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From: wrs@k.cs.cmu.edu (Walter Smith)
Newsgroups: net.micro.mac,net.micro.atari16
Subject: Re: Flopping line in MacPaint
Message-ID: <1050@k.cs.cmu.edu>
Date: Tue, 12-Aug-86 23:56:49 EDT
Article-I.D.: k.1050
Posted: Tue Aug 12 23:56:49 1986
Date-Received: Wed, 13-Aug-86 09:12:29 EDT
References: <2693@mit-hermes.ARPA> <1232@trwrb.UUCP>
Distribution: net
Organization: Carnegie-Mellon University, CS/RI
Lines: 29
Summary: MacPaint does not XOR
Xref: mnetor net.micro.mac:6589 net.micro.atari16:1586

In article <1232@trwrb.UUCP>, sansom@trwrb.UUCP (Richard Sansom) writes:
> 
> In article <2693@mit-hermes.ARPA> jpexg@mit-hermes.ARPA (John Purbrick) writes:
> >One thing that the Mac does when running MacPaint is let you start a line
> >by clicking on a point, then flopping the other end around with the mouse
> >...
> 
> The technique used is called XOR (eXclusive-OR).
> ...

Most programs do use XOR.  MacDraw, for instance, leaves things in OR mode
until you start to manipulate an object, then redraws that object in XOR to
allow rubber-band feedback.

However, if you look closely at a line being drawn in MacPaint, you will see
that XOR mode is NOT being used.  The line is solid black all the way
through, whatever the original contents of the screen.  Apparently, MacPaint
saves the original bitmap not only for Undo, but also for redrawing the
screen contents during rubber-banding.  The CopyBits is carefully
synchronized to the vertical retrace time for maximum smoothness.

Another example of attention to detail (there are a lot of them in
MacPaint), something Microsoft should probably investigate.

- Walt
-- 
         Walter Smith, CS undergraduate, Carnegie-Mellon University
       uucp: ...!seismo!cmu-cs-k!wrs          ARPA: wrs@k.cs.cmu.edu
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