why.  the region field of gdkeventexpose allows you to redraw less than the traditional gdkeventregion.area.

in gtk+ 1.x, the gdkeventexpose structure only had an area field to let you determine the region that you needed to redraw. in gtk+ 2.x, this field exists for compatibility and as a simple interface. however, there is also a region field which contains a fine-grained region. the area field is simply the bounding rectangle of the region.

widgets that are very expensive to re-render, such as an image editor, may prefer to use the gdkeventexpose.region field to paint as little as possible. widgets that just use a few drawing primitives, such as labels and buttons, may prefer to use the traditional gdkeventexpose.area field for simplicity.

regions have an internal representation that is accessible as a list of rectangles. to turn the gdkeventexpose.region field into such a list, use gdk_region_get_rectangles():

static gboolean
my_widget_expose_event_handler (gtkwidget *widget, gdkeventexpose *event)
{
  gdkrectangle *rects;
  int n_rects;
  int i;

  gdk_region_get_rectangles (event->region, &rects, &n_rects);

  for (i = 0; i < n_rects; i++)
    {
      /* repaint rectangle: (rects[i].x, rects[i].y), 
       *                    (rects[i].width, rects[i].height)
       */
    }

  g_free (rects);

  return false;
}