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Image Composition Example

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:
<!DOCTYPE RCC><RCC version="1.0">
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.

The following code example is written in c++.
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.

The following code example is written in c++.
    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 QLabels equalLabel and resultLabel are created and resultLabel's minimumWidth is set.

The following code example is written in c++.
    destinationButton = new QToolButton;

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

    resultLabel = new QLabel;
We connect the following signals to their corresponding slots:

The following code example is written in c++.
    connect(sourceButton, SIGNAL(clicked()), this, SLOT(chooseSource()));
    connect(operatorComboBox, SIGNAL(activated(int)),
            this, SLOT(recalculateResult()));
    connect(destinationButton, SIGNAL(clicked()),
            this, SLOT(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.

The following code example is written in c++.
    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".

The following code example is written in c++.
    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.

The following code example is written in c++.
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.

The following code example is written in c++.
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.

The following code example is written in c++.
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 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.

The following code example is written in c++.
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.

The following code example is written in c++.
void ImageComposer::loadImage(const QString &fileName, QImage *image,
                              QToolButton *button)

    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.

The following code example is written in c++.
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.

The following code example is written in c++.
QPoint ImageComposer::imagePos(const QImage &image) const
    return QPoint((resultSize.width() - image.width()) / 2,
                  (resultSize.height() - image.height()) / 2);

The main() Function

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

The following code example is written in c++.
int main(int argc, char *argv[])

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

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Qt Jambi 4.5.2_01