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/*!
    \example painting/imagecomposition
    \title Image Composition Example

    The Image Composition example lets the user combine images
    together using any composition mode supported by QPainter, described
    in detail in \l{QPainter#Composition Modes}{Composition Modes}.

    \image imagecomposition-example.png

    \section1 Setting Up The Resource File

    The Image Composition example requires two source images,
    \e butterfly.png and \e checker.png that are embedded within
    \e imagecomposition.qrc. The file contains the following code:

    \quotefile examples/painting/imagecomposition/imagecomposition.qrc

    For more information on resource files, see \l{The Qt Resource System}.

    \section1 ImageComposer Class Definition

    The \c ImageComposer class is a subclass of QWidget that implements three
    private slots, \c chooseSource(), \c chooseDestination(), and
    \c recalculateResult().

    \snippet examples/painting/imagecomposition/imagecomposer.h 0

    In addition, \c ImageComposer consists of five private functions,
    \c addOp(), \c chooseImage(), \c loadImage(), \c currentMode(), and
    \c imagePos(), as well as private instances of QToolButton, QComboBox,
    QLabel, and QImage.

    \snippet examples/painting/imagecomposition/imagecomposer.h 1

    \section1 ImageComposer Class Implementation

    We declare a QSize object, \c resultSize, as a static constant with width
    and height equal to 200.

    \snippet examples/painting/imagecomposition/imagecomposer.cpp 0

    Within the constructor, we instantiate a QToolButton object,
    \c sourceButton and set its \l{QAbstractButton::setIconSize()}{iconSize}
    property to \c resultSize. The \c operatorComboBox is instantiated and
    then populated using the \c addOp() function. This function accepts a
    QPainter::CompositionMode, \a mode, and a QString, \a name, representing
    the name of the composition mode.

    \snippet examples/painting/imagecomposition/imagecomposer.cpp 1

    The \c destinationButton is instantiated and its
    \l{QAbstractButton::setIconSize()}{iconSize} property is set to
    \c resultSize as well. The \l{QLabel}s \c equalLabel and \c resultLabel
    are created and \c{resultLabel}'s \l{QWidget::setMinimumWidth()}
    {minimumWidth} is set.

    \snippet examples/painting/imagecomposition/imagecomposer.cpp 2

    We connect the following signals to their corresponding slots:
    \list
        \o \c{sourceButton}'s \l{QPushButton::clicked()}{clicked()} signal is
            connected to \c chooseSource(),
        \o \c{operatorComboBox}'s \l{QComboBox::activated()}{activated()}
            signal is connected to \c recalculateResult(), and
        \o \c{destinationButton}'s \l{QToolButton::clicked()}{clicked()} signal
            is connected to \c chooseDestination().
    \endlist

    \snippet examples/painting/imagecomposition/imagecomposer.cpp 3

    A QGridLayout, \c mainLayout, is used to place all the widgets. Note
    that \c{mainLayout}'s \l{QLayout::setSizeConstraint()}{sizeConstraint}
    property is set to QLayout::SetFixedSize, which means that
    \c{ImageComposer}'s size cannot be resized at all.

    \snippet examples/painting/imagecomposition/imagecomposer.cpp 4

    We create a QImage, \c resultImage, and we invoke \c loadImage() twice
    to load both the image files in our \e imagecomposition.qrc file. Then,
    we set the \l{QWidget::setWindowTitle()}{windowTitle} property to
    "Image Composition".

    \snippet examples/painting/imagecomposition/imagecomposer.cpp 5

    The \c chooseSource() and \c chooseDestination() functions are
    convenience functions that invoke \c chooseImage() with specific
    parameters.

    \snippet examples/painting/imagecomposition/imagecomposer.cpp 6
    \codeline
    \snippet examples/painting/imagecomposition/imagecomposer.cpp 7

    The \c chooseImage() function loads an image of the user's choice,
    depending on the \a title, \a image, and \a button.

    \snippet examples/painting/imagecomposition/imagecomposer.cpp 10

    The \c recalculateResult() function is used to calculate amd display the
    result of combining the two images together with the user's choice of
    composition mode.

    \snippet examples/painting/imagecomposition/imagecomposer.cpp 8

    The \c addOp() function adds an item to the \c operatorComboBox using
    \l{QComboBox}'s \l{QComboBox::addItem()}{addItem} function. This function
    accepts a QPainter::CompositionMode, \a mode, and a QString, \a name. The
    rectangle is filled with Qt::Transparent and both the \c sourceImage and
    \c destinationImage are painted, before displaying it on \c resultLabel.

    \snippet examples/painting/imagecomposition/imagecomposer.cpp 9

    The \c loadImage() function paints a transparent background using
    \l{QPainter::fillRect()}{fillRect()} and draws \c image in a
    centralized position using \l{QPainter::drawImage()}{drawImage()}.
    This \c image is then set as the \c{button}'s icon.

    \snippet examples/painting/imagecomposition/imagecomposer.cpp 11

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

    \snippet examples/painting/imagecomposition/imagecomposer.cpp 12

    We use the \c imagePos() function to ensure that images loaded onto the
    QToolButton objects, \c sourceButton and \c destinationButton, are
    centralized.

    \snippet examples/painting/imagecomposition/imagecomposer.cpp 13

    \section1 The \c main() Function

    The \c main() function instantiates QApplication and \c ImageComposer
    and invokes its \l{QWidget::show()}{show()} function.

    \snippet examples/painting/imagecomposition/main.cpp 0

    */