Unable to paint on Gtk drawing_area with Cairo

Question

I am trying to paint a point on drawing_area in Gtk with cairo. I have two functions, do_drawing function to paint the background on draw signal, and the second one do_drawPoint to draw a point.

void painter::do_drawing(cairo_t *cr)
{
cairo_set_source_rgb(cr, 1, 1, 1);
cairo_paint(cr);    
}

void painter::do_pointDraw(cairo_t *cr)
{
cairo_set_source_rgb(cr, 0, 0, 0);
cairo_arc(cr, 150, 150, 10, 0, 2 * M_PI);
cairo_fill(cr);    
}

Now, the drawPoint function is called whenever user clicks on the drawing_area.

void drawingArea::drawPoint()
{
g_signal_connect(area, "button-press-event", 
  G_CALLBACK(clicked), NULL);
std::cout<<"drawPoint"<<std::endl;
}

gboolean clicked(GtkWidget *widget, GdkEventButton *event, cairo_t *cr,
gpointer user_data)
{
if (event->button == 1) {
    x = event->x;
    y = event->y;
}
std::cout<<x<<" and "<<y<<std::endl;
ptr->do_pointDraw(cr);
gtk_widget_queue_draw(widget);
}
return TRUE;
}

But after printing values of x and y, the program terminates with segmentation fault.

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff74ac3ae in cairo_set_source_rgb ()

What might be causing this? Am I proceeding correctly?

You might want to recheck the code you pasted: it probably can't compile as it is (please fix the indenting if you do). — Jussi Kukkonen

Jussi Kukkonen Jussi Kukkonen · Accepted Answer · 2014-08-01T11:43:08

It's hard to read the code (with broken indentation and with only snippets provided), but some things to note:

button-press-event handler signature is incorrect. The cairo context is not there.
Drawing should happen only inside the draw signal handler of GtkDrawingArea. The idea of drawing on button press is just not how Cairo works: user interaction should change the state of your app, draw-handler should draw based on the current app state.
button-press-event handler is connected inside drawPoint (). This sounds very wrong.

I suggest you try reading the DrawingArea docs and start with the simple example it gives, and then start adding more complex things when that works.

Unable to paint on Gtk drawing_area with Cairo

1 Answers