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| 6 | <title>Qt Tutorial - Chapter 9: With Cannon You Can</title>
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| 30 | <font color="#004faf">Functions</font></a>
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| 31 | </td>
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| 32 | <td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>Qt Tutorial - Chapter 9: With Cannon You Can</h1>
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| 33 |
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| 34 |
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| 35 | <p> <center><img src="t9.png" alt="Screenshot of tutorial nine"></center>
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| 36 | <p> In this example we become graphic by drawing a cute little blue
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| 37 | cannon. Only cannon.cpp differs from the previous chapter.
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| 38 | <p> <ul>
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| 39 | <li> <a href="t9-lcdrange-h.html">t9/lcdrange.h</a> contains the LCDRange
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| 40 | class definition.
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| 41 | <li> <a href="t9-lcdrange-cpp.html">t9/lcdrange.cpp</a> contains the LCDRange
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| 42 | implementation.
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| 43 | <li> <a href="t9-cannon-h.html">t9/cannon.h</a> contains the CannonField class
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| 44 | definition.
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| 45 | <li> <a href="t9-cannon-cpp.html">t9/cannon.cpp</a> contains the CannonField
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| 46 | implementation.
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| 47 | <li> <a href="t9-main-cpp.html">t9/main.cpp</a> contains MyWidget and main.
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| 48 | </ul>
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| 49 | <p> <h2> Line-by-line Walkthrough
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| 50 | </h2>
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| 51 | <a name="1"></a><p> <h3> <a href="t9-cannon-cpp.html">t9/cannon.cpp</a>
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| 52 | </h3>
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| 53 | <a name="1-1"></a><p>
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| 54 |
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| 55 | <p> <pre> <a name="x2346"></a>void CannonField::<a href="qwidget.html#paintEvent">paintEvent</a>( <a href="qpaintevent.html">QPaintEvent</a> * )
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| 56 | {
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| 57 | <a href="qpainter.html">QPainter</a> p( this );
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| 58 | </pre>
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| 59 | <p> We'll now start to use <a href="qpainter.html">QPainter</a> in earnest. We create a painter that
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| 60 | operates on this widget.
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| 61 | <p> <pre> <a name="x2343"></a> p.<a href="qpainter.html#setBrush">setBrush</a>( blue );
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| 62 | </pre>
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| 63 | <p> When QPainter fills a rectangle, a circle, or whatever, it fills the
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| 64 | shape using its brush. Here we set it to use a blue brush. (We
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| 65 | could also use a pattern.)
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| 66 | <p> <pre> <a name="x2344"></a> p.<a href="qpainter.html#setPen">setPen</a>( NoPen );
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| 67 | </pre>
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| 68 | <p> And the edges of what QPainter draws are drawn using the pen. Here we
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| 69 | set it to NoPen, meaning that there will be no special edge when we
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| 70 | draw something; the blue brush will go all the way to the edges of
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| 71 | the things we draw.
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| 72 | <p> <pre> <a name="x2345"></a> p.<a href="qpainter.html#translate">translate</a>( 0, rect().bottom() );
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| 73 | </pre>
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| 74 | <p> The <a href="qpainter.html#translate">QPainter::translate</a>() function translates the coordinate
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| 75 | system of the QPainter; i.e., it moves it by an offset. Here we set
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| 76 | the (0, 0) point to the bottom-left corner of the widget. The x and
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| 77 | y directions remain unchanged, i.e., all the y coordinates inside the
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| 78 | widget are now negative (see <a href="coordsys.html">The Coordinate
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| 79 | System</a> for more information about Qt's coordinate system).
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| 80 | <p> <pre> <a name="x2340"></a> p.<a href="qpainter.html#drawPie">drawPie</a>( QRect(-35, -35, 70, 70), 0, 90*16 );
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| 81 | </pre>
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| 82 | <p> The drawPie() function draws a pie shape inside the specified
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| 83 | rectangle using a start angle and an arc length. The angles are
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| 84 | specified in 1/16th of a degree. Zero degrees is at the 3 o'clock
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| 85 | position. The drawing direction is counter-clockwise. Here we draw a
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| 86 | quarter of a circle in the bottom-left corner of the widget. The pie
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| 87 | is filled with blue and has no outline.
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| 88 | <p> <pre> <a name="x2342"></a> p.<a href="qpainter.html#rotate">rotate</a>( -ang );
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| 89 | </pre>
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| 90 | <p> The <a href="qpainter.html#rotate">QPainter::rotate</a>() function rotates the coordinate system of the
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| 91 | <a href="qpainter.html">QPainter</a> around the origin. The rotation argument is a <tt>float</tt> given
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| 92 | in degrees (not given in 1/16th of a degree as above) and clockwise.
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| 93 | Here we rotate the coordinate system <tt>ang</tt> degrees counter-clockwise.
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| 94 | <p> <pre> <a name="x2341"></a> p.<a href="qpainter.html#drawRect">drawRect</a>( QRect(33, -4, 15, 8) );
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| 95 | </pre>
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| 96 | <p> The <a href="qpainter.html#drawRect">QPainter::drawRect</a>() function draws the specified rectangle. Here
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| 97 | we draw the barrel of the cannon.
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| 98 | <p> It can often be difficult to envision the resulting drawing when the
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| 99 | coordinate system has been transformed (translated, rotated, scaled, or
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| 100 | sheared) as above.
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| 101 | <p> In this case the coordinate system is first translated and then rotated.
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| 102 | If the rectangle <a href="qrect.html">QRect</a>(33, -4, 15, 8) had been drawn in the translated
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| 103 | coordinate system, it would have looked like this:
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| 104 | <p> <center><img src="t9_1.png" alt="The cannon translated but not rotated"></center>
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| 105 | <p> Note that the rectangle is clipped by the border of the CannonField
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| 106 | widget. When we rotate the coordinate system, for instance 60
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| 107 | degrees, the rectangle will be rotated around (0, 0), which is the
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| 108 | bottom-left corner because we have translated the coordinate system.
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| 109 | The result looks like this:
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| 110 | <p> <center><img src="t9_2.png" alt="The cannon translated and rotated"></center>
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| 111 | <p> We're done, except that we haven't explained why Windows didn't dither
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| 112 | this time.
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| 113 | <p>
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| 114 |
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| 115 | <pre> int main( int argc, char **argv )
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| 116 | {
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| 117 | <a name="x2347"></a> QApplication::<a href="qapplication.html#setColorSpec">setColorSpec</a>( QApplication::CustomColor );
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| 118 | <a href="qapplication.html">QApplication</a> a( argc, argv );
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| 119 | </pre>
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| 120 | <p> We tell Qt that we want a different color-allocation strategy for this
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| 121 | program. There is no single correct color-allocation strategy. Because
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| 122 | this program uses an unusual yellow but not many colors, <tt>CustomColor</tt> is best. There are several other allocation strategies; you can read about them in the <a href="qapplication.html#setColorSpec">QApplication::setColorSpec</a>()
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| 123 | documentation.
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| 124 | <p> Mostly you can ignore this, since the default is good. Occasionally
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| 125 | some applications with unusual color use look bad; changing the
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| 126 | allocation strategy often helps then.
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| 127 | <p> <h2> Behavior
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| 128 | </h2>
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| 129 | <a name="2"></a><p> When the slider is operated the angle of the drawn cannon changes
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| 130 | accordingly.
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| 131 | <p> The Q on the Quit button is now underlined, and Alt+Q does what you
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| 132 | think it does. If you do not know why, you didn't do the exercises in
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| 133 | Chapter 8.
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| 134 | <p> You may notice that the cannon flickers annoyingly, especially on a
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| 135 | slow machine. We'll fix this in the next chapter.
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| 136 | <p> (See <a href="tutorial1-07.html#compiling">Compiling</a> for how to create a
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| 137 | makefile and build the application.)
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| 138 | <p> <h2> Exercises
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| 139 | </h2>
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| 140 | <a name="3"></a><p> Set a different pen instead of NoPen. Set a patterned brush.
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| 141 | <p> Try "Q&uit" or "Qu&it" as button text instead of "&Quit". What
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| 142 | happens?
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| 143 | <p> You're now ready for <a href="tutorial1-10.html">Chapter 10.</a>
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| 144 | <p> [<a href="tutorial1-08.html">Previous tutorial</a>]
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| 145 | [<a href="tutorial1-10.html">Next tutorial</a>]
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| 146 | [<a href="tutorial.html">Main tutorial page</a>]
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| 147 | <p>
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| 150 | <table width=100% cellspacing=0 border=0><tr>
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| 151 | <td>Copyright © 2007
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| 152 | <a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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| 153 | <td align=right><div align=right>Qt 3.3.8</div>
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| 154 | </table></div></address></body>
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| 155 | </html>
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