In addition to drawing straight lines Cairo allows you to easily
draw curved lines (technically a cubic Bézier spline) using the
Cairo::Context::curve_to()
and
Cairo::Context::rel_curve_to()
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.
This simple application draws a curve with Cairo and displays the control points for each end of the curve.
File: myarea.h
(For use with gtkmm 4)
#ifndef GTKMM_EXAMPLE_MYAREA_H
#define GTKMM_EXAMPLE_MYAREA_H
#include <gtkmm/drawingarea.h>
class MyArea : public Gtk::DrawingArea
{
public:
MyArea();
virtual ~MyArea();
protected:
void on_draw(const Cairo::RefPtr<Cairo::Context>& cr, int width, int height);
};
#endif // GTKMM_EXAMPLE_MYAREA_H
File: main.cc
(For use with gtkmm 4)
#include "myarea.h"
#include <gtkmm/application.h>
#include <gtkmm/window.h>
class ExampleWindow : public Gtk::Window
{
public:
ExampleWindow();
protected:
MyArea m_area;
};
ExampleWindow::ExampleWindow()
{
set_title("DrawingArea");
set_child(m_area);
}
int main(int argc, char** argv)
{
auto app = Gtk::Application::create("org.gtkmm.example");
return app->make_window_and_run<ExampleWindow>(argc, argv);
}
File: myarea.cc
(For use with gtkmm 4)
#include "myarea.h"
#include <cairomm/context.h>
MyArea::MyArea()
{
set_draw_func(sigc::mem_fun(*this, &MyArea::on_draw));
}
MyArea::~MyArea()
{
}
void MyArea::on_draw(const Cairo::RefPtr<Cairo::Context>& cr, int width, int 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();
}
The only difference between this example and the straight line
example is in the on_draw()
function,
but there are a few new concepts and functions introduced here, so
let's examine them briefly.
We make a call to
Cairo::Context::scale()
, 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.
The call to Cairo::Context::curve_to()
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
set_source_rgb()
called
set_source_rgba()
. This function takes a
fourth argument specifying the alpha value of the color (valid
values are between 0 and 1).