Path: utzoo!utgpu!attcan!uunet!lll-winken!lll-lcc!ames!mailrus!iuvax!rutgers!apple!lsr
From: lsr@Apple.COM (Larry Rosenstein)
Newsgroups: comp.sys.mac.programmer
Subject: Re: color "xor"
Message-ID: <318@internal.Apple.COM>
Date: 15 Dec 88 03:01:23 GMT
References: <4XdeYOy00UgCNdM4Bj@andrew.cmu.edu>
Organization: Advanced Technology Group, Apple Computer
Lines: 24

In article <4XdeYOy00UgCNdM4Bj@andrew.cmu.edu> kw1r+@andrew.cmu.edu (Kevin Whitley) writes:
>1) If the same obect is drawn twice (with the same foreground & background
>colors) in xor, the screen should remain unchanged.
>2) Drawing in xor against pixels in the background color should give the
>foreground color.  (Drawing against colors of any other pixel can be undefined).

You can achieve (2) by using HiliteMode (p 61 of Inside Mac volume 5).  You
can pass hiliteMode instead of patXOR as your drawing mode.  The black bits
in the source determine which bits in the destination are affected (as in
XOR mode).  Those bits that are in the current background color are made to
be the current highlight color.

If you wanted to do this to a rectangular area, for example, you would set
the PenMode to hiliteMode, the PenPat to black, and use PaintRect.

To reverse the operation, you have to set up the highlight and background
colors appropriately.  (This is not like XOR where 2 successive calls undo
each other.)  In that sense, it doesn't directly satisfy (1), but it can be
made to do so.

-- 
		 Larry Rosenstein,  Object Specialist
 Apple Computer, Inc.  20525 Mariani Ave, MS 46-B  Cupertino, CA 95014
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