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From: thibaud@dabchick.stanford.EDU
Newsgroups: comp.windows.x
Subject: about GCxor
Message-ID: <9104242202.AA05459@dabchick.Stanford.EDU>
Date: 24 Apr 91 22:02:44 GMT
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 69

my problem is the following :

I am trying to set the GC function to GCxor in order to do rubberbanding.
Then I am displaying a series of rectangles to check what is the effect
of the GXxor.

The first rectangle ( R1 ) is displayed with GXcopy ( and appears on the screen ).
the second ( R2 ) is drawn with GXxor on white background and doesn't appear. why ?
the third ( R3 ) is drawn with GXxor on top of R1. It doesn't erase it. why ?

================================================================================
#include <X11/Xlib.h>
#include <stdlib.h>


main()

{
  Display *display;
  int screen;
  Window window;
  GC gc;
  XEvent event;
  XGCValues GCValues;
  
  if( (display = XOpenDisplay( NULL )) == NULL ) exit(0);
  screen = DefaultScreen( display );
  
  window = XCreateSimpleWindow( display, RootWindow( display, screen ),
			       0,0,200,200,
			       1,
          		       BlackPixel( display, screen ),
			       WhitePixel( display, screen ) );
  gc = XCreateGC( display, RootWindow( display, screen ), 0, NULL );
  XSetForeground( display, gc, BlackPixel(display,screen) );
  XSetBackground( display, gc, WhitePixel(display,screen) );
  XSelectInput( display, window, ExposureMask );
  
  XMapWindow( display, window );

  XNextEvent( display, &event ); /* to be sur that next drawings will appear */

  /* the GC is still GXcopy */
  XDrawRectangle( display, window, gc, 10, 10, 30, 30 );
  XFlush( display );
  sleep(2);

  /* here I am changing the GC to GCxor */
  XGetGCValues( display, gc, GCFunction, &GCValues );
  GCValues.function = 0x6;
  XChangeGC( display, gc, GCFunction, &GCValues );
  XGetGCValues( display, gc, GCFunction, &GCValues );
  
  XDrawRectangle( display, window, gc, 40, 40, 30, 30 );
  XFlush( display );
  sleep(2);

  XDrawRectangle( display, window, gc, 10, 10, 30, 30 );
  XFlush( display );
  sleep(2);

  XCloseDisplay( display );
  exit(0);
}
================================================================================

Thank you for your help.

Thibaud