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/*!

    \example painting/transformations

    \title Transformations Example

    The Transformations example shows how transformations influence

    the way that QPainter renders graphics primitives. In particular

    it shows how the order of transformations affect the result.

    \image transformations-example.png

    The application allows the user to manipulate the rendering of a

    shape by changing the translation, rotation and scale of

    QPainter's coordinate system.

    The example consists of two classes and a global enum:

    \list

    \o The \c RenderArea class controls the rendering of a given shape.

    \o The \c Window class is the application's main window.

    \o The \c Operation enum describes the various transformation

       operations available in the application.

    \endlist

    First we will take a quick look at the \c Operation enum, then we

    will review the \c RenderArea class to see how a shape is

    rendered. Finally, we will take a look at the Transformations

    application's features implemented in the \c Window class.

    \section1 Transformation Operations

    Normally, the QPainter operates on the associated device's own

    coordinate system, but it also has good support for coordinate

    transformations.

    The default coordinate system of a paint device has its origin at

    the top-left corner. The x values increase to the right and the y

    values increase downwards. You can scale the coordinate system by

    a given offset using the QPainter::scale() function, you can

    rotate it clockwise using the QPainter::rotate() function and you

    can translate it (i.e. adding a given offset to the points) using

    the QPainter::translate() function. You can also twist the

    coordinate system around the origin (called shearing) using the

    QPainter::shear() function.

    All the tranformation operations operate on QPainter's

    tranformation matrix that you can retrieve using the

    QPainter::worldTransform() function. A matrix transforms a point in the

    plane to another point. For more information about the

    transformation matrix, see the \l {The Coordinate System} and

    QTransform documentation.

    \snippet examples/painting/transformations/renderarea.h 0

    The global \c Operation enum is declared in the \c renderarea.h

    file and describes the various transformation operations available

    in the Transformations application.

    \section1 RenderArea Class Definition

    The \c RenderArea class inherits QWidget, and controls the

    rendering of a given shape.

    \snippet examples/painting/transformations/renderarea.h 1

    We declare two public functions, \c setOperations() and

    \c setShape(), to be able to specify the \c RenderArea widget's shape

    and to transform the coordinate system the shape is rendered

    within.

    We reimplement the QWidget's \l

    {QWidget::minimumSizeHint()}{minimumSizeHint()} and \l

    {QWidget::sizeHint()}{sizeHint()} functions to give the \c

    RenderArea widget a reasonable size within our application, and we

    reimplement the QWidget::paintEvent() event handler to draw the

    render area's shape applying the user's transformation choices.

    \snippet examples/painting/transformations/renderarea.h 2

    We also declare several convenience functions to draw the shape,

    the coordinate system's outline and the coordinates, and to

    transform the painter according to the chosen transformations.

    In addition, the \c RenderArea widget keeps a list of the

    currently applied transformation operations, a reference to its

    shape, and a couple of convenience variables that we will use when

    rendering the coordinates.

    \section1 RenderArea Class Implementation

    The \c RenderArea widget controls the rendering of a given shape,

    including the transformations of the coordinate system, by

    reimplementing the QWidget::paintEvent() event handler. But first

    we will take a quick look at the constructor and at the functions

    that provides access to the \c RenderArea widget:

    \snippet examples/painting/transformations/renderarea.cpp 0

    In the constructor we pass the parent parameter on to the base

    class, and customize the font that we will use to render the

    coordinates. The QWidget::font() funtion returns the font

    currently set for the widget. As long as no special font has been

    set, or after QWidget::setFont() is called, this is either a

    special font for the widget class, the parent's font or (if this

    widget is a top level widget) the default application font.

    After ensuring that the font's size is 12 points, we extract the

    rectangles enclosing the coordinate letters, 'x' and 'y',  using the

    QFontMetrics class.

    QFontMetrics provides functions to access the individual metrics

    of the font, its characters, and for strings rendered in the

    font. The QFontMetrics::boundingRect() function returns the

    bounding rectangle of the given character relative to the

    left-most point on the base line.

    \snippet examples/painting/transformations/renderarea.cpp 1

    \codeline

    \snippet examples/painting/transformations/renderarea.cpp 2

    In the \c setShape() and \c setOperations() functions we update

    the \c RenderArea widget by storing the new value or values

    followed by a call to the QWidget::update() slot which schedules a

    paint event for processing when Qt returns to the main event loop.

    \snippet examples/painting/transformations/renderarea.cpp 3

    \codeline

    \snippet examples/painting/transformations/renderarea.cpp 4

    We reimplement the QWidget's \l

    {QWidget::minimumSizeHint()}{minimumSizeHint()} and \l

    {QWidget::sizeHint()}{sizeHint()} functions to give the \c

    RenderArea widget a reasonable size within our application. The

    default implementations of these functions returns an invalid size

    if there is no layout for this widget, and returns the layout's

    minimum size or preferred size, respectively, otherwise.

    \snippet examples/painting/transformations/renderarea.cpp 5

    The \c paintEvent() event handler recieves the \c RenderArea

    widget's paint events. A paint event is a request to repaint all

    or part of the widget. It can happen as a result of

    QWidget::repaint() or QWidget::update(), or because the widget was

    obscured and has now been uncovered, or for many other reasons.

    First we create a QPainter for the \c RenderArea widget. The \l

    {QPainter::RenderHint}{QPainter::Antialiasing} render hint

    indicates that the engine should antialias edges of primitives if

    possible. Then we erase the area that needs to be repainted using

    the QPainter::fillRect() function.

    We also translate the coordinate system with an constant offset to

    ensure that the original shape is renderend with a suitable

    margin.

    \snippet examples/painting/transformations/renderarea.cpp 6

    Before we start to render the shape, we call the QPainter::save()

    function.

    QPainter::save() saves the current painter state (i.e. pushes the

    state onto a stack) including the current coordinate system. The

    rationale for saving the painter state is that the following call

    to the \c transformPainter() function will transform the

    coordinate system depending on the currently chosen transformation

    operations, and we need a way to get back to the original state to

    draw the outline.

    After transforming the coordinate system, we draw the \c

    RenderArea's shape, and then we restore the painter state using

    the QPainter::restore() function (i.e. popping the saved state off

    the stack).

    \snippet examples/painting/transformations/renderarea.cpp 7

    Then we draw the square outline.

    \snippet examples/painting/transformations/renderarea.cpp 8

    Since we want the coordinates to correspond with the coordinate

    system the shape is rendered within, we must make another call to

    the \c transformPainter() function.

    The order of the painting operations is essential with respect to

    the shared pixels. The reason why we don't render the coordinates

    when the coordinate system already is transformed to render the

    shape, but instead defer their rendering to the end, is that we

    want the coordinates to appear on top of the shape and its

    outline.

    There is no need to save the QPainter state this time since

    drawing the coordinates is the last painting operation.

    \snippet examples/painting/transformations/renderarea.cpp 9

    \codeline

    \snippet examples/painting/transformations/renderarea.cpp 10

    \codeline

    \snippet examples/painting/transformations/renderarea.cpp 11

    The \c drawCoordinates(), \c drawOutline() and \c drawShape() are

    convenience functions called from the \c paintEvent() event

    handler. For more information about QPainter's basic drawing

    operations and how to display basic graphics primitives, see the

    \l {painting/basicdrawing}{Basic Drawing} example.

    \snippet examples/painting/transformations/renderarea.cpp 12

    The \c transformPainter() convenience function is also called from

    the \c paintEvent() event handler, and transforms the given

    QPainter's coordinate system according to the user's

    transformation choices.

    \section1 Window Class Definition

    The \c Window class is the Transformations application's main

    window.

    The application displays four \c RenderArea widgets. The left-most

    widget renders the shape in QPainter's default coordinate system,

    the others render the shape with the chosen transformation in

    addition to all the transformations applied to the \c RenderArea

    widgets to their left.

    \snippet examples/painting/transformations/window.h 0

    We declare two public slots to make the application able to

    respond to user interaction, updating the displayed \c RenderArea

    widgets according to the user's transformation choices.

    The \c operationChanged() slot updates each of the \c RenderArea

    widgets applying the currently chosen transformation operations, and

    is called whenever the user changes the selected operations. The

    \c shapeSelected() slot updates the \c RenderArea widgets' shapes

    whenever the user changes the preferred shape.

    \snippet examples/painting/transformations/window.h 1

    We also declare a private convenience function, \c setupShapes(),

    that is used when constructing the \c Window widget, and we

    declare pointers to the various components of the widget. We

    choose to keep the available shapes in a QList of \l

    {QPainterPath}s. In addition we declare a private enum counting

    the number of displayed \c RenderArea widgets except the widget

    that renders the shape in QPainter's default coordinate system.

    \section1 Window Class Implementation

    In the constructor we create and initialize the application's

    components:

    \snippet examples/painting/transformations/window.cpp 0

    First we create the \c RenderArea widget that will render the

    shape in the default coordinate system. We also create the

    associated QComboBox that allows the user to choose among four

    different shapes: A clock, a house, a text and a truck. The shapes

    themselves are created at the end of the constructor, using the

    \c setupShapes() convenience function.

    \snippet examples/painting/transformations/window.cpp 1

    Then we create the \c RenderArea widgets that will render their

    shapes with coordinate tranformations. By default the applied

    operation is \gui {No Transformation}, i.e. the shapes are

    rendered within the default coordinate system. We create and

    initialize the associated \l {QComboBox}es with items

    corresponding to the various transformation operations decribed by

    the global \c Operation enum.

    We also connect the \l {QComboBox}es' \l

    {QComboBox::activated()}{activated()} signal to the \c

    operationChanged() slot to update the application whenever the

    user changes the selected transformation operations.

    \snippet examples/painting/transformations/window.cpp 2

    Finally, we set the layout for the application window using the

    QWidget::setLayout() function, construct the available shapes

    using the private \c setupShapes() convenience function, and make

    the application show the clock shape on startup using the public

    \c shapeSelected() slot before we set the window title.

    \snippet examples/painting/transformations/window.cpp 3

    \snippet examples/painting/transformations/window.cpp 4

    \snippet examples/painting/transformations/window.cpp 5

    \snippet examples/painting/transformations/window.cpp 6

    \dots

    \snippet examples/painting/transformations/window.cpp 7

    The \c setupShapes() function is called from the constructor and

    create the QPainterPath objects representing the shapes that are

    used in the application. For construction details, see the \l

    {painting/transformations/window.cpp}{window.cpp} example

    file. The shapes are stored in a QList. The QList::append()

    function inserts the given shape at the end of the list.

    We also connect the associated QComboBox's \l

    {QComboBox::activated()}{activated()} signal to the \c

    shapeSelected() slot to update the application when the user

    changes the preferred shape.

    \snippet examples/painting/transformations/window.cpp 8

    The public \c operationChanged() slot is called whenever the user

    changes the selected operations.

    We retrieve the chosen transformation operation for each of the

    transformed \c RenderArea widgets by querying the associated \l

    {QComboBox}{QComboBoxes}. The transformed \c RenderArea widgets

    are supposed to render the shape with the transformation specified

    by its associated combobox \e {in addition to} all the

    transformations applied to the \c RenderArea widgets to its

    left. For that reason, for each widget we query, we append the

    associated operation to a QList of transformations which we apply

    to the widget before proceeding to the next.

    \snippet examples/painting/transformations/window.cpp 9

    The \c shapeSelected() slot is called whenever the user changes

    the preferred shape, updating the \c RenderArea widgets using

    their public \c setShape() function.

    \section1 Summary

    The Transformations example shows how transformations influence

    the way that QPainter renders graphics primitives. Normally, the

    QPainter operates on the device's own coordinate system, but it

    also has good support for coordinate transformations. With the

    Transformations application you can scale, rotate and translate

    QPainter's coordinate system.  The order in which these

    tranformations are applied is essential for the result.

    All the tranformation operations operate on QPainter's

    tranformation matrix. For more information about the

    transformation matrix, see the \l {The Coordinate System} and

    QTransform documentation.

    The Qt reference documentation provides several painting

    demos. Among these is the \l {demos/affine}{Affine

    Transformations} demo that shows Qt's ability to perform

    transformations on painting operations. The demo also allows the

    user to experiment with the various transformation operations.

*/