Clipping-Masks in Gdk

[Stephan Krings <slothkri zedat fu-berlin de>]

Hello out there,

could anyone please explain me while the code appended does not
work. Forgive me if this is a stupid question, but although I'm
beginning to feel comfortable with Gtk and Gdk, I don't know much
about the underlying Xlib-calls and -mechanisms (maybe I need to
invest in one of those expensive Xlib-books), and I'd guess this is
the problem.

I hope my intention is clear, when reading the code: I want to create
an offscreen clipping-mask for a window or pixmap. Unfortunatly I'm
not sure, how to create the GdkBitmap in memory (it works, when
loading the bitmap from a file with gdk_bitmap_create_from_data for
example).

Oh yes, the program crashes with an abort-signal and BadMatch error
message.

So, how do I create this magic GdkBitmap or mask in memory?

Thank you all in advance,

Stephan

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

int main (int argc, char **argv) {

  GtkWidget *window;
  GtkWidget *drawarea;

  GdkPixmap *mask;
  GdkGC *gc;
  GdkColor red;
  GdkColor white;

  gtk_init(&argc, &argv);

  drawarea = gtk_drawing_area_new();
  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
  gtk_container_add(GTK_CONTAINER (window), drawarea);
  gtk_widget_show(GTK_WIDGET (drawarea));
  gtk_widget_show(GTK_WIDGET (window));

  gdk_color_parse("red", &red);
  gdk_color_alloc(gtk_widget_get_colormap(drawarea), &red);

  gdk_color_parse("white", &white);
  gdk_color_alloc(gtk_widget_get_colormap(drawarea), &white);

  gc = gdk_gc_new(drawarea->window);
  gdk_gc_copy(gc, drawarea->style->black_gc);

  gdk_gc_set_foreground(gc, &white);

  mask = gdk_pixmap_new(drawarea->window,
			drawarea->allocation.width,
			drawarea->allocation.height,
			-1);

  gdk_draw_rectangle(mask, gc, TRUE, 10, 10, 20, 20);

  gdk_gc_set_foreground(gc, &red);

  gdk_gc_set_clip_mask(gc, mask);
  gdk_gc_set_clip_origin(gc, 0, 0);

  gdk_draw_rectangle(drawarea->window, gc, TRUE, 0, 0,
		     drawarea->allocation.width,
		     drawarea->allocation.height);

  gtk_main();

  return 0;
}
---------------------------------------------------------------------------

-- 
Stephan Krings                                       Freie Universitaet Berlin
AG Kommunikationsserver      Zentraleinrichtung fuer Datenverarbeitung (ZEDAT)
slothkri@zedat.fu-berlin.de                                      Fabeckstr. 32
Tel: +49 30 838 3239                   				  14195 Berlin