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Path: utzoo!mnetor!seismo!mcvax!ukc!msp
From: msp@ukc.ac.uk (M.S.Parsons)
Newsgroups: net.sources.bugs
Subject: Bug in Miranda code (To generate Escher "Square Limit" picture)
Message-ID: <1735@eagle.ukc.ac.uk>
Date: Fri, 25-Jul-86 10:03:06 EDT
Article-I.D.: eagle.1735
Posted: Fri Jul 25 10:03:06 1986
Date-Received: Sat, 26-Jul-86 06:06:51 EDT
Reply-To: msp@ukc.ac.uk (M.S.Parsons)
Organization: U of Kent at Canterbury, Canterbury, UK
Lines: 24

Whoops! The Miranda code I recently sent to draw the M.C.Escher "squarelimit"
picture had a bug. A classic case: a "+" instead of a "-".

This is the correct version of the rot function (I hope):

|| rot rotates an image by th radians anti-clockwise, about the image centre.
rot :: num -> image -> image
rot th i = tr middle (imap rotp (tr (negpoint middle) i))
           where
           rotp p = Pt (coord cos) (coord sin)
                    where
                    coord f = l * (f (a - th))
                              where
                              (Po a l) = ctop p

(Note for those running out of heap space: The only rotations involved in the 
drawing are multiples of 90 degrees, so this general rotate could be replaced 
by a much more efficient specific routine.)


------------------------------------------------------------------------------
Mike Parsons    UUCP: ..!seismo!mcvax!ukc!msp             JANET: msp@uk.ac.ukc
                MAIL: Computing Lab, University of Kent, Canterbury, Kent, UK.