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# imagecomposer.cpp Example File
painting/imagecomposition/imagecomposer.cpp

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     #include <QtGui>

     #include "imagecomposer.h"

     static const QSize resultSize(200, 200);

     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"));

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

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

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

         connect(sourceButton, SIGNAL(clicked()), this, SLOT(chooseSource()));
         connect(operatorComboBox, SIGNAL(activated(int)),
                 this, SLOT(recalculateResult()));
         connect(destinationButton, SIGNAL(clicked()),
                 this, SLOT(chooseDestination()));

         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);

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

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

         setWindowTitle(tr("Image Composition"));
     }

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

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

     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));
     }

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

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

     void ImageComposer::loadImage(const QString &fileName, QImage *image,
                                   QToolButton *button)
     {
         image->load(fileName);

         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();
     }

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

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