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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>
Qt Tutorial - Chapter 9: With Cannon You Can
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<p>
<h1 align=center>Chapter 9: With Cannon You Can</h1><br clear="all">
<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">lcdrange.h</a> contains the LCDRange
class definition
<li><a href="t9-lcdrange-cpp.html">lcdrange.cpp</a> contains the LCDRange
implementation
<li><a href="t9-cannon-h.html">cannon.h</a> contains the CannonField class
definition
<li><a href="t9-cannon-cpp.html">cannon.cpp</a> contains the CannonField
implementation
<li><a href="t9-main-cpp.html">main.cpp</a> contains MyWidget and main.
<li><a href="t9-makefile.html">Makefile</a> contains some rules for
generating the meta object information necessary for <a
href="signalsandslots.html">signal/slot creation.</a>
</ul>
<p>
<h2>Line by Line Walk-Through</h2>
<p>
<h3><a href="t9-cannon-cpp.html">cannon.cpp</a></h3> <pre>
void CannonField::paintEvent( <a href="qpaintevent.html">QPaintEvent</a> * )
{
<a href="qpainter.html">QPainter</a> p( this );
</pre>
<p>
We'll now start to use QPainter in earnest. We create a painter that
operates on this widget. <pre>
p.<a href="qpainter.html#3e0cc8">setBrush</a>( blue );
</pre>
<p>
When QPainter fills a rectangle, a circle or something, it fills the
shape using its brush. Here, we set it to use a blue brush. (We
could also use a pattern.) <pre>
p.<a href="qpainter.html#0183e4">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. <pre>
p.<a href="qpainter.html#eb778c">translate</a>( 0, <a href="qwidget.html#75ae71">rect</a>().bottom() );
</pre>
<p>
The <a href="qpainter.html#eb778c">QPainter::translate()</a> function translates the coordinate system
of the QPainter, i.e. 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). <pre>
p.<a href="qpainter.html#3ca7a2">drawPie</a>( <a href="qrect.html">QRect</a>(-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. <pre>
p.<a href="qpainter.html#b5205c">rotate</a>( -ang );
</pre>
<p>
The QPainter::rotate() function rotates the coordinate system of the
QPainter around the origin. The rotation argument is a <code>float</code> given
in degrees (not given in 1/16th of a degree as above) and clockwise.
Here we rotate the coordinate system <code>ang</code> degrees counter-clockwise. <pre>
p.<a href="qpainter.html#4c0077">drawRect</a>( <a href="qrect.html">QRect</a>(33, -4, 15, 8) );
</pre>
<p>
The QPainter::drawRect() 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, then rotated.
If the rectangle QRect(33, -4, 15, 8) had been drawn in the translated
coordinate system, it would have looked like this:
<p>
<img src="t9_1.png" alt="The cannon, translated but not rotated">
<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, since we have translated the coordinate system.
The result looks like this:
<p>
<img src="t9_2.png" alt="The cannon, translated and rotated">
<p>
We're done, except that we haven't explained why Windows didn't dither
this time. <pre>
int main( int argc, char **argv )
{
<a href="qapplication.html#1ee2d1">QApplication::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. Since
this program uses an unusual yellow but not many colors, <code>CustomColor</code> is best. There are several other allocation strategies.
You can read about them in the <a href="qapplication.html#1ee2d1">QApplication::setColorSpec()</a>
documentation.
<p>
Mostly, you can ignore this - the default is good. Occasionally some
applications with unusual color use look bad, and often changing the
allocation strategy helps.
<p>
<h2>Behavior</h2>
<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 eight.
<p>
You may notice that the cannon flickers annoyingly, especially on a
slow machine. We'll fix this in the next chapter.
<p>
<h2>Exercises</h2>
<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 may now go on to <a href="t10.html">chapter ten.</a>
<p>
[<a href="t8.html">Previous tutorial</a>]
[<a href="t10.html">Next tutorial</a>]
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<td>Copyright 2001 Trolltech<td><a href="http://www.trolltech.com/trademarks.html">Trademarks</a>
<td align="right"><div align="right">Qt version 2.3.2</div>
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