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<th width="60%" align="center">Chapter 17. The Drawing Area Widget</th>
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<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="sec-cairo-curved-lines"></a>Drawing Curved Lines</h2></div></div></div>
<p>
In addition to drawing straight lines Cairo allows you to easily
draw curved lines (technically a cubic Bézier spline) using the
<code class="methodname">Cairo::Context::curve_to()</code> and
<code class="methodname">Cairo::Context::rel_curve_to()</code> functions.
These functions take coordinates for a destination point as well as
coordinates for two 'control' points. This is best explained using
an example, so let's dive in.
</p>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="cairo-example-curves"></a>Example</h3></div></div></div>
<p>
This simple application draws a curve with Cairo and displays
the control points for each end of the curve.
</p>
<div class="figure">
<a name="figure-drawingarea-curve"></a><p class="title"><b>Figure 17.4. Drawing Area - Lines</b></p>
<div class="figure-contents"><div class="screenshot"><div><img src="figures/drawingarea_curve.png" alt="Drawing Area - Lines"></div></div></div>
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<br class="figure-break"><p><a class="ulink" href="http://git.gnome.org/browse/gtkmm-documentation/tree/examples/book/drawingarea/curve?h=gtkmm-3-24" target="_top">Source Code</a></p>
<p>File: <code class="filename">myarea.h</code> (For use with gtkmm 3, not gtkmm 2)
</p>
<pre class="programlisting">
#ifndef GTKMM_EXAMPLE_MYAREA_H
#define GTKMM_EXAMPLE_MYAREA_H
#include <gtkmm/drawingarea.h>
class MyArea : public Gtk::DrawingArea
{
public:
MyArea();
virtual ~MyArea();
protected:
//Override default signal handler:
bool on_draw(const Cairo::RefPtr<Cairo::Context>& cr) override;
};
#endif // GTKMM_EXAMPLE_MYAREA_H
</pre>
<p>File: <code class="filename">main.cc</code> (For use with gtkmm 3, not gtkmm 2)
</p>
<pre class="programlisting">
#include "myarea.h"
#include <gtkmm/application.h>
#include <gtkmm/window.h>
int main(int argc, char** argv)
{
auto app = Gtk::Application::create(argc, argv, "org.gtkmm.example");
Gtk::Window win;
win.set_title("DrawingArea");
MyArea area;
win.add(area);
area.show();
return app->run(win);
}
</pre>
<p>File: <code class="filename">myarea.cc</code> (For use with gtkmm 3, not gtkmm 2)
</p>
<pre class="programlisting">
#include "myarea.h"
#include <cairomm/context.h>
MyArea::MyArea()
{
}
MyArea::~MyArea()
{
}
bool MyArea::on_draw(const Cairo::RefPtr<Cairo::Context>& cr)
{
Gtk::Allocation allocation = get_allocation();
const int width = allocation.get_width();
const int height = allocation.get_height();
double x0=0.1, y0=0.5, // start point
x1=0.4, y1=0.9, // control point #1
x2=0.6, y2=0.1, // control point #2
x3=0.9, y3=0.5; // end point
// scale to unit square (0 to 1 width and height)
cr->scale(width, height);
cr->set_line_width(0.05);
// draw curve
cr->move_to(x0, y0);
cr->curve_to(x1, y1, x2, y2, x3, y3);
cr->stroke();
// show control points
cr->set_source_rgba(1, 0.2, 0.2, 0.6);
cr->move_to(x0, y0);
cr->line_to (x1, y1);
cr->move_to(x2, y2);
cr->line_to (x3, y3);
cr->stroke();
return true;
}
</pre>
<p>
The only difference between this example and the straight line
example is in the <code class="methodname">on_draw()</code> function,
but there are a few new concepts and functions introduced here, so
let's examine them briefly.
</p>
<p>
We make a call to
<code class="methodname">Cairo::Context::scale()</code>, passing in the width
and height of the drawing area. This scales the user-space
coordinate system such that the width and height of the widget
are both equal to 1.0 'units'. There's no particular reason to
scale the coordinate system in this case, but sometimes it can make
drawing operations easier.
</p>
<p>
The call to <code class="methodname">Cairo::Context::curve_to()</code> should
be fairly self-explanatory. The first pair of coordinates define
the control point for the beginning of the curve. The second set
of coordinates define the control point for the end of the curve,
and the last set of coordinates define the destination point. To
make the concept of control points a bit easier to visualize, a
line has been drawn from each control point to the end-point on the
curve that it is associated with. Note that these control point
lines are both translucent. This is achieved with a variant of
<code class="methodname">set_source_rgb()</code> called
<code class="methodname">set_source_rgba()</code>. This function takes a
fourth argument specifying the alpha value of the color (valid
values are between 0 and 1).
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