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From: cjmchale@cs.tcd.ie (Ciaran McHale)
Newsgroups: comp.windows.x
Subject: Re: overlay function in X
Message-ID: <1991Feb22.144752.17371@cs.tcd.ie>
Date: 22 Feb 91 14:47:52 GMT
References: <9102220729.AA00513@lightning.McRCIM.McGill.EDU>
Organization: DSG, Dept. of Comp. Sci., Trinity College, Dublin.
Lines: 65

In <9102220729.AA00513@lightning.McRCIM.McGill.EDU>
mouse@lightning.mcrcim.mcgill.EDU writes:

>>> In case there's anyone out there who cares, something I would really
>>> like in X is an overlay function of the sort
>
>>> 	pixel_out = (pixel_1 == 0 ? pixel_2 : pixel_1);
>
>Me too, though I'd prefer for the 0 to be variable.
>
>> Try XCopyPlane.  I think it does exactly what you want.
>
>No, what that does is
>
>	pixel_out = ((pixel_1 & bit) == 0) ? color_A : color_B;

Consider the case where you want to display a satellite image
in a window and overlay some contour lines on it. This can
be achieved by the following:

#define	Bitmap	Pixmap
	Window		win;			/* window to display stuff in */
	Pixmap		image_pixmap;		/* for satellite image */
	Bitmap		contour_bitmap;		/* for contour lines */
	GC	gc;

	... /* initialisation; create the Window, Pixmap, Bitmap and GC */
	... /* draw the satellite image into image_pixmap */
	... /* draw the contour lines into contour_bitmap */


	/* expose handling code */

	/* redraw the satellite image to the window */
	XSetClipMask(dpy, gc, None);	/* no clipmask */
	XCopyArea(dpy, image_pixmap, win, gc, ...);

	/* overlay the contour lines on top of the satellite image */
	XSetClipMask(dpy, gc, contour_bitmap);
	XCopyPlane(dpy, contour_bitmap, win, gc, ..., 1L);


The above code gives an overlay because the clipmask of
(the GC used in) XCopyPlane() is the same as its source
pixmap.

Things to watch out for:

	1. Set the foreground of gc to be the colour that
	   you want the contour lines to be drawn in.
	2. You'll need a different GC for drawing the
	   contour lines into the contour_bitmap since this
	   is a different depth than the image_pixmap/window
	   (unless you're on a monochrome display).

Disclaimer: I haven't tried any of this. But from my
understanding of Xlib I think it should work.


Ciaran.
-- 
Ciaran McHale		"Verbosity says it all"			      ____
Department of Computer Science, Trinity College, Dublin 2, Ireland.   \  /
Telephone: +353-1-772941 ext 1538	FAX: +353-1-772204	       \/
Telex: 93782 TCD EI			email: cjmchale@cs.tcd.ie