Path: chuka.playstation.co.uk!news
From: "Jon Prestidge  (Jon@surfed.to)" <jon@surfed.to>
Newsgroups: scee.yaroze.freetalk.english
Subject: Arc tangent
Date: Thu, 30 Aug 2001 21:52:45 +0100
Organization: PlayStation Net Yaroze (SCEE)
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OK everyone.  There have been a couple of threads that could do with this
routine of mine I think.  I am a mean get and don't usually give much source
code away, but I was starting to feel a bit too guilty on this one.  I did
spend many hours writing this so keep it to yourselves and give me a mention
if you use it.  It's not too accurate however (I do have a more up-to-date
version that I've developed that is more accurate, but it is also slower so
I think this is the one best suited).

Just find dx and dy and pass `em to this routine and sit-back and have this
routine do all the work for you.  Get it while it's hot.

Jon


long auto_arc_tan( long v_dx, long v_dy )
   {
   // (c) Jon Prestidge 1997    Do not copy, do not distribute.
   // Version 1.0
   // automatic arc tangent routine.  Just plug-in dx and dy and
   // get back a full angle between 0 and 4096.
   //
   long grad, ang, 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;

   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;
   }