NetNews Usenet Archive 1993 #1/NN_1993_1.iso/spool/comp/graphics/13422

Text File | 1993-01-05 | 1.7 KB | 39 lines

Newsgroups: comp.graphics
Path: sparky!uunet!zaphod.mps.ohio-state.edu!swrinde!cs.utexas.edu!torn!news.ccs.queensu.ca!hausner
From: [e-mail] (Alejo Hausner)
Subject: Re: fast bitmap rotation
Message-ID: <[e-mail]>
Sender: [e-mail] (Netnews control)
Organization: M.Sc, C.S, Queen's, Kingston, Canada.
References: <[e-mail]> <[e-mail]>
Date: Tue, 5 Jan 1993 22:39:50 GMT
Lines: 27

In article <[e-mail]> [e-mail] (MCCORMICK JOSHUA C) writes:

> From article <[e-mail]>, by
> [e-mail] (Renee Teunissen):
>>
>> I have the following "problem". for a Photo montage package i wrote a
>> rotation tool for full color objects [bitmap images]. this program
>> runs unther windows 3.1. but rotating these bitmaps goes very slow, eg
>> if I do the same rotation [same image size and pixel depth] in
>> PhotoStyler is goes much faster. anyone any smart idears about a
>> pointer to a fast rotation algorithm ??
>
>x' = x * cos (radians) - y * sin (radians)
>y' = y * cos (radians) + x * sin (radians)
>

I think Foley and van Dam point out that a rotation can be decomposed
into two shears, one about the x axis and the other about the y.  If
your pixel array is n x n, each of the shears can be done in O(n) time
if the displacements for each row or column are pre-computed.  There
will be O(n x n) additions.  However, the usual method you give uses
O(n x n) multiplications, so it can be a lot slower.

I forget the way to decompose the rotation into two shears, but I can
look it up.

Alejo Hausner ([e-mail])