Re: the right way to draw in a drawingarea

[Rob Benton <rob benton conwaycorp net>]

Paul Davis wrote:
> Foster, Gareth wrote:
>
> > > the display flickers, even when drawing a simple square.
> > >
> > >   
> >
> >
> > Doesn't this often mean you need to be double buffering?
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> >  
> This is a point I am completely unsure about.  I haven't the foggiest on 
> Gtk internals and the ramifications of a simple call such as 
> draw_rectangle.  I've used double buffering in both OpenGL and Java.  
> I've just never really seen any real reference to it in gtk.  Now that 
> I've had a couple beers, I seem to rememeber some reference to getting 
> an offscreen graphics context, drawing to that, and then switching the 
> main gc with it back and forth.  But I could be completely making that 
> up. ( the beer )
>
> Double buffering isn't really that hard of a concept.  I'm just not sure 
> of if its needed.  Because its more than possible that gtk has its own 
> buffering scheme burried somewhere.  I've actually written a couple 
> programs to animate image data using Gtkmm.  I've never had a problem 
> with flickering that wasn't because I was doing something silly like 
> clearing and then redrawing.  So the answer is I dunno.  Its possible.  
> If I get time I'll write a test program to try and play with the idea.
>
> And if I had thought to take a moment to google something as silly as 
> gtkmm double buffering, I would have noticed that the gtkmm docs specify 
> that widgets are double buffered by default.  In which case, no double 
> buffering will not solve this problem cause its already being done.  
> Which goes to show that yes, I was making up the graphics context 
> thing.  Must be something from java I was thinking of.
> http://www.gtkmm.org/docs/gtkmm-2.4/docs/reference/html/classGtk_1_1Widget.html#a175 
>
>
> In other news, Unleashed wasn't half bad if anyone cares.
>
> Cheers,
> Paul
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In my experience you always need double-buffering when you draw to a 
Gtk::DrawingArea.  I have not seen that 
Gtk::Widget::set_double_buffered() function before.  But you are not 
drawing directly to the Gtk::DrawingArea.  Instead you are drawing to a 
Gdk::Drawable such as a pixmap.  Here's how I do it in my code:

bool my_expose_event(GdkEventExpose * e)
{
	Glib::RefPtr<Gdk::Window> window = _drawarea.get_window();
	_pixmap->draw_rectangle(_gc, true, 0, 0, size, size);
	window->draw_drawable(_gc, _pixmap, e->area.x, e->area.y, e->area.x, 
e->area.y, e->area.width, e->area.height);

	return true;
}


And I can't speak for Unleashed but Shadow of the Colossus is pretty 
awesome. :)