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/*!

    \example painting/concentriccircles

    \title Concentric Circles Example

    The Concentric Circles example shows the improved rendering

    quality that can be obtained using floating point precision and

    anti-aliasing when drawing custom widgets. The example also shows

    how to do simple animations.

    The application's main window displays several widgets which are

    drawn using the various combinations of precision and

    anti-aliasing.

    \image concentriccircles-example.png

    Anti-aliasing is one of QPainter'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.

    The difference between floating point precision and integer

    precision is a matter of accuracy, and is visible in the

    application's main window: Even though the logic that is

    calculating the circles' geometry is the same, floating points

    ensure that the white spaces between each circle are of the same

    size, while integers make two and two circles appear as if they

    belong together. The reason is that the integer based precision

    rely on rounding off non-integer calculations.

    The example consists of two classes:

    \list

    \o \c CircleWidget is a custom widget which renders several animated

       concentric circles.

    \o \c Window is the application's main window displaying four \c

       {CircleWidget}s drawn using different combinations of precision

       and aliasing.

    \endlist

    First we will review the CircleWidget class, then we will take a

    look at the Window class.

    \section1 CircleWidget Class Definition

    The CircleWidget class inherits QWidget, and is a custom widget

    which renders several animated concentric circles.

    \snippet examples/painting/concentriccircles/circlewidget.h 0

    We declare the \c floatBased and \c antialiased variables to hold

    whether an instance of the class should be rendered with integer

    or float based precision, and whether the rendering should be

    anti-aliased or not. We also declare functions setting each of

    these variables.

    In addition we reimplement the QWidget::paintEvent() function to

    apply the various combinations of precision and anti-aliasing when

    rendering, and to support the animation. We reimplement the

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

    give the widget a reasonable size within our application.

    We declare the private \c nextAnimationFrame() slot, and the

    associated \c frameNo variable holding the number of "animation

    frames" for the widget, to facilitate the animation.

    \section1 CircleWidget Class Implementation

    In the constructor we make the widget's rendering integer based

    and aliased by default:

    \snippet examples/painting/concentriccircles/circlewidget.cpp 0

    We initialize the widget's \c frameNo variable, and set the

    widget's background color using the QWidget::setBackgroundColor()

    function which takes a \l {QPalette::ColorRole}{color role} as

    argument; the QPalette::Base color role is typically white.

    Then we set the widgets size policy using the

    QWidget::setSizePolicy() function. QSizePolicy::Expanding means

    that the widget's \l {QWidget::sizeHint()}{sizeHint()} is a

    sensible size, but that the widget can be shrunk and still be

    useful. The widget can also make use of extra space, so it should

    get as much space as possible.

    \snippet examples/painting/concentriccircles/circlewidget.cpp 1

    \codeline

    \snippet examples/painting/concentriccircles/circlewidget.cpp 2

    The public \c setFloatBased() and \c setAntialiased() functions

    update the widget's rendering preferences, i.e. whether the widget

    should be rendered with integer or float based precision, and

    whether the rendering should be anti-aliased or not.

    The functions also generate a paint event by calling the

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

    the new rendering preferences.

    \snippet examples/painting/concentriccircles/circlewidget.cpp 3

    \codeline

    \snippet examples/painting/concentriccircles/circlewidget.cpp 4

    The default implementations of the QWidget::minimumSizeHint() and

    QWidget::sizeHint() functions return invalid sizes if there is no

    layout for the widget, otherwise they return the layout's minimum and

    preferred size, respectively.

    We reimplement the functions to give the widget minimum and

    preferred sizes which are reasonable within our application.

    \snippet examples/painting/concentriccircles/circlewidget.cpp 5

    The nextAnimationFrame() slot simply increments the \c frameNo

    variable's value, and calls the QWidget::update() function which

    schedules a paint event for processing when Qt returns to the main

    event loop.

    \snippet examples/painting/concentriccircles/circlewidget.cpp 6

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

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

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

    reimplement the event handler to apply the various combinations of

    precision and anti-aliasing when rendering the widget, and to

    support the animation.

    First, we create a QPainter for the widget, and set its

    antialiased flag to the widget's preferred aliasing. We also

    translate the painters coordinate system, preparing to draw the

    widget's cocentric circles. The translation ensures that the

    center of the circles will be equivalent to the widget's center.

    \snippet examples/painting/concentriccircles/circlewidget.cpp 7

    When painting a circle, we use the number of "animation frames" to

    determine the alpha channel of the circle's color. The alpha

    channel specifies the color's transparency effect, 0 represents a

    fully transparent color, while 255 represents a fully opaque

    color.

    \snippet examples/painting/concentriccircles/circlewidget.cpp 8

    If the calculated alpha channel is fully transparent, we don't

    draw anything since that would be equivalent to drawing a white

    circle on a white background. Instead we skip to the next circle

    still creating a white space. If the calculated alpha channel is

    fully opaque, we set the pen (the QColor passed to the QPen

    constructor is converted into the required QBrush by default) and

    draw the circle. If the widget's preferred precision is float

    based, we specify the circle's bounding rectangle using QRectF and

    double values, otherwise we use QRect and integers.

    The animation is controlled by the public \c nextAnimationFrame()

    slot: Whenever the \c nextAnimationFrame() slot is called the

    number of frames is incremented and a paint event is

    scheduled. Then, when the widget is repainted, the alpha-blending

    of the circles' colors change and the circles appear as animated.

    \section1 Window Class Definition

    The Window class inherits QWidget, and is the application's main

    window rendering four \c {CircleWidget}s using different

    combinations of precision and aliasing.

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

    We declare the various components of the main window, i.e the text

    labels and a double array that will hold reference to the four \c

    {CircleWidget}s. In addition we declare the private \c

    createLabel() function to simplify the constructor.

    \section1 Window Class Implementation

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

    In the constructor, we first create the various labels and put

    them in a QGridLayout.

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

    Then we create a QTimer. The QTimer class is a high-level

    programming interface for timers, and provides repetitive and

    single-shot timers.

    We create a timer to facilitate the animation of our concentric

    circles; when we create the four CircleWidget instances (and add

    them to the layout), we connect the QTimer::timeout() signal to

    each of the widgets' \c nextAnimationFrame() slots.

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

    Before we set the layout and window title for our main window, we

    make the timer start with a timeout interval of 100 milliseconds,

    using the QTimer::start() function. That means that the

    QTimer::timeout() signal will be emitted, forcing a repaint of the

    four \c {CircleWidget}s, every 100 millisecond which is the reason

    the circles appear as animated.

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

    The private \c createLabel() function is implemented to simlify

    the constructor.

*/