Re: XORing the GC

[Tony Denault <denault hawaii edu>]

On Mon, 19 Mar 2001, Donald G Plugge wrote:

> I'm testing out Gtk+ on the VMS environment.  The source code was
> recently ported to VMS and I've been messing around with the example
> programs.  I took the simple-scribble.c program and modified it to
> perform rubberbanding, however I'm having trouble with XORing the
> results. I want to eliminate the artifacts that result from simply
> negating the old line with the background color and rather flip it back
> to whatever it was originally.


I am doing this in my GTK image viewer. I use rubbberbanding to allow the
user to specify a group of pixel by rubberbanding a box. My code looks
like:

1. When creating my application widgetw, I also create an GC used for
the rubbingbanding draw routines. The set_line_attributes make the line 3
pixel wide:

  /* Xor gc for all stats box */
   if( (Xor_gc = gdk_gc_new(base_window->window)) == NULL )
      printf("Error unable to obtain new GC: Xor_gc\n");
   gdk_gc_set_function( Xor_gc, GDK_XOR);
   gdk_gc_set_line_attributes( Xor_gc, 3, GDK_LINE_SOLID, GDK_CAP_BUTT,  GDK_JOIN_MITER);


2. I have a function that draws an XOR rectangle on a drawing area:

 void draw_xor_box_on_canvas( dpinx, x, y, wid, hgt )
   int dpinx;            /* Identifies canvas to xor on     */
   int x, y, wid, hgt;   /* Draw a box around these pixels  */
{
   int      box_x, box_y, box_wid, box_hgt;
   float    pixel_wid;

   struct dpy_t *dp = &Dpy[dpinx];

   /* determine position of upper-left pixel */
   pixel_wid = dp->image_zoom > 0 ? dp->image_zoom : 1.0/(1.0+abs(dp->image_zoom));
   box_x = ((x - dp->image_offx) * pixel_wid) + 0.5;
   box_y = ((y - dp->image_offy) * pixel_wid) + 0.5;

   box_wid = wid * pixel_wid;
   box_hgt = hgt * pixel_wid;
   /* Adjust values so rectangle is draw around the pixels */
   box_x--; box_y--; box_wid++; box_hgt++;

   /* Draw it */
   gdk_gc_set_foreground( Xor_gc, &CM.colors[CM_GREEN] );
   gdk_draw_rectangle( dp->data_drawingarea->window, Xor_gc, FALSE, box_x, box_y, box_wid, box_hgt);

 }

Notice I gdk_gc_set_foregound color. In 8 bit mode, I don't think it does
anything, but in 32-bit mode black pixel XOR to green. I mostly view
images in grayscale so I the green XOR rubberbanding is nice.

3. When first rendering the image I put the xor box on it:

 draw_xor_box_on_canvas( dpinx, x,  y, wid, hgt);

4. When application notice the user rubberbanding it erase the old box,
and put a new one on:


    draw_xor_box_on_canvas( dpinx, oldx,  oldy, oldwid, oldhgt);

    draw_xor_box_on_canvas( dpinx, x,  y, wid, hgt);


Hope this helps,

Tony

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