Qt 5.13 | Qt Widgets | Image Composition Example

Qt 5.13.2 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