From: murray AT southeast DOT net (Murray Stokely)
Newsgroups: comp.os.msdos.djgpp
Subject: math optimization
Date: Sat, 14 Dec 1996 08:48:58 GMT
Organization: ACiD Productions
Lines: 49
Message-ID: <32b26866.241305048@nntp.southeast.net>
Reply-To: murray AT southeast DOT net
NNTP-Posting-Host: ts7-000.southeast.net
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp


 	This is a little code snippet from a vesa lens effect I coded,
The diameter and magnification of the lens are adjustable in realtime
via the +/- keys so I need the lens calculating function as fast as
possible.  Someone gave me a good tip with my sqrt problem earlier in
using the difference of squares to take out one of the multiplies but
I'd like to take it a step further.  This routine is actualy faster
than I expected it would be on my 486dx4, but still every clock cycle
counts ;)  I know theres lots of room for improvement, so any tips
would be appreciated.

( I'll eventualy convert all doubles/floats to 16.16 fixed point, so
skip that obvious MAJOR optimization )

void calculate_tfm(int diameter, char magnification)
{
        int a,b,x,y,z,s;
        int radius;
        z=0;
        radius=diameter/2;
//        s=round(sqrt(abs((radius*radius) -
(magnification*magnification))));
// difference of squares - (only one imul instruction)
        s=round(sqrt(abs((radius-magnification) *
(radius+magnification))));
        for(y=-radius;y<diameter-radius;y++)
        {
                for(x=-radius;x<diameter-radius;x++)
                {
                        if (((x*x) + (y*y)) >= (s*s))
                        {
                                a=x;
                                b=y;
                        } else {
//
z=round(sqrt((radius*radius)-(x*x)-(y*y)));

z=round(sqrt((radius-x)*(radius+x)-(y*y)));
                                a=round(x*magnification/z);
                                b=round(y*magnification/z);
                        }

tfm[(y+radius)*diameter+(x+radius)]=(b+radius)*diameter+(a+radius);
                } // end of for x
        } // end of for y
}

Murray Stokely ( murray AT southeast DOT net )
http://www.cdrom.com/pub/artpacks