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Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxt!houxm!ihnp4!stolaf!umn-cs!mmm!schley
From: schley@mmm.UUCP (Steve Schley)
Newsgroups: net.micro.mac
Subject: Re: Re: Deficiencies in QuickDraw support fo
Message-ID: <172@mmm.UUCP>
Date: Mon, 8-Jul-85 13:57:28 EDT
Article-I.D.: mmm.172
Posted: Mon Jul  8 13:57:28 1985
Date-Received: Fri, 12-Jul-85 01:43:54 EDT
References: <202@harvard.UUCP> <26700020@inmet.UUCP> <218@harvard.ARPA>
Organization: 3M Company, St. Paul, Minn.
Lines: 34

Ben Hyde posed this problem:
> ... You have two line segments
> which are known to cross, one "behind" the other.  I want to "cut out" a
> little bit of the one which is "behind", so it looks like this:
> 
> 	\      / <- line b
> 	 \    /
> 	  \  /
> 	   \ 
> 	    \
> 	  /  \
> 	 /    \
> 	/      \ <- line a
> 
> Now, the way that occurred to me was to subtract a region of fixed
> width around line a from the clipping region while drawing line b.
> Now what I will have to do is compute the points where line b crosses
> the boundaries of that region myself.  Anyone got a better way?

Sure do: use White-Out!  Here's how I do it...

Assuming that your application can always draw (or redraw) line a after
line b, simply draw line b, then change pen color to background, change
pen size to a few pixels wider than the lines being drawn, offset the
endpoints  of line a by half the new wider line width, and draw this
new line a.  Finally, reset the pen and line attributes and draw the
real line b.

Sure, this isn't as clean as the region technique which handles the
hidden line problem on its own, but it works, and making regions around
lines isn't always clean, either.-- 
	Steve Schley

	ihnp4!mmm!schley