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+<title>Qt Tutorial - Chapter 9: With Cannon You Can</title>
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+<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>
+
+
+<p> <center><img src="t9.png" alt="Screenshot of tutorial nine"></center>
+<p> In this example we become graphic by drawing a cute little blue
+cannon. Only cannon.cpp differs from the previous chapter.
+<p> <ul>
+<li> <a href="t9-lcdrange-h.html">t9/lcdrange.h</a> contains the LCDRange
+class definition.
+<li> <a href="t9-lcdrange-cpp.html">t9/lcdrange.cpp</a> contains the LCDRange
+implementation.
+<li> <a href="t9-cannon-h.html">t9/cannon.h</a> contains the CannonField class
+definition.
+<li> <a href="t9-cannon-cpp.html">t9/cannon.cpp</a> contains the CannonField
+implementation.
+<li> <a href="t9-main-cpp.html">t9/main.cpp</a> contains MyWidget and main.
+</ul>
+<p> <h2> Line-by-line Walkthrough
+</h2>
+<a name="1"></a><p> <h3> <a href="t9-cannon-cpp.html">t9/cannon.cpp</a>
+</h3>
+<a name="1-1"></a><p>
+
+<p> <pre> <a name="x2346"></a>void CannonField::<a href="qwidget.html#paintEvent">paintEvent</a>( <a href="qpaintevent.html">QPaintEvent</a> * )
+ {
+ <a href="qpainter.html">QPainter</a> p( this );
+</pre>
+<p> We'll now start to use <a href="qpainter.html">QPainter</a> in earnest. We create a painter that
+operates on this widget.
+<p> <pre> <a name="x2343"></a> p.<a href="qpainter.html#setBrush">setBrush</a>( blue );
+</pre>
+<p> When QPainter fills a rectangle, a circle, or whatever, it fills the
+shape using its brush. Here we set it to use a blue brush. (We
+could also use a pattern.)
+<p> <pre> <a name="x2344"></a> p.<a href="qpainter.html#setPen">setPen</a>( NoPen );
+</pre>
+<p> And the edges of what QPainter draws are drawn using the pen. Here we
+set it to NoPen, meaning that there will be no special edge when we
+draw something; the blue brush will go all the way to the edges of
+the things we draw.
+<p> <pre> <a name="x2345"></a> p.<a href="qpainter.html#translate">translate</a>( 0, rect().bottom() );
+</pre>
+<p> The <a href="qpainter.html#translate">QPainter::translate</a>() function translates the coordinate
+system of the QPainter; i.e., it moves it by an offset. Here we set
+the (0, 0) point to the bottom-left corner of the widget. The x and
+y directions remain unchanged, i.e., all the y coordinates inside the
+widget are now negative (see <a href="coordsys.html">The Coordinate
+System</a> for more information about Qt's coordinate system).
+<p> <pre> <a name="x2340"></a> p.<a href="qpainter.html#drawPie">drawPie</a>( QRect(-35, -35, 70, 70), 0, 90*16 );
+</pre>
+<p> The drawPie() function draws a pie shape inside the specified
+rectangle using a start angle and an arc length. The angles are
+specified in 1/16th of a degree. Zero degrees is at the 3 o'clock
+position. The drawing direction is counter-clockwise. Here we draw a
+quarter of a circle in the bottom-left corner of the widget. The pie
+is filled with blue and has no outline.
+<p> <pre> <a name="x2342"></a> p.<a href="qpainter.html#rotate">rotate</a>( -ang );
+</pre>
+<p> The <a href="qpainter.html#rotate">QPainter::rotate</a>() function rotates the coordinate system of the
+<a href="qpainter.html">QPainter</a> around the origin. The rotation argument is a <tt>float</tt> given
+in degrees (not given in 1/16th of a degree as above) and clockwise.
+Here we rotate the coordinate system <tt>ang</tt> degrees counter-clockwise.
+<p> <pre> <a name="x2341"></a> p.<a href="qpainter.html#drawRect">drawRect</a>( QRect(33, -4, 15, 8) );
+</pre>
+<p> The <a href="qpainter.html#drawRect">QPainter::drawRect</a>() function draws the specified rectangle. Here
+we draw the barrel of the cannon.
+<p> It can often be difficult to envision the resulting drawing when the
+coordinate system has been transformed (translated, rotated, scaled, or
+sheared) as above.
+<p> In this case the coordinate system is first translated and then rotated.
+If the rectangle <a href="qrect.html">QRect</a>(33, -4, 15, 8) had been drawn in the translated
+coordinate system, it would have looked like this:
+<p> <center><img src="t9_1.png" alt="The cannon translated but not rotated"></center>
+<p> Note that the rectangle is clipped by the border of the CannonField
+widget. When we rotate the coordinate system, for instance 60
+degrees, the rectangle will be rotated around (0, 0), which is the
+bottom-left corner because we have translated the coordinate system.
+The result looks like this:
+<p> <center><img src="t9_2.png" alt="The cannon translated and rotated"></center>
+<p> We're done, except that we haven't explained why Windows didn't dither
+this time.
+<p>
+
+<pre> int main( int argc, char **argv )
+ {
+ <a name="x2347"></a> QApplication::<a href="qapplication.html#setColorSpec">setColorSpec</a>( QApplication::CustomColor );
+ <a href="qapplication.html">QApplication</a> a( argc, argv );
+</pre>
+<p> We tell Qt that we want a different color-allocation strategy for this
+program. There is no single correct color-allocation strategy. Because
+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>()
+documentation.
+<p> Mostly you can ignore this, since the default is good. Occasionally
+some applications with unusual color use look bad; changing the
+allocation strategy often helps then.
+<p> <h2> Behavior
+</h2>
+<a name="2"></a><p> When the slider is operated the angle of the drawn cannon changes
+accordingly.
+<p> The Q on the Quit button is now underlined, and Alt+Q does what you
+think it does. If you do not know why, you didn't do the exercises in
+Chapter 8.
+<p> You may notice that the cannon flickers annoyingly, especially on a
+slow machine. We'll fix this in the next chapter.
+<p> (See <a href="tutorial1-07.html#compiling">Compiling</a> for how to create a
+makefile and build the application.)
+<p> <h2> Exercises
+</h2>
+<a name="3"></a><p> Set a different pen instead of NoPen. Set a patterned brush.
+<p> Try "Q&uit" or "Qu&it" as button text instead of "&Quit". What
+happens?
+<p> You're now ready for <a href="tutorial1-10.html">Chapter 10.</a>
+<p> [<a href="tutorial1-08.html">Previous tutorial</a>]
+[<a href="tutorial1-10.html">Next tutorial</a>]
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