I'm looking for a drag-and-zoom box. Where to start?

["Paul van Haren" <paul vanharen zonnet nl>]

Hi,

In my application I have a map, implemented as a GdkPixbuf, that I can manipulate and display. Now I want to 
extend this app with a zoom function as follows: the user clicks left and draws a non-filled rectangle while 
keeping the left button pressed. As soon as the left mouse button is depressed, the action is completed and I 
can execute the zoom function. The new view will show what is inside the drawn rectangle. 

However, I have no idea where to start to look for such a function. Should I be using low level primitives 
such as gdk_draw_rectangle and implement by own backing store, or is there a more convenient way to get it 
implemented.

Any help is appreciated. Thank, Paul van Haren