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216 lines
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216 lines
13 KiB
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<!-- /home/espenr/tmp/qt-3.3.8-espenr-2499/qt-x11-free-3.3.8/doc/coordsys.doc:36 -->
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<title>The Coordinate System</title>
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<font color="#004faf">Home</font></a>
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<font color="#004faf">Grouped Classes</font></a>
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<font color="#004faf">Functions</font></a>
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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>The Coordinate System</h1>
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<p> A <a href="ntqpaintdevice.html">paint device</a> in TQt is a drawable 2D
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surface. <a href="ntqwidget.html">TQWidget</a>, <a href="ntqpixmap.html">TQPixmap</a>, <a href="ntqpicture.html">TQPicture</a> and <a href="ntqprinter.html">TQPrinter</a> are all
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paint devices. A <a href="ntqpainter.html">TQPainter</a> is an object which can draw on such
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devices.
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<p> The default coordinate system of a paint device has its origin at the
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top left corner. X increases to the right and Y increases downwards.
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The unit is one pixel on pixel-based devices and one point on
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printers.
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<p> <h2> An Example
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</h2>
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<a name="1"></a><p> The illustration below shows a highly magnified portion of the top
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left corner of a paint device.
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<p> <center><img src="coordsys.png"></center>
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<p> The rectangle and the line were drawn by this code (with the grid
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added and colors touched up in the illustration):
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<p> <pre>
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void MyWidget::paintEvent( <a href="qpaintevent.html">TQPaintEvent</a> * )
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{
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<a href="ntqpainter.html">TQPainter</a> p( this );
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p.<a href="ntqpainter.html#setPen">setPen</a>( darkGray );
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p.<a href="ntqpainter.html#drawRect">drawRect</a>( 1,2, 5,4 );
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p.<a href="ntqpainter.html#setPen">setPen</a>( lightGray );
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p.<a href="ntqpainter.html#drawLine">drawLine</a>( 9,2, 7,7 );
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}
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</pre>
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<p> Note that all of the pixels drawn by drawRect() are inside the size
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specified (5*4 pixels). This is different from some toolkits; in TQt
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the size you specify exactly encompasses the pixels drawn. This
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applies to all the relevant functions in <a href="ntqpainter.html">TQPainter</a>.
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<p> Similarly, the drawLine() call draws both endpoints of the line, not
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just one.
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<p> Here are the classes that relate most closely to the coordinate
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system:
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<p> <center><table cellpadding="4" cellspacing="2" border="0">
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<tr bgcolor="#f0f0f0"> <td valign="top"><a href="ntqpoint.html">TQPoint</a>
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<td valign="top">A single 2D point in the coordinate system. Most functions in
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TQt that deal with points can accept either a <a href="ntqpoint.html">TQPoint</a> argument
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or two ints, for example <a href="ntqpainter.html#drawPoint">TQPainter::drawPoint</a>().
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<tr bgcolor="#d0d0d0"> <td valign="top"><a href="ntqsize.html">TQSize</a>
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<td valign="top">A single 2D vector. Internally, TQPoint and <a href="ntqsize.html">TQSize</a> are the same,
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but a point is not the same as a size, so both classes exist.
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Again, most functions accept either a TQSize or two ints, for
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example <a href="ntqwidget.html#resize">TQWidget::resize</a>().
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<tr bgcolor="#f0f0f0"> <td valign="top"><a href="ntqrect.html">TQRect</a>
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<td valign="top">A 2D rectangle. Most functions accept either a <a href="ntqrect.html">TQRect</a> or four
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ints, for example <a href="ntqwidget.html#setGeometry">TQWidget::setGeometry</a>().
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<tr bgcolor="#d0d0d0"> <td valign="top"><a href="ntqregion.html">TQRegion</a>
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<td valign="top">An arbitrary set of points, including all the normal set
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operations, e.g. <a href="ntqregion.html#intersect">TQRegion::intersect</a>(), and also a less
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usual function to return a list of rectangles whose union is
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equal to the region. <a href="ntqregion.html">TQRegion</a> is used e.g. by <a href="ntqpainter.html#setClipRegion">TQPainter::setClipRegion</a>(), <a href="ntqwidget.html#repaint">TQWidget::repaint</a>() and <a href="qpaintevent.html#region">TQPaintEvent::region</a>().
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<tr bgcolor="#f0f0f0"> <td valign="top"><a href="ntqpainter.html">TQPainter</a>
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<td valign="top">The class that paints. It can paint on any device with the
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same code. There are differences between devices, <a href="ntqprinter.html#newPage">TQPrinter::newPage</a>() is a good example, but <a href="ntqpainter.html">TQPainter</a> works the
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same way on all devices.
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<tr bgcolor="#d0d0d0"> <td valign="top"><a href="ntqpaintdevice.html">TQPaintDevice</a>
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<td valign="top">A device on which TQPainter can paint. There are two internal
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devices, both pixel-based, and two external devices, <a href="ntqprinter.html">TQPrinter</a> and <a href="ntqpicture.html">TQPicture</a> (which records TQPainter commands to a
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file or other <a href="ntqiodevice.html">TQIODevice</a>, and plays them back). Other
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devices can be defined.
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</table></center>
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<p> <h2> Transformations
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</h2>
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<a name="2"></a><p> Although TQt's default coordinate system works as described above, <a href="ntqpainter.html">TQPainter</a> also supports arbitrary transformations.
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<p> This transformation engine is a three-step pipeline, closely following
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the model outlined in books such as
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<a href="http://www.amazon.com/exec/obidos/ASIN/0201848406/trolltech/t">Foley & Van Dam</a> and the
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<a href="http://www.amazon.com/exec/obidos/ASIN/0201604582/trolltech/t">OpenGL Programming Guide.</a> Refer to those for in-depth
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coverage; here we give just a brief overview and an example.
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<p> The first step uses the world <a href="ntqwmatrix.html#TransformationMode">transformation matrix</a>. Use this matrix
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to orient and position your objects in your model. TQt provides
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methods such as <a href="ntqpainter.html#rotate">TQPainter::rotate</a>(), <a href="ntqpainter.html#scale">TQPainter::scale</a>(), <a href="ntqpainter.html#translate">TQPainter::translate</a>() and so on to operate on this matrix.
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<p> <a href="ntqpainter.html#save">TQPainter::save</a>() and <a href="ntqpainter.html#restore">TQPainter::restore</a>() save and restore this
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matrix. You can also use <a href="ntqwmatrix.html">TQWMatrix</a> objects, <a href="ntqpainter.html#worldMatrix">TQPainter::worldMatrix</a>() and <a href="ntqpainter.html#setWorldMatrix">TQPainter::setWorldMatrix</a>() to store and
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use named matrices.
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<p> The second step uses the window. The window describes the view
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boundaries in model coordinates. The matrix positions the <em>objects</em>
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and <a href="ntqpainter.html#setWindow">TQPainter::setWindow</a>() positions the <em>window</em>, deciding what
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coordinates will be visible. (If you have 3D experience, the window
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is what's usually called projection in 3D.)
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<p> The third step uses the viewport. The viewport too, describes the view
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boundaries, but in device coordinates. The viewport and the windows
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describe the same rectangle, but in different coordinate systems.
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<p> On-screen, the default is the entire <a href="ntqwidget.html">TQWidget</a> or <a href="ntqpixmap.html">TQPixmap</a> where
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you are drawing, which is usually appropriate. For printing this
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function is vital, since very few printers can print over the entire
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physical page.
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<p> So each object to be drawn is transformed into model
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coordinates using <a href="ntqpainter.html#worldMatrix">TQPainter::worldMatrix</a>(), then positioned
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on the drawing device using <a href="ntqpainter.html#window">TQPainter::window</a>() and
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<a href="ntqpainter.html#viewport">TQPainter::viewport</a>().
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<p> It is perfectly possible to do without one or two of the stages. If,
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for example, your goal is to draw something scaled, then just using <a href="ntqpainter.html#scale">TQPainter::scale</a>() makes perfect sense. If your goal is to use a
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fixed-size coordinate system, <a href="ntqpainter.html#setWindow">TQPainter::setWindow</a>() is
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ideal. And so on.
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<p> Here is a short example that uses all three mechanisms: the function
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that draws the clock face in the <a href="aclock-example.html">aclock/aclock.cpp</a> example. We
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recommend compiling and running the example before you read any
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further. In particular, try resizing the window to different sizes.
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<p>
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<pre> void AnalogClock::drawClock( <a href="ntqpainter.html">TQPainter</a> *paint )
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{
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<a name="x2275"></a> paint-><a href="ntqpainter.html#save">save</a>();
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</pre>
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<p> Firstly, we save the painter's state, so that the calling function
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is guaranteed not to be disturbed by the transformations we're going
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to use.
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<p> <pre> <a name="x2277"></a> paint-><a href="ntqpainter.html#setWindow">setWindow</a>( -500,-500, 1000,1000 );
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</pre>
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<p> We set the model coordinate system we want a 1000*1000 window where
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0,0 is in the middle.
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<p> <pre> <a name="x2278"></a> <a href="ntqrect.html">TQRect</a> v = paint-><a href="ntqpainter.html#viewport">viewport</a>();
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<a name="x2282"></a><a name="x2279"></a> int d = TQMIN( v.<a href="ntqrect.html#width">width</a>(), v.<a href="ntqrect.html#height">height</a>() );
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</pre>
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<p> The device may not be square and we want the clock to be, so we find
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its current viewport and compute its shortest side.
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<p> <pre> <a name="x2280"></a><a name="x2276"></a> paint-><a href="ntqpainter.html#setViewport">setViewport</a>( v.<a href="ntqrect.html#left">left</a>() + (v.<a href="ntqrect.html#width">width</a>()-d)/2,
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<a name="x2281"></a> v.<a href="ntqrect.html#top">top</a>() + (v.<a href="ntqrect.html#height">height</a>()-d)/2, d, d );
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</pre>
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<p> Then we set a new square viewport, centered in the old one.
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<p> We're now done with our view. From this point on, when we draw in a
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1000*1000 area around 0,0, what we draw will show up in the largest
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possible square that'll fit in the output device.
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<p> Time to start drawing.
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<p> <pre> <a href="ntqpointarray.html">TQPointArray</a> pts;
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</pre>
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<p> <em>pts</em> is just a temporary variable to hold some points.
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<p> Next come three drawing blocks, one for the hour hand, one for the
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minute hand and finally one for the clock face itself. First we draw
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the hour hand:
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<p> <pre> paint-><a href="ntqpainter.html#save">save</a>();
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<a name="x2274"></a> paint-><a href="ntqpainter.html#rotate">rotate</a>( 30*(time.hour()%12-3) + time.minute()/2 );
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</pre>
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<p> We save the painter and then rotate it so that one axis points along
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the hour hand.
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<p> <pre> pts.setPoints( 4, -20,0, 0,-20, 300,0, 0,20 );
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<a name="x2271"></a> paint-><a href="ntqpainter.html#drawConvexPolygon">drawConvexPolygon</a>( pts );
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</pre>
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<p> We set <em>pts</em> to a four-point polygon that looks like the hour hand at
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three o'clock, and draw it. Because of the rotation, it's drawn
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pointed in the right direction.
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<p> <pre> <a name="x2273"></a> paint-><a href="ntqpainter.html#restore">restore</a>();
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</pre>
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<p> We restore the saved painter, undoing the rotation. We could also
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call rotate( -30 ) but that might introduce rounding errors, so it's
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better to use save() and restore(). Next, the minute hand, drawn
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almost the same way:
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<p> <pre> paint-><a href="ntqpainter.html#save">save</a>();
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paint-><a href="ntqpainter.html#rotate">rotate</a>( (time.minute()-15)*6 );
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pts.setPoints( 4, -10,0, 0,-10, 400,0, 0,10 );
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paint-><a href="ntqpainter.html#drawConvexPolygon">drawConvexPolygon</a>( pts );
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paint-><a href="ntqpainter.html#restore">restore</a>();
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</pre>
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<p> The only differences are how the rotation angle is computed and the
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shape of the polygon.
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<p> The last part to be drawn is the clock face itself.
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<p> <pre> for ( int i=0; i<12; i++ ) {
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<a name="x2272"></a> paint-><a href="ntqpainter.html#drawLine">drawLine</a>( 440,0, 460,0 );
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paint-><a href="ntqpainter.html#rotate">rotate</a>( 30 );
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}
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</pre>
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<p> Twelve short hour lines at thirty-degree intervals. At the end of
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that, the painter is rotated in a way which isn't very useful, but
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we're done with painting so that doesn't matter.
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<p> <pre> paint-><a href="ntqpainter.html#restore">restore</a>();
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}
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</pre>
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<p> The final line of the function restores the painter, so that the
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caller won't be affected by all the transformations we've done.
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<p>
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<!-- eof -->
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<p><address><hr><div align=center>
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<table width=100% cellspacing=0 border=0><tr>
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<td>Copyright © 2007
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<a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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<td align=right><div align=right>TQt 3.3.8</div>
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</table></div></address></body>
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</html>
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