Drawing Curved Lines

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.

Figure 17.4. Drawing Area - Lines

Drawing Area - Lines

Source Code

File: myarea.h (For use with gtkmm 3, not gtkmm 2)


#include <gtkmm/drawingarea.h>

class MyArea : public Gtk::DrawingArea
  virtual ~MyArea();

  //Override default signal handler:
  bool on_draw(const Cairo::RefPtr<Cairo::Context>& cr) override;


File: main.cc (For use with gtkmm 3, not gtkmm 2)

#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;

   MyArea area;

   return app->run(win);

File: myarea.cc (For use with gtkmm 3, not gtkmm 2)

#include "myarea.h"
#include <cairomm/context.h>



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);

  // draw curve
  cr->move_to(x0, y0);
  cr->curve_to(x1, y1, x2, y2, x3, y3);
  // 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);

  return true;

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).