and QRect.

The QBrush class defines the fill pattern of shapes drawn by QPainter. A brush has a style, a color, a gradient and a texture.

A QRect can be constructed with a set of left, top, width and height integers:

QRect::QRect(int x, int y, int width, int height)

QRect::QRect(const QPoint & topLeft, const QSize & size)

QRect r1(100, 200, 11, 16);
QRect r2(QPoint(100, 200), QSize(11, 16));

Qt->Qt Gui Application:

We will override QWidget::paintEvent() so that we can create customized widgets and have complete control over their appearance.

Here is the modified dialog.h:

// dialog.h

#ifndef DIALOG_H
#define DIALOG_H

#include <QDialog>
#include <QPainter>
#include <QRect>
#include <QBrush>

namespace Ui {
class Dialog;
}

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

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

#endif // DIALOG_H

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

Here is the implementation file, dialog.cpp:

#include "dialog.h"
#include "ui_dialog.h"

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

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

void Dialog:: paintEvent(QPaintEvent *e)
{
    QPainter painter(this);

    int left = 100;
    int top = 50;
    int width = 200;
    int height = 150;

    // r1 and r2 are same
    QRect r1(left, top, width, height);
    QRect r2(QPoint(left, top), QSize(width, height));

    painter.setPen(Qt::blue);
    painter.drawRect(r1);
    // r1 as a bounding box
    painter.drawEllipse(r1);
}

In the paintEvent(), we created QPainter object. Then, we constructed a rectangle in two ways, and draw the rectangle and an ellipse with the rectangle as a bounding box.

If we run the code, we get:

We can use a QBrush.
Brushes do fill and Pens do outline.

Here we reimplemented the paintEvent() again:

void Dialog:: paintEvent(QPaintEvent *e)
{
    QPainter painter(this);

    int left = 100;
    int top = 50;
    int width = 200;
    int height = 150;

    // r1 and r2 are same
    QRect r1(left, top, width, height);
    QRect r2(QPoint(left, top), QSize(width, height));

    painter.setPen(Qt::blue);

    QBrush brush(Qt::red, Qt::CrossPattern);

    painter.drawRect(r1);
    painter.fillRect(r1, brush);

    // r1 as a bounding box
    painter.drawEllipse(r1);


}

Run the code, then we will have: