Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!brunix!jfh
From: jfh@cs.brown.edu (John Forbes Hughes)
Newsgroups: comp.graphics
Subject: Re: Foley and van Dam book II
Message-ID: <45359@brunix.UUCP>
Date: 20 Jul 90 00:09:13 GMT
References: <25553@mimsy.umd.edu> <3777@csccat.UUCP>
Sender: news@brunix.UUCP
Reply-To: jfh@cs.brown.edu (John Forbes Hughes)
Organization: Brown University Department of Computer Science
Lines: 40

In article <3777@csccat.UUCP> larry@csccat.UUCP (Larry Spence) writes:
>In article <25553@mimsy.umd.edu> rmr@tove.cs.umd.edu (Randy M. Rohrer) writes:
>>
>>        The new edition of "Foley and van Dam" is available.
>>I just received it directly from Addison Wesley last Thursday.
>>It should be appearing in bookstores at any time.
>
> [... stuff deleted ...]
>
>A memorable specification:
>
>   (on polygon interior conventions)
>   "Another rule is the nonexterior rule... if we think of the curve
>    as a fence, the interior is the region in which animals can be 
>    penned up." [p. 965]
>
>I'm having a little trouble implementing from their description %> ...

I don't mean to sound too defensive here, but I was the guy who wrote that
particular sentence. I'm not very proud of it, I admit, but the ellipsis
leaves out two things: the reference to a figure, in which an example of the
rule is shown, and the more precise description: "...[a seed point] distant
from the polyline [is chosen]. Any point that can be connected to this seed
point by a path that does not intersect the polyline is said to be outside..."

I *could* have been more precise and said "Consider the setwise difference of
the plane and the polyline. In this set, the component of the point at infinity
is defined to be *outside* the polyline; the complement of the outside in the
plane is the inside." [Where "point at infinity" is defined as in typical math 
texts.] Still, I thought that my description was more enlightening as is. 

The poster asked for an address to which corrections could be mailed. In
a few days, when the authors have gotten a chance to discuss it, I'll
try to post news containing such an address. 

By the way, the book also contains an answer to an earlier question, on how to
scan convert a parabola--it actually contains a Bresenahm/midpoint-style
scan conversion algorithm for an arbitrary conic. 

-John Hughes