Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!purdue!haven!cvl!elsie!ado
From: ado@elsie.UUCP (Arthur David Olson)
Newsgroups: comp.windows.x
Subject: Color Frame Buffer tile padding--bug or feature?
Message-ID: <9074@elsie.UUCP>
Date: 13 Feb 89 20:13:28 GMT
Followup-To: poster
Organization: NIH-LEC, Bethesda, MD
Lines: 71

XV11R3's "cfbPadPixmap" function (in "cfbpixmap.c") pads narrow tiling
Pixmaps if 32 is an even multiple of their widths.  This means that if you
	1.  create a "narrow" Pixmap with a call such as
		tile = XCreatePixmap(display, window, 1, 16, 8);
	2.  fill the Pixmap with some desired data
	3.  associate the Pixmap with a GC with a call such as
		XSetTile(display, tiling_gc, tile)
	4.  use the GC to tile a region
and then try to
	1.  refill the Pixmap with new data
	2.  reassociate the Pixmap with a GC to update the tiling
you're in trouble--you will presumably only rewrite the first
column of a Pixmap that's been widened to 32 columns, so the tiling will be
done differently from what I (at least) would expect.

(I've attached sample code that, when run, shows what the situation is.
If you put the code in a file named "try.c" and then
	cc try.c -lX11 -o try
note how the correct output you get from typing the command
	try +
differs from the output you get when you type
	try
unadorned.)

Two questions.  Is this behavior documented somewhere that I missed?
And should it be considered a bug?  If you have answers, I'd appreciate
hearing from you by electronic mail.
-- 
	Arthur David Olson    ado@ncifcrf.gov    ADO is a trademark of Ampex.

#include "X11/Xlib.h"

int
main(argc, argv)
char *	argv[];
{
	register GC		default_gc;
	register Display *	display;
	register int		i;
	register Window		root_window;
	register int		screen_number;
	register Pixmap		tile;
	register GC		tiling_gc;
	register Window		window;

	display = XOpenDisplay((char *) 0);
	XSynchronize(display, 1);
	root_window = XDefaultRootWindow(display);
	screen_number = XDefaultScreen(display);
	default_gc = XDefaultGC(display, screen_number);
	window = XCreateSimpleWindow(display, root_window,
		0, 0, 100, 100, 0, (unsigned long) 0, (unsigned long) 0);
	XMapWindow(display, window);
	(void) getchar();

	tiling_gc = XCreateGC(display, window, 0, (XGCValues *) 0);
	XCopyGC(display, default_gc, (unsigned long) ~0, tiling_gc);
	XSetFillStyle(display, tiling_gc, FillTiled);

	tile = XCreatePixmap(display, window, 1, 100, 8);

	XSetTile(display, tiling_gc, tile);
	XFillRectangle(display, window, tiling_gc, 0, 0, 1, 1);
	for (i = 0; i < 100; ++i)
		if (((i / 4) % 2) == 0)
			XDrawPoint(display, tile, default_gc, 0, i);
	XSetTile(display, tiling_gc, tile);
	XFillRectangle(display, window, tiling_gc, 0, 0, 100, 100);
	(void) getchar();
	return 0;
}