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29 \example painting/concentriccircles
30 \title Concentric Circles Example
32 \brief The Concentric Circles example shows the improved rendering
33 quality that can be obtained using floating point precision and
34 anti-aliasing when drawing custom widgets. The example also shows
35 how to do simple animations.
37 The application's main window displays several widgets which are
38 drawn using the various combinations of precision and
41 \image concentriccircles-example.png
43 Anti-aliasing is one of QPainter's render hints. The
44 QPainter::RenderHints are used to specify flags to QPainter that
45 may, or may not, be respected by any given
46 engine. QPainter::Antialiasing indicates that the engine should
47 anti-alias the edges of primitives if possible, i.e. put
48 additional pixels around the original ones to smooth the edges.
50 The difference between floating point precision and integer
51 precision is a matter of accuracy, and is visible in the
52 application's main window: Even though the logic that is
53 calculating the circles' geometry is the same, floating points
54 ensure that the white spaces between each circle are of the same
55 size, while integers make two and two circles appear as if they
56 belong together. The reason is that the integer based precision
57 rely on rounding off non-integer calculations.
59 The example consists of two classes:
62 \li \c CircleWidget is a custom widget which renders several animated
64 \li \c Window is the application's main window displaying four \c
65 {CircleWidget}s drawn using different combinations of precision
69 First we will review the CircleWidget class, then we will take a
70 look at the Window class.
72 \section1 CircleWidget Class Definition
74 The CircleWidget class inherits QWidget, and is a custom widget
75 which renders several animated concentric circles.
77 \snippet painting/concentriccircles/circlewidget.h 0
79 We declare the \c floatBased and \c antialiased variables to hold
80 whether an instance of the class should be rendered with integer
81 or float based precision, and whether the rendering should be
82 anti-aliased or not. We also declare functions setting each of
85 In addition we reimplement the QWidget::paintEvent() function to
86 apply the various combinations of precision and anti-aliasing when
87 rendering, and to support the animation. We reimplement the
88 QWidget::minimumSizeHint() and QWidget::sizeHint() functions to
89 give the widget a reasonable size within our application.
91 We declare the private \c nextAnimationFrame() slot, and the
92 associated \c frameNo variable holding the number of "animation
93 frames" for the widget, to facilitate the animation.
95 \section1 CircleWidget Class Implementation
97 In the constructor we make the widget's rendering integer based
98 and aliased by default:
100 \snippet painting/concentriccircles/circlewidget.cpp 0
102 We initialize the widget's \c frameNo variable, and set the
103 widget's background color using the QWidget::setBackgroundColor()
104 function which takes a \l {QPalette::ColorRole}{color role} as
105 argument; the QPalette::Base color role is typically white.
107 Then we set the widgets size policy using the
108 QWidget::setSizePolicy() function. QSizePolicy::Expanding means
109 that the widget's \l {QWidget::sizeHint()}{sizeHint()} is a
110 sensible size, but that the widget can be shrunk and still be
111 useful. The widget can also make use of extra space, so it should
112 get as much space as possible.
114 \snippet painting/concentriccircles/circlewidget.cpp 1
116 \snippet painting/concentriccircles/circlewidget.cpp 2
118 The public \c setFloatBased() and \c setAntialiased() functions
119 update the widget's rendering preferences, i.e. whether the widget
120 should be rendered with integer or float based precision, and
121 whether the rendering should be anti-aliased or not.
123 The functions also generate a paint event by calling the
124 QWidget::update() function, forcing a repaint of the widget with
125 the new rendering preferences.
127 \snippet painting/concentriccircles/circlewidget.cpp 3
129 \snippet painting/concentriccircles/circlewidget.cpp 4
131 The default implementations of the QWidget::minimumSizeHint() and
132 QWidget::sizeHint() functions return invalid sizes if there is no
133 layout for the widget, otherwise they return the layout's minimum and
134 preferred size, respectively.
136 We reimplement the functions to give the widget minimum and
137 preferred sizes which are reasonable within our application.
139 \snippet painting/concentriccircles/circlewidget.cpp 5
141 The nextAnimationFrame() slot simply increments the \c frameNo
142 variable's value, and calls the QWidget::update() function which
143 schedules a paint event for processing when Qt returns to the main
146 \snippet painting/concentriccircles/circlewidget.cpp 6
148 A paint event is a request to repaint all or part of the
149 widget. The \c paintEvent() function is an event handler that can
150 be reimplemented to receive the widget's paint events. We
151 reimplement the event handler to apply the various combinations of
152 precision and anti-aliasing when rendering the widget, and to
153 support the animation.
155 First, we create a QPainter for the widget, and set its
156 antialiased flag to the widget's preferred aliasing. We also
157 translate the painters coordinate system, preparing to draw the
158 widget's cocentric circles. The translation ensures that the
159 center of the circles will be equivalent to the widget's center.
161 \snippet painting/concentriccircles/circlewidget.cpp 7
163 When painting a circle, we use the number of "animation frames" to
164 determine the alpha channel of the circle's color. The alpha
165 channel specifies the color's transparency effect, 0 represents a
166 fully transparent color, while 255 represents a fully opaque
169 \snippet painting/concentriccircles/circlewidget.cpp 8
171 If the calculated alpha channel is fully transparent, we don't
172 draw anything since that would be equivalent to drawing a white
173 circle on a white background. Instead we skip to the next circle
174 still creating a white space. If the calculated alpha channel is
175 fully opaque, we set the pen (the QColor passed to the QPen
176 constructor is converted into the required QBrush by default) and
177 draw the circle. If the widget's preferred precision is float
178 based, we specify the circle's bounding rectangle using QRectF and
179 double values, otherwise we use QRect and integers.
181 The animation is controlled by the public \c nextAnimationFrame()
182 slot: Whenever the \c nextAnimationFrame() slot is called the
183 number of frames is incremented and a paint event is
184 scheduled. Then, when the widget is repainted, the alpha-blending
185 of the circles' colors change and the circles appear as animated.
187 \section1 Window Class Definition
189 The Window class inherits QWidget, and is the application's main
190 window rendering four \c {CircleWidget}s using different
191 combinations of precision and aliasing.
193 \snippet painting/concentriccircles/window.h 0
195 We declare the various components of the main window, i.e., the text
196 labels and a double array that will hold reference to the four \c
197 {CircleWidget}s. In addition we declare the private \c
198 createLabel() function to simplify the constructor.
200 \section1 Window Class Implementation
202 \snippet painting/concentriccircles/window.cpp 0
204 In the constructor, we first create the various labels and put
205 them in a QGridLayout.
207 \snippet painting/concentriccircles/window.cpp 1
209 Then we create a QTimer. The QTimer class is a high-level
210 programming interface for timers, and provides repetitive and
213 We create a timer to facilitate the animation of our concentric
214 circles; when we create the four CircleWidget instances (and add
215 them to the layout), we connect the QTimer::timeout() signal to
216 each of the widgets' \c nextAnimationFrame() slots.
218 \snippet painting/concentriccircles/window.cpp 2
220 Before we set the layout and window title for our main window, we
221 make the timer start with a timeout interval of 100 milliseconds,
222 using the QTimer::start() function. That means that the
223 QTimer::timeout() signal will be emitted, forcing a repaint of the
224 four \c {CircleWidget}s, every 100 millisecond which is the reason
225 the circles appear as animated.
227 \snippet painting/concentriccircles/window.cpp 3
229 The private \c createLabel() function is implemented to simlify