Gtk3 widgets with background image

#include <gtk/gtk.h>
#include <cairo.h>

#define WINDOW_W 200
#define WINDOW_H 200


static cairo_surface_t *scale_surface
( cairo_surface_t *old_surface, gdouble new_w, gdouble new_h,
  gboolean smooth )
{
    gint old_width = cairo_image_surface_get_width(old_surface);
    gint old_height = cairo_image_surface_get_height(old_surface);

    cairo_surface_t *new_surface = cairo_surface_create_similar
        (old_surface, CAIRO_CONTENT_COLOR_ALPHA,
        (gint)(new_w), (gint)(new_h));
    cairo_t *cr = cairo_create (new_surface);

    cairo_scale (cr, new_w/old_width, new_h/old_height);
    cairo_set_source_surface (cr, old_surface, 0, 0);

    if (!smooth)
    {
        cairo_pattern_set_filter
            (cairo_get_source(cr), CAIRO_FILTER_NEAREST);
    }
    cairo_set_operator (cr, CAIRO_OPERATOR_SOURCE);
    cairo_paint (cr);
    cairo_destroy (cr);

    return new_surface;
}


/* The expose event handler for the window.
 *
 * This function performs the actual compositing of the event box onto
 * the already-existing background of the window at 70% normal opacity.
 */
static gboolean
window_draw_event   (GtkWidget      *widget,
                     cairo_t        *cr,
                     gpointer        data)
{
  GtkWidget *child;
  /* get our child (in this case, the event box) */
  child = gtk_bin_get_child (GTK_BIN (widget));
  /* draw the background image first */
  cairo_set_source_surface
    (cr, (cairo_surface_t*)data, 0, 0);
  cairo_set_operator (cr, CAIRO_OPERATOR_OVER);
  cairo_paint(cr);
  /* now draw the event box */
  gint border_width =
    gtk_container_get_border_width(GTK_CONTAINER(widget));
  gdk_cairo_set_source_window (cr, gtk_widget_get_window(child),
                               border_width,
                               border_width);
  /* composite, with a 70% opacity */
  cairo_paint_with_alpha (cr, 0.7);
  /* we're done */
  return FALSE;
}
int
main (int argc, char **argv)
{
  GtkWidget *window, *event, *button1, *button2, *checkbox, *box;
  GdkScreen *screen;
  GdkRGBA transparent = { 0, 0, 0, 0 };

  gtk_init (&argc, &argv);

  cairo_surface_t *bg_image =
    cairo_image_surface_create_from_png("background.png");
  cairo_surface_t *bg_scaled_image
    = scale_surface(bg_image, WINDOW_W, WINDOW_H, TRUE);
  cairo_surface_destroy(bg_image);
  /* Make the widgets */
  button1 = gtk_button_new_with_label ("Label");
  button2 = gtk_button_new_from_stock (GTK_STOCK_ABOUT);
  checkbox = gtk_check_button_new_with_label ("Checkbox");

  box = gtk_vbox_new(FALSE, 8);
  gtk_box_pack_start(GTK_BOX(box), button1, FALSE, FALSE, 8);
  gtk_box_pack_start(GTK_BOX(box), button2, FALSE, FALSE, 8);
  gtk_box_pack_start(GTK_BOX(box), checkbox, FALSE, FALSE, 8);

  event = gtk_event_box_new ();
  window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  screen = gtk_widget_get_screen (event);
  gtk_widget_override_background_color(event, 0, &transparent);
  /* Put them inside one another */
  gtk_container_set_border_width (GTK_CONTAINER (window), 10);
  gtk_container_add (GTK_CONTAINER (window), event);
  gtk_container_add (GTK_CONTAINER (event), box);
  /* Since rescaling the background is expensive, you better fix the widget size */
  gtk_widget_set_size_request(window, WINDOW_W, WINDOW_H);
  gtk_window_set_resizable(GTK_WINDOW(window), FALSE);
  /* Realise and show everything */
  gtk_widget_show_all (window);

  /* Set the event box GdkWindow to be composited.
   * Obviously must be performed after event box is realised.
   */
  gdk_window_set_composited (gtk_widget_get_window(event), TRUE);
  /* Set up the compositing handler.
   * Note that we do _after_ so that the normal (red) background is drawn
   * by gtk before our compositing occurs.
   */
  g_signal_connect_after (window, "draw",
                          G_CALLBACK (window_draw_event), bg_scaled_image);
  g_signal_connect_swapped(G_OBJECT(window), "destroy",
        G_CALLBACK(gtk_main_quit), G_OBJECT(window));

  gtk_main ();
  return 0;
}