Path: chuka.playstation.co.uk!news
From: sceetech <ps_yaroze@interactive.sony.com>
Newsgroups: scee.yaroze.programming.libraries
Subject: Re: Rotation Matrix Functions : The Solution?
Date: Thu, 10 Apr 1997 09:19:38 +0100
Organization: SCEE
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Message-ID: <334CA29A.539A@interactive.sony.com>
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Michael Enoch wrote:
> 
> > Take the rows (or columns) one at a time calculate the length
> > sqrt(x*x+y*y+z*z)
> > and divide the x,y,z by the length
> >
> > The problem is that 4096 is one and sqrt is slow.
> > All this means is that before the divide you have to multiply by 4096,
> > and for the sqrt, only doing one row in a frame and not every frame.
> >
> > Stuart
> 
> Thanks for the explanation, I made this from it and tried it within my
> code:
> 
> length=sqrt(Matrix->m[i][0]*Matrix->m[i][0]+
>         Matrix->m[i][1]*Matrix->m[i][1]+
>         Matrix->m[i][2]*Matrix->m[i][2]);
> Matrix->m[i][0]=(Matrix->m[i][0]*ONE)/length;
> Matrix->m[i][1]=(Matrix->m[i][1]*ONE)/length;
> Matrix->m[i][2]=(Matrix->m[i][2]*ONE)/length;
> 
> I repeated it 3 times changing i to equal 0,1,or 2 for the rows, I wasn't
> too bothered about speed at the moment so I just did all 3 rows every
> frame.  What I was wondering is whether I got it right? The reason I ask is
> that the object still changes shape/whatever slightly. Am I just being
> picky or have I done something wrong?  It doesn't seem as bad as before but
> I thought I'd ask anyway to make sure.
> 
>         Thanks for your trouble,
>                 Mike.

You get this problem because of maths inaccuracies on the PS ( because
of the 16 bit values ) The normalisation process will oscillate is you
use it continuously.
The solution is only to normalise the matrix when you change the
orientation of your scene/object. Then, when the picture is still you
won't see any changes occuring.

Colin.