Scribble Example▲

We reimplement the mouse event handlers to implement drawing, the paint event handler to update the application and the resize event handler to optimize the application's appearance. In addition we reimplement the close event handler to intercept the close events before terminating the application.

The example also demonstrates how to use QPainter to draw an image in real time, as well as to repaint widgets.

Screenshot of the Scribble example

With the Scribble application the users can draw an image. The File menu gives the users the possibility to open and edit an existing image file, save an image and exit the application. While drawing, the Options menu allows the users to choose the pen color and pen width, as well as clear the screen. In addition the Help menu provides the users with information about the Scribble example in particular, and about Qt in general.

The example consists of two classes:

  • ScribbleArea is a custom widget that displays a QImage and allows to the user to draw on it.

  • MainWindow provides a menu above the ScribbleArea.

We will start by reviewing the ScribbleArea class. Then we will review the MainWindow class, which uses ScribbleArea.

ScribbleArea Class Definition▲

 
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class ScribbleArea : public QWidget
{
    Q_OBJECT

public:
    ScribbleArea(QWidget *parent = nullptr);

    bool openImage(const QString &fileName);
    bool saveImage(const QString &fileName, const char *fileFormat);
    void setPenColor(const QColor &newColor);
    void setPenWidth(int newWidth);

    bool isModified() const { return modified; }
    QColor penColor() const { return myPenColor; }
    int penWidth() const { return myPenWidth; }

public slots:
    void clearImage();
    void print();

protected:
    void mousePressEvent(QMouseEvent *event) override;
    void mouseMoveEvent(QMouseEvent *event) override;
    void mouseReleaseEvent(QMouseEvent *event) override;
    void paintEvent(QPaintEvent *event) override;
    void resizeEvent(QResizeEvent *event) override;

private:
    void drawLineTo(const QPoint &endPoint);
    void resizeImage(QImage *image, const QSize &newSize);

    bool modified;
    bool scribbling;
    int myPenWidth;
    QColor myPenColor;
    QImage image;
    QPoint lastPoint;
};

The ScribbleArea class inherits from QWidget. We reimplement the mousePressEvent(), mouseMoveEvent() and mouseReleaseEvent() functions to implement the drawing. We reimplement the paintEvent() function to update the scribble area, and the resizeEvent() function to ensure that the QImage on which we draw is at least as large as the widget at any time.

We need several public functions: openImage() loads an image from a file into the scribble area, allowing the user to edit the image; save() writes the currently displayed image to file; clearImage() slot clears the image displayed in the scribble area. We need the private drawLineTo() function to actually do the drawing, and resizeImage() to change the size of a QImage. The print() slot handles printing.

We also need the following private variables:

  • modified is true if there are unsaved changes to the image displayed in the scribble area.

  • scribbling is true while the user is pressing the left mouse button within the scribble area.

  • penWidth and penColor hold the currently set width and color for the pen used in the application.

  • image stores the image drawn by the user.

  • lastPoint holds the position of the cursor at the last mouse press or mouse move event.

ScribbleArea Class Implementation▲

 
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ScribbleArea::ScribbleArea(QWidget *parent)
    : QWidget(parent)
{
    setAttribute(Qt::WA_StaticContents);
    modified = false;
    scribbling = false;
    myPenWidth = 1;
    myPenColor = Qt::blue;
}

In the constructor, we set the Qt::WA_StaticContents attribute for the widget, indicating that the widget contents are rooted to the top-left corner and don't change when the widget is resized. Qt uses this attribute to optimize paint events on resizes. This is purely an optimization and should only be used for widgets whose contents are static and rooted to the top-left corner.

 
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bool ScribbleArea::openImage(const QString &fileName)
{
    QImage loadedImage;
    if (!loadedImage.load(fileName))
        return false;

    QSize newSize = loadedImage.size().expandedTo(size());
    resizeImage(&loadedImage, newSize);
    image = loadedImage;
    modified = false;
    update();
    return true;
}

In the openImage() function, we load the given image. Then we resize the loaded QImage to be at least as large as the widget in both directions using the private resizeImage() function and we set the image member variable to be the loaded image. At the end, we call QWidget::update() to schedule a repaint.

 
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bool ScribbleArea::saveImage(const QString &fileName, const char *fileFormat)
{
    QImage visibleImage = image;
    resizeImage(&visibleImage, size());

    if (visibleImage.save(fileName, fileFormat)) {
        modified = false;
        return true;
    } else {
        return false;
    }
}

The saveImage() function creates a QImage object that covers only the visible section of the actual image and saves it using QImage::save(). If the image is successfully saved, we set the scribble area's modified variable to false, because there is no unsaved data.

 
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void ScribbleArea::setPenColor(const QColor &newColor)
{
    myPenColor = newColor;
}

void ScribbleArea::setPenWidth(int newWidth)
{
    myPenWidth = newWidth;
}

The setPenColor() and setPenWidth() functions set the current pen color and width. These values will be used for future drawing operations.

 
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void ScribbleArea::clearImage()
{
    image.fill(qRgb(255, 255, 255));
    modified = true;
    update();
}

The public clearImage() slot clears the image displayed in the scribble area. We simply fill the entire image with white, which corresponds to RGB value (255, 255, 255). As usual when we modify the image, we set modified to true and schedule a repaint.

 
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void ScribbleArea::mousePressEvent(QMouseEvent *event)
{
    if (event->button() == Qt::LeftButton) {
        lastPoint = event->pos();
        scribbling = true;
    }
}

void