Path: chuka.playstation.co.uk!news
From: "Matt Verran" <matt@hermitgames.com>
Newsgroups: scee.yaroze.freetalk.english
Subject: Re: Sine & Cosine Lookup Tables
Date: Tue, 10 Dec 2002 17:25:53 -0800
Organization: PlayStation Net Yaroze (SCEE)
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Jon Prestidge has kindly passed on this code, remember to thank him if you
use it!

long auto_arc_tan( long v_dx, long v_dy )
   {
   // V1
   //
   // (C) Jon Prestidge, 1998.
   //
   long grad, ang, deg, abs_dx, abs_dy, half;

   abs_dx = v_dx;
   if ( abs_dx < 0 ) abs_dx *= -1;
   abs_dy = v_dy;
   if ( abs_dy < 0 ) abs_dy *= -1;

   // find the gradient between 0 and one (or 0 and 4096 in this
impelmentation)....
   if ( abs_dy <= abs_dx )
      {
      if ( abs_dx == 0 )


         // exact angle can be returned... also this prevents divide by 0
error...
         if ( v_dy >= 0 )
            return 0;
         else
            return 2048;
         }
      grad = abs_dy;
      grad <<= 12;
      grad /= abs_dx;
      half = 1;
      }
   else
      {
      if ( abs_dy == 0 )


         // exact angle can be returned... also this prevents divide by 0
error...
         if ( v_dx >= 0 )
            return 1024;
         else
            return 3072;
         }
      grad = abs_dx;
      grad <<= 12;
      grad /= abs_dy;
      half = 0;
      }

   ang = ( grad * 2409 ) >> 12;
   ang *= 4096 - ang;
   ang += grad * 3097;
   ang >>= 15;

   deg = ang * 90 / 1024;

   if ( v_dx >= 0 )
      {
      if ( v_dy >= 0 )
         {
         if ( half ) ang = 1024 - ang;
         }
      else
         {
         if ( half )
            ang += 1024;
         else
            ang = 2048 - ang;
         }
      }
   else
      {
      if ( v_dy >= 0 )
         {
         if ( half )
            ang += 3072;
         else
            ang = 4096 - ang;
         }
      else
         {
         if ( half )
            ang = 3072 - ang;
         else
            ang += 2048;
         }
      }

   return ang;
   }