Image Composition Example

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:

 <!DOCTYPE RCC><RCC version="1.0">
 <qresource>
     <file>images/butterfly.png</file>
     <file>images/checker.png</file>
 </qresource>
 </RCC>

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
 {
     Q_OBJECT

 public:
     ImageComposer();

 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.

 private:
     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 QPainter::CompositionMode, mode, and a QString, name, representing the name of the composition mode.

 ImageComposer::ImageComposer()
 {
     sourceButton = new QToolButton;
     sourceButton->setIconSize(resultSize);

     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 QLabels equalLabel and resultLabel are created and resultLabel's minimumWidth is set.

     destinationButton = new QToolButton;
     destinationButton->setIconSize(resultSize);

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

     resultLabel = new QLabel;
     resultLabel->setMinimumWidth(resultSize.width());

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 QLayout::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);
     mainLayout->setSizeConstraint(QLayout::SetFixedSize);
     setLayout(mainLayout);

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.setCompositionMode(QPainter::CompositionMode_Source);
     painter.fillRect(resultImage.rect(), Qt::transparent);
     painter.setCompositionMode(QPainter::CompositionMode_SourceOver);
     painter.drawImage(0, 0, destinationImage);
     painter.setCompositionMode(mode);
     painter.drawImage(0, 0, sourceImage);
     painter.setCompositionMode(QPainter::CompositionMode_DestinationOver);
     painter.fillRect(resultImage.rect(), Qt::white);
     painter.end();

     resultLabel->setPixmap(QPixmap::fromImage(resultImage));
 }

The addOp() function adds an item to the operatorComboBox using QComboBox's addItem function. This function accepts a QPainter::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)
 {
     image->load(fileName);

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

     QImage fixedImage(resultSize, QImage::Format_ARGB32_Premultiplied);
     QPainter painter(&fixedImage);
     painter.setCompositionMode(QPainter::CompositionMode_Source);
     painter.fillRect(fixedImage.rect(), Qt::transparent);
     painter.setCompositionMode(QPainter::CompositionMode_SourceOver);
     painter.drawImage(imagePos(*image), *image);
     painter.end();
     button->setIcon(QPixmap::fromImage(fixedImage));

     *image = fixedImage;

     recalculateResult();
 }

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

 QPainter::CompositionMode ImageComposer::currentMode() const
 {
     return (QPainter::CompositionMode)
            operatorComboBox->itemData(operatorComboBox->currentIndex()).toInt();
 }

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() Function

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

 int main(int argc, char *argv[])
 {
     Q_INIT_RESOURCE(imagecomposition);

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

Example project @ code.qt.io