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From: avjewe@cvl.UUCP
Newsgroups: comp.sys.mac
Subject: Re: Rotation Algorithm needed
Message-ID: <2442@cvl.umd.edu>
Date: Mon, 31-Aug-87 07:42:57 EDT
Article-I.D.: cvl.2442
Posted: Mon Aug 31 07:42:57 1987
Date-Received: Tue, 1-Sep-87 03:45:44 EDT
References: <2180@mulga.oz>
Reply-To: avjewe@cvl.UUCP (Andrew V Jewell)
Organization: Center for Automation Research, Univ. of Md.
Lines: 27
Keywords: Rotation, Mac

In article <2180@mulga.oz> jteh@mulga.oz (J.T. Teh) writes:
>I need a rotation algorithm that will allow me to rotate the contents of
>a bounding rectangle at a given angle. Currently, I am using the standard
>algorithm of rotating the point around the origin with
>	[ cosX -sinX ][ x ]
>	[ sinX  cosX ][ y ]
>but this causes the resultant image to have "holes" in it if I am rotating

Your algorithm is correct, just run it backwards. That is for each
point in the destination rectangle, calculate which point in the
original maps to it. Presto! no holes.

As for your speed problem, I think your out of luck. There are three
things I can think of (probobly many I can't) none of which are very
helpful. 
1) special cases (30, 45, 90 and such)
2) rotate objects rather than dots if available
3) rather than the sin and cosine functions, use pre-filled arrays
   (e.g. sin[45] instead of sin(45). This only works if you have a
    well defined small domain)


Hope this helps, 

     Andy Jewell

disclaimer: I don't really know much about this stuff