Re: drawables.

["Jonathon Jongsma" <jonathon jongsma gmail com>]

On 6/19/06, Morten Bo Nielsen <morten bo nielsen topsil com> wrote:
> hi
>
> I'm currently trying to make an application that retrieves an image
> from, say, jpeg, and I would like to draw some lines on top of this
> image.
>
> I have found this snippet in the tutotial
> (http://www.gtkmm.org/docs/gtkmm-2.4/docs/tutorial/html/ch15s06.html):
>
> bool myarea::on_expose_event(GdkEventExpose* ev)
> {
> Glib::RefPtr<Gdk::PixBuf> image =
> Gdk::PixBuf::create_from_file("myimage.png");
> image->render_to_drawable(get_window(), get_style()->get_black_gc(),
> 0, 0, 100, 80, image->get_width(), image->get_height(), // draw the
> whole image (from 0,0 to the full width,height) at 100,80 in the window
> Gdk::RGB_DITHER_NONE, 0, 0);
> return true;
> }
>
> It works. But I don't want to draw directly on the window, and every
> example I find the gdk::drawable used is from get_window().

First, I just want to make sure that you're aware of the distinction
between Gtk::Window and Gdk::Window.  (I apologize if you know all of
this already -- I have no idea what level of experience you have with
Gtk / gtkmm so I don't want to make any assumptions).  A
Gtk::DrawingArea contains its own Gdk::Window (which is just a
rectangular region on the screen), but this has nothing to do with the
Gtk::Window widget that it is displayed in.  The Gdk::Window is the
window you're getting when you call the get_window() function, not the
Gtk::Window.

> I don't understand why Gtk::DrawingArea is not gdk::drawable.

it is.  You just have to get its Gdk::Window to do the actual drawing
(Gdk::Window inherits from Gdk::Drawable).

> The question is what gtk-widget to use. Apparently only pixmap and
> bitmap are gdk::drawable, but they are usuable for "offscreen"
> rendering.
>
> Help, pseudocode, links to examples would be much appreciated.

Does that help, or did I just repeat things you already knew?

Jonner