Image Composition Example

Shows how composition modes work in QPainter .

The Image Composition example lets the user combine images together using any composition mode supported by QPainter , described in detail in Composition Modes .


Setting Up The Resource File

The Image Composition example requires two source images, butterfly.png and checker.png that are embedded within imagecomposition.qrc . The file contains the following code:

<Code snippet "/data/snapshot-qt5full-5.15/qt5/qtbase/painting/imagecomposition/imagecomposition.qrc" not found>

For more information on resource files, see The Qt Resource System .

ImageComposer Class Definition

The ImageComposer class is a subclass of QWidget that implements three private slots, chooseSource() , chooseDestination() , and recalculateResult() .

class ImageComposer : public QWidget


private slots:
    void chooseSource();
    void chooseDestination();
    void recalculateResult();

In addition, ImageComposer consists of five private functions, addOp() , chooseImage() , loadImage() , currentMode() , and imagePos() , as well as private instances of QToolButton , QComboBox , QLabel , and QImage .

    void addOp(QPainter::CompositionMode mode, const QString &name);
    void chooseImage(const QString &title, QImage *image, QToolButton *button);
    void loadImage(const QString &fileName, QImage *image, QToolButton *button);
    QPainter::CompositionMode currentMode() const;
    QPoint imagePos(const QImage &image) const;

    QToolButton *sourceButton;
    QToolButton *destinationButton;
    QComboBox *operatorComboBox;
    QLabel *equalLabel;
    QLabel *resultLabel;

    QImage sourceImage;
    QImage destinationImage;
    QImage resultImage;

ImageComposer Class Implementation

We declare a QSize object, resultSize , as a static constant with width and height equal to 200.

static const QSize resultSize(200, 200);

Within the constructor, we instantiate a QToolButton object, sourceButton and set its iconSize property to resultSize . The operatorComboBox is instantiated and then populated using the addOp() function. This function accepts a CompositionMode , mode , and a QString , name , representing the name of the composition mode.

    sourceButton = new QToolButton;

    operatorComboBox = new QComboBox;
    addOp(QPainter::CompositionMode_SourceOver, tr("SourceOver"));
    addOp(QPainter::CompositionMode_DestinationOver, tr("DestinationOver"));
    addOp(QPainter::CompositionMode_Clear, tr("Clear"));
    addOp(QPainter::CompositionMode_Source, tr("Source"));
    addOp(QPainter::CompositionMode_Destination, tr("Destination"));
    addOp(QPainter::CompositionMode_SourceIn, tr("SourceIn"));
    addOp(QPainter::CompositionMode_DestinationIn, tr("DestinationIn"));
    addOp(QPainter::CompositionMode_SourceOut, tr("SourceOut"));
    addOp(QPainter::CompositionMode_DestinationOut, tr("DestinationOut"));
    addOp(QPainter::CompositionMode_SourceAtop, tr("SourceAtop"));
    addOp(QPainter::CompositionMode_DestinationAtop, tr("DestinationAtop"));
    addOp(QPainter::CompositionMode_Xor, tr("Xor"));
    addOp(QPainter::CompositionMode_Plus, tr("Plus"));
    addOp(QPainter::CompositionMode_Multiply, tr("Multiply"));
    addOp(QPainter::CompositionMode_Screen, tr("Screen"));
    addOp(QPainter::CompositionMode_Overlay, tr("Overlay"));
    addOp(QPainter::CompositionMode_Darken, tr("Darken"));
    addOp(QPainter::CompositionMode_Lighten, tr("Lighten"));
    addOp(QPainter::CompositionMode_ColorDodge, tr("ColorDodge"));
    addOp(QPainter::CompositionMode_ColorBurn, tr("ColorBurn"));
    addOp(QPainter::CompositionMode_HardLight, tr("HardLight"));
    addOp(QPainter::CompositionMode_SoftLight, tr("SoftLight"));
    addOp(QPainter::CompositionMode_Difference, tr("Difference"));
    addOp(QPainter::CompositionMode_Exclusion, tr("Exclusion"));

The destinationButton is instantiated and its iconSize property is set to resultSize as well. The QLabel s equalLabel and resultLabel are created and resultLabel ‘s minimumWidth is set.

destinationButton = new QToolButton;

equalLabel = new QLabel(tr("="));

resultLabel = new QLabel;

We connect the following signals to their corresponding slots:

  • sourceButton’s clicked() signal is connected to chooseSource(),

  • operatorComboBox’s activated() signal is connected to recalculateResult(), and

  • destinationButton’s clicked() signal is connected to chooseDestination().

connect(sourceButton, &QAbstractButton::clicked,
        this, &ImageComposer::chooseSource);
connect(operatorComboBox, QOverload<int>::of(&QComboBox::activated),
        this, &ImageComposer::recalculateResult);
connect(destinationButton, &QAbstractButton::clicked,
        this, &ImageComposer::chooseDestination);

A QGridLayout , mainLayout , is used to place all the widgets. Note that mainLayout ‘s sizeConstraint property is set to SetFixedSize , which means that ImageComposer ‘s size cannot be resized at all.

QGridLayout *mainLayout = new QGridLayout;
mainLayout->addWidget(sourceButton, 0, 0, 3, 1);
mainLayout->addWidget(operatorComboBox, 1, 1);
mainLayout->addWidget(destinationButton, 0, 2, 3, 1);
mainLayout->addWidget(equalLabel, 1, 3);
mainLayout->addWidget(resultLabel, 0, 4, 3, 1);

We create a QImage , resultImage , and we invoke loadImage() twice to load both the image files in our imagecomposition.qrc file. Then, we set the windowTitle property to “Image Composition”.

    resultImage = QImage(resultSize, QImage::Format_ARGB32_Premultiplied);

    loadImage(":/images/butterfly.png", &sourceImage, sourceButton);
    loadImage(":/images/checker.png", &destinationImage, destinationButton);

    setWindowTitle(tr("Image Composition"));

The chooseSource() and chooseDestination() functions are convenience functions that invoke chooseImage() with specific parameters.

void ImageComposer::chooseSource()
    chooseImage(tr("Choose Source Image"), &sourceImage, sourceButton);

void ImageComposer::chooseDestination()
    chooseImage(tr("Choose Destination Image"), &destinationImage, destinationButton);

The chooseImage() function loads an image of the user’s choice, depending on the title , image , and button .

void ImageComposer::chooseImage(const QString &title, QImage *image,
                                QToolButton *button)
    QString fileName = QFileDialog::getOpenFileName(this, title);
    if (!fileName.isEmpty())
        loadImage(fileName, image, button);

The recalculateResult() function is used to calculate amd display the result of combining the two images together with the user’s choice of composition mode.

void ImageComposer::recalculateResult()
    QPainter::CompositionMode mode = currentMode();

    QPainter painter(&resultImage);
    painter.fillRect(resultImage.rect(), Qt::transparent);
    painter.drawImage(0, 0, destinationImage);
    painter.drawImage(0, 0, sourceImage);
    painter.fillRect(resultImage.rect(), Qt::white);


The addOp() function adds an item to the operatorComboBox using QComboBox ‘s addItem function. This function accepts a CompositionMode , mode , and a QString , name . The rectangle is filled with Qt::Transparent and both the sourceImage and destinationImage are painted, before displaying it on resultLabel .

void ImageComposer::addOp(QPainter::CompositionMode mode, const QString &name)
    operatorComboBox->addItem(name, mode);

The loadImage() function paints a transparent background using fillRect() and draws image in a centralized position using drawImage() . This image is then set as the button ‘s icon.

void ImageComposer::loadImage(const QString &fileName, QImage *image,
                              QToolButton *button)

    // Scale the image to given size
    *image = image->scaled(resultSize, Qt::KeepAspectRatio);

    QImage fixedImage(resultSize, QImage::Format_ARGB32_Premultiplied);
    QPainter painter(&fixedImage);
    painter.fillRect(fixedImage.rect(), Qt::transparent);
    painter.drawImage(imagePos(*image), *image);

    *image = fixedImage;


The currentMode() function returns the composition mode currently selected in operatorComboBox .

QPainter::CompositionMode ImageComposer::currentMode() const
    return (QPainter::CompositionMode)

We use the imagePos() function to ensure that images loaded onto the QToolButton objects, sourceButton and destinationButton , are centralized.

QPoint ImageComposer::imagePos(const QImage &image) const
    return QPoint((resultSize.width() - image.width()) / 2,
                  (resultSize.height() - image.height()) / 2);

The main()


The main() function instantiates QApplication and ImageComposer and invokes its show() function.

int main(int argc, char *argv[])

    QApplication app(argc, argv);
    ImageComposer composer;;
    return app.exec();

Example project @