Drawing Curved Lines
| Chapter 16. The Drawing Area Widget |

## 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.

### Example

This simple application draws a curve with Cairo and displays the control points for each end of the curve.

Figure 16.3. Drawing Area - Lines

Source Code

File: `myarea.h`

    #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:
      virtual bool on_expose_event(GdkEventExpose* event);
    };

    #endif // GTKMM_EXAMPLE_MYAREA_H

File: `main.cc`

    #include "myarea.h"
    #include <gtkmm/main.h>
    #include <gtkmm/window.h>

    int main(int argc, char** argv)
    {
       Gtk::Main kit(argc, argv);

       Gtk::Window win;
       win.set_title("DrawingArea");

       MyArea area;
       win.add(area);
       area.show();

       Gtk::Main::run(win);

       return 0;
    }

File: `myarea.cc`

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

    MyArea::MyArea()
    {
    }

    MyArea::~MyArea()
    {
    }

    bool MyArea::on_expose_event(GdkEventExpose* event)
    {
      // This is where we draw on the window
      Glib::RefPtr<Gdk::Window> window = get_window();
      if(window)
      {
        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

        Cairo::RefPtr<Cairo::Context> cr = window->create_cairo_context();
        // clip to the area indicated by the expose event so that we only redraw
        // the portion of the window that needs to be redrawn
        cr->rectangle(event->area.x, event->area.y,
                event->area.width, event->area.height);
        cr->clip();

        // scale to unit square (0 to 1 with 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;
    }

The only difference between this example and the straight line example is in the `on_expose_event()` function, but there are a few new concepts and functions introduced here, so let's examine them briefly.

Note that we clip to the area that needs re-exposing just as we did in the last example. After clipping, however, 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 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 draw 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).

Drawing Straight Lines |  | Drawing Arcs and Circles