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From: umueller@iiic.ethz.ch (Urban Dominik Mueller)
Newsgroups: comp.sys.amiga.programmer
Subject: Re: Clicking on irregular shapes?
Message-ID: <27927@neptune.inf.ethz.ch>
Date: 9 Apr 91 13:13:27 GMT
References: <1991Apr05.104141.2118@cs.ruu.nl> <27867@neptune.inf.ethz.ch> <1079@cbmger.UUCP>
Sender: news@neptune.inf.ethz.ch
Reply-To: umueller@iiic.ethz.ch (Urban Dominik Mueller)
Organization: Departement Informatik, ETH, Zurich
Lines: 36

>I always have moot feelings with such analytical algorithms. Often
>you have to compare for equality of real numbers, which is normally
>a NO-NO. You would have to convert coordinates to integers only to
>do this algorithm on them.

It's not that bad, in order to find out whether a point is a local
maximum or minimum, you need only check for less-equal. Additionally,
you can often work with integer arithmetics when it comes to gfx. For
example the guy who started this thread is programming a game, I doubt
he works with reals.

>Look for another example at this shape, would it be
>processed correctly?
>      /\
>     /  \       /\
>    / O  \_____/  \
>   /_______________\
>
>The O is thought to be on the same y coordinate as the horiz. line

Yes, it works. If you have defined a local minimum to be a point whose
neighbours have higher *or same* y-coordinates, you've got two minima
here, which aren't counted. This works even of that horizontal line 
consists of more than one edge.
But it's really good to be suspicious, I know a lot of people (including
my CS prof) who have trouble getting that problem right... Not counting
all the bad solutions that were posted on this net. Ah yes, I have to
join that guy who recommended the Sedgewick book, it's a must for any
good programmer.

>Best regards, Dr. Peter Kittel  // E-Mail to  \\  Only my personal opinions.. 
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