- Qt 5.15
- Qt Widgets
- Image Composition Example
- Qt 5.15.18 Reference Documentation

# Image Composition Example

Shows how composition modes work in QPainter.

#### 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