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/*!

    \example painting/painterpaths

    \title Painter Paths Example

    The Painter Paths example shows how painter paths can be used to

    build complex shapes for rendering.

    \image painterpaths-example.png

    The QPainterPath class provides a container for painting

    operations, enabling graphical shapes to be constructed and

    reused.

    A painter path is an object composed of a number of graphical

    building blocks (such as rectangles, ellipses, lines, and curves),

    and can be used for filling, outlining, and clipping. The main

    advantage of painter paths over normal drawing operations is that

    complex shapes only need to be created once, but they can be drawn

    many times using only calls to QPainter::drawPath().

    The example consists of two classes:

    \list

        \o The \c RenderArea class which is a custom widget displaying

           a single painter path.

        \o The \c Window class which is the applications main window

           displaying several \c RenderArea widgets, and allowing the user

           to manipulate the painter paths' filling, pen, color

           and rotation angle.

    \endlist

    First we will review the \c Window class, then we will take a look

    at the \c RenderArea class.

    \section1 Window Class Definition

    The \c Window class inherits QWidget, and is the applications main

    window displaying several \c RenderArea widgets, and allowing the

    user to manipulate the painter paths' filling, pen, color and

    rotation angle.

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

    We declare three private slots to respond to user input regarding

    filling and color: \c fillRuleChanged(), \c fillGradientChanged()

    and \c penColorChanged().

    When the user changes the pen width and the rotation angle, the

    new value is passed directly on to the \c RenderArea widgets using

    the QSpinBox::valueChanged() signal. The reason why we must

    implement slots to update the filling and color, is that QComboBox

    doesn't provide a similar signal passing the new value as

    argument; so we need to retrieve the new value, or values, before

    we can update the \c RenderArea widgets.

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

    We also declare a couple of private convenience functions: \c

    populateWithColors() populates a given QComboBox with items

    corresponding to the color names Qt knows about, and \c

    currentItemData() returns the current item for a given QComboBox.

    \snippet examples/painting/painterpaths/window.h 2

    Then we declare the various components of the main window

    widget. We also declare a convenience constant specifying the

    number of \c RenderArea widgets.

    \section1 Window Class Implementation

    In the implementation of the \c Window class we first declare the

    constant \c Pi with six significant figures:

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

    In the constructor, we then define the various painter paths and

    create corresponding \c RenderArea widgets which will render the

    graphical shapes:

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

    We construct a rectangle with sharp corners using the

    QPainterPath::moveTo() and QPainterPath::lineTo()

    functions.

    QPainterPath::moveTo() moves the current point to the point passed

    as argument. A painter path is an object composed of a number of

    graphical building blocks, i.e. subpaths. Moving the current point

    will also start a new subpath (implicitly closing the previously

    current path when the new one is started). The

    QPainterPath::lineTo() function adds a straight line from the

    current point to the given end point. After the line is drawn, the

    current point is updated to be at the end point of the line.

    We first move the current point starting a new subpath, and we

    draw three of the rectangle's sides. Then we call the

    QPainterPath::closeSubpath() function which draws a line to the

    beginning of the current subpath. A new subpath is automatically

    begun when the current subpath is closed. The current point of the

    new path is (0, 0). We could also have called

    QPainterPath::lineTo() to draw the last line as well, and then

    explicitly start a new subpath using the QPainterPath::moveTo()

    function.

    QPainterPath also provide the QPainterPath::addRect() convenience

    function, which adds a given rectangle to the path as a closed

    subpath. The rectangle is added as a clockwise set of lines. The

    painter path's current position after the rect has been added is

    at the top-left corner of the rectangle.

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

    Then we construct a rectangle with rounded corners. As before, we

    use the QPainterPath::moveTo() and QPainterPath::lineTo()

    functions to draw the rectangle's sides. To create the rounded

    corners we use the QPainterPath::arcTo() function.

    QPainterPath::arcTo() creates an arc that occupies the given

    rectangle (specified by a QRect or the rectangle's coordinates),

    beginning at the given start angle and extending the given degrees

    counter-clockwise. Angles are specified in degrees. Clockwise arcs

    can be specified using negative angles.  The function connects the

    current point to the starting point of the arc if they are not

    already connected.

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

    We also use the QPainterPath::arcTo() function to construct the

    ellipse path. First we move the current point starting a new

    path. Then we call QPainterPath::arcTo() with starting angle 0.0

    and 360.0 degrees as the last argument, creating an ellipse.

    Again, QPainterPath provides a convenience function (

    QPainterPath::addEllipse()) which creates an ellipse within a

    given bounding rectangle and adds it to the painter path. If the

    current subpath is closed, a new subpath is started. The ellipse

    is composed of a clockwise curve, starting and finishing at zero

    degrees (the 3 o'clock position).

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

    When constructing the pie chart path we continue to use a

    combination of the mentioned functions: First we move the current

    point, starting a new subpath. Then we create a line from the

    center of the chart to the arc, and the arc itself. When we close

    the subpath, we implicitly construct the last line back to the

    center of the chart.

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

    Constructing a polygon is equivalent to constructing a rectangle.

    QPainterPath also provide the QPainterPath::addPolygon()

    convenience function which adds the given polygon to the path as a

    new subpath. Current position after the polygon has been added is

    the last point in polygon.

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

    Then we create a path consisting of a group of subpaths: First we

    move the current point, and create a circle using the

    QPainterPath::arcTo() function with starting angle 0.0, and 360

    degrees as the last argument, as we did when we created the

    ellipse path. Then we move the current point again, starting a

    new subpath, and construct three sides of a square using the

    QPainterPath::lineTo() function.

    Now, when we call the QPainterPath::closeSubpath() fucntion the

    last side is created. Remember that the

    QPainterPath::closeSubpath() function draws a line to the

    beginning of the \e current subpath, i.e the square.

    QPainterPath provide a convenience function,

    QPainterPath::addPath() which adds a given path to the path that

    calls the function.

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

    When creating the text path, we first create the font. Then we set

    the font's style strategy which tells the font matching algorithm

    what type of fonts should be used to find an appropriate default

    family. QFont::ForceOutline forces the use of outline fonts.

    To construct the text, we use the QPainterPath::addText() function

    which adds the given text to the path as a set of closed subpaths

    created from the supplied font. The subpaths are positioned so

    that the left end of the text's baseline lies at the specified

    point.

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

    To create the Bezier path, we use the QPainterPath::cubicTo()

    function which adds a Bezier curve between the current point and

    the given end point with the given control point.  After the curve

    is added, the current point is updated to be at the end point of

    the curve.

    In this case we omit to close the subpath so that we only have a

    simple curve. But there is still a logical line from the curve's

    endpoint back to the beginning of the subpath; it becomes visible

    when filling the path as can be seen in the applications main

    window.

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

    The final path that we construct shows that you can use

    QPainterPath to construct rather complex shapes using only the

    previous mentioned QPainterPath::moveTo(), QPainterPath::lineTo()

    and QPainterPath::closeSubpath() functions.

    \snippet examples/painting/painterpaths/window.cpp 10

    Now that we have created all the painter paths that we need, we

    create a corresponding \c RenderArea widget for each. In the end,

    we make sure that the number of render areas is correct using the

    Q_ASSERT() macro.

    \snippet examples/painting/painterpaths/window.cpp 11

    Then we create the widgets associated with the painter paths' fill

    rule.

    There are two available fill rules in Qt: The Qt::OddEvenFill rule

    determine whether a point is inside the shape by drawing a

    horizontal line from the point to a location outside the shape,

    and count the number of intersections. If the number of

    intersections is an odd number, the point is inside the

    shape. This rule is the default.

    The Qt::WindingFill rule determine whether a point is inside the

    shape by drawing a horizontal line from the point to a location

    outside the shape. Then it determines whether the direction of the

    line at each intersection point is up or down. The winding number

    is determined by summing the direction of each intersection. If

    the number is non zero, the point is inside the shape.

    The Qt::WindingFill rule can in most cases be considered as the

    intersection of closed shapes.

    \snippet examples/painting/painterpaths/window.cpp 12

    We also create the other widgets associated with the filling, the

    pen and the rotation angle.

    \snippet examples/painting/painterpaths/window.cpp 16

    We connect the comboboxes \l {QComboBox::activated()}{activated()}

    signals to the associated slots in the \c Window class, while we

    connect the spin boxes \l

    {QSpinBox::valueChanged()}{valueChanged()} signal directly to the

    \c RenderArea widget's respective slots.

    \snippet examples/painting/painterpaths/window.cpp 17

    We add the \c RenderArea widgets to a separate layout which we

    then add to the main layout along with the rest of the widgets.

    \snippet examples/painting/painterpaths/window.cpp 18

    Finally, we initialize the \c RenderArea widgets by calling the \c

    fillRuleChanged(), \c fillGradientChanged() and \c

    penColorChanged() slots, and we set the inital pen width and

    window title.

    \snippet examples/painting/painterpaths/window.cpp 19

    \codeline

    \snippet examples/painting/painterpaths/window.cpp 20

    \codeline

    \snippet examples/painting/painterpaths/window.cpp 21

    The private slots are implemented to retrieve the new value, or

    values, from the associated comboboxes and update the RenderArea

    widgets.

    First we determine the new value, or values, using the private \c

    currentItemData() function and the qvariant_cast() template

    function. Then we call the associated slot for each of the \c

    RenderArea widgets to update the painter paths.

    \snippet examples/painting/painterpaths/window.cpp 22

    The \c populateWithColors() function populates the given combobox

    with items corresponding to the color names Qt knows about

    provided by the static QColor::colorNames() function.

    \snippet examples/painting/painterpaths/window.cpp 23

    The \c currentItemData() function simply return the current item

    of the given combobox.

    \section1 RenderArea Class Definition

    The \c RenderArea class inherits QWidget, and is a custom widget

    displaying a single painter path.

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

    We declare several public slots updating the \c RenderArea

    widget's associated painter path. In addition we reimplement the

    QWidget::minimumSizeHint() and QWidget::sizeHint() functions to

    give the \c RenderArea widget a reasonable size within our

    application, and we reimplement the QWidget::paintEvent() event

    handler to draw its painter path.

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

    Each instance of the \c RenderArea class has a QPainterPath, a

    couple of fill colors, a pen width, a pen color and a rotation

    angle.

    \section1 RenderArea Class Implementation

    The constructor takes a QPainterPath as argument (in addition to

    the optional QWidget parent):

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

    In the constructor we initialize the \c RenderArea widget with the

    QPainterPath parameter as well as initializing the pen width and

    rotation angle. We also set the widgets \l

    {QWidget::backgroundRole()}{background role}; QPalette::Base is

    typically white.

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

    \codeline

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

    Then we reimplement the QWidget::minimumSizeHint() and

    QWidget::sizeHint() functions to give the \c RenderArea widget a

    reasonable size within our application.

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

    \codeline

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

    \codeline

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

    \codeline

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

    \codeline

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

    The various public slots updates the \c RenderArea widget's

    painter path by setting the associated property and make a call to

    the QWidget::update() function, forcing a repaint of the widget

    with the new rendering preferences.

    The QWidget::update() slot does not cause an immediate repaint;

    instead it schedules a paint event for processing when Qt returns

    to the main event loop.

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

    A paint event is a request to repaint all or parts of the

    widget. The paintEvent() function is an event handler that can be

    reimplemented to receive the widget's paint events. We reimplement

    the event handler to render the \c RenderArea widget's painter

    path.

    First, we create a QPainter for the \c RenderArea instance, and

    set the painter's render hints. The QPainter::RenderHints are used

    to specify flags to QPainter that may, or may not, be respected by

    any given engine. QPainter::Antialiasing indicates that the engine

    should anti-alias the edges of primitives if possible, i.e. put

    additional pixels around the original ones to smooth the edges.

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

    Then we scale the QPainter's coordinate system to ensure that the

    painter path is rendered in the right size, i.e that it grows with

    the \c RenderArea widget when the application is resized. When we

    constructed the various painter paths, they were all rnedered

    within a square with a 100 pixel width wich is equivalent to \c

    RenderArea::sizeHint(). The QPainter::scale() function scales the

    coordinate system by the \c RenderArea widget's \e current width

    and height divided by 100.

    Now, when we are sure that the painter path has the right size, we

    can translate the coordinate system to make the painter path

    rotate around the \c RenderArea widget's center. After we have

    performed the rotation, we must remember to translate the

    coordinate system back again.

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

    Then we set the QPainter's pen with the instance's rendering

    preferences. We create a QLinearGradient and set its colors

    corresponding to the \c RenderArea widget's fill colors. Finally,

    we set the QPainter's brush (the gradient is automatically

    converted into a QBrush), and draw the \c RenderArea widget's

    painter path using the QPainter::drawPath() function.

*/