Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!caip!topaz!ll-xn!mit-amt!mit-eddie!mit-hermes!jpexg
From: jpexg@mit-hermes.ARPA (John Purbrick)
Newsgroups: net.micro.mac,net.micro.atari16
Subject: Re: Re: Flopping line in MacPaint (rubberband line)
Message-ID: <2694@mit-hermes.ARPA>
Date: Sat, 16-Aug-86 00:02:26 EDT
Article-I.D.: mit-herm.2694
Posted: Sat Aug 16 00:02:26 1986
Date-Received: Sun, 17-Aug-86 10:29:52 EDT
References: <2693@mit-hermes.ARPA> <1232@trwrb.UUCP> <208@bridge2.UUCP>
Distribution: net
Organization: The MIT AI Lab, Cambridge, MA
Lines: 23
Xref: watmath net.micro.mac:7353 net.micro.atari16:1647

> The rubberband line effect is easy to get on the ST.  GEM supports
> an XOR writing mode, where each graphic that you place on the screen
> is automatically XORed with the existing pattern....
> The rubberband line is a simple call to polyline with the XOR writing
> mode enabled....

I got no less than 5 replies to my original posting, only one of which 
disagreed with the above. The problem is, if you XOR, then of course 
any existing line crossed by the rubber-band (or "flopping") line will
get a hole in it where the new line crosses. I thought I looked at a 
friend's Mac while doing this, and saw no such hole, but I'll check again.
Here's the lone dissenter--name removed as it was private mail:

 Actually, MacPaint keeps the old screen buffer and copies it back each
 time you move the line to restore the original image.  It doesn't do
 XOR--if you notice, the line always looks like it will when you finish
 moving it.  Bill Atkinson has a special routine that copies his "save"
 bitmap VERY quickly to the screen for just this kind of application,
 thus the fixed window size.

So what does everyone else think of this? To me, it has the ring of truth
about it.
						--jp