From: Michal Mertl <XMERM05 AT vse DOT cz>
Newsgroups: comp.os.msdos.djgpp
Subject: Ellipse algorithmus finally found
Date: Wed, 16 Apr 1997 17:22:52 -0700
Organization: VSE Praha
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Message-ID: <33556D5C.3CAB@vse.cz>
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CC: shawn AT talula DOT demon DOT co DOT uk
To: djgpp AT delorie DOT com
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I was looking (not too hard) for ellipse algorithmus for long time,
finally found one and because there was question about it some time ago
I post this code. It's from jlib graphics library and it 100% works. I
post only one general version of it changed a bit to use Allegro
function names to make my post shorter. I think it can be of some
interest for Shawn too.

--> CUT HERE <--
/*
   ** Copyright (C) 1995  Jonathan Paul Griffiths.  All rights reserved.
   **
   ** You may do anything with this code you wish EXCEPT sell it. You
may sell
   ** any software you create using this code,  but you MUST NOT charge
for
   ** the code itself.  See the file "jlib.doc" for detailed terms. 
 */
#include <jlib.h>

#define FAST_2_MUL(x) ((x)<<1)
#define FAST_3_MUL(x) (((x)<<1)+x)
#define FAST_4_DIV(x) ((x)>>2)

void buff_draw_ellipse(BITMAP * buff, int x0, int y0, int a, int b, int
c)
{
   int x, y, y1, aa, bb, d, g, h;

   aa = a * a;
   bb = b * b;

   h = (FAST_4_DIV (aa)) - b * aa + bb;
   g = (FAST_4_DIV (9 * aa)) - (FAST_3_MUL (b * aa)) + bb;
   x = 0;
   y = b;

   while (g < 0) {
      putpixel(buff, x0 + x, y0 + y, c);
      putpixel(buff, x0 - x, y0 + y, c);
      putpixel(buff, x0 - x, y0 - y, c);
      putpixel(buff, x0 + x, y0 - y, c);

      if (h < 0) {
	 d = ((FAST_2_MUL (x)) + 3) * bb;
	 g += d;
      }
      else {
	 d = ((FAST_2_MUL (x)) + 3) * bb - FAST_2_MUL ((y - 1) * aa);
	 g += (d + (FAST_2_MUL (aa)));
	 --y;
      }

      h += d;
      ++x;
   }

   y1 = y;
   h = (FAST_4_DIV (bb)) - a * bb + aa;
   x = a;
   y = 0;

   while (y <= y1) {
      putpixel(buff, x0 + x, y0 + y, c);
      putpixel(buff, x0 - x, y0 + y, c);
      putpixel(buff, x0 - x, y0 - y, c);
      putpixel(buff, x0 + x, y0 - y, c);

      if (h < 0) {
	 h += ((FAST_2_MUL (y)) + 3) * aa;
      }
      else {
	 h += (((FAST_2_MUL (y) + 3) * aa) - (FAST_2_MUL (x - 1) * bb));
	 --x;
      }
      ++y;
   }

}
--> CUT HERE <--

I hope someone find it as usefull as I did.
-- 

    //\\  //\\     ..
   //  \\//  \\
  //    \/    \\   || |/\/\  /---\
 //            \\  || | | |  |___'
//              \\ || | | |  \___.
        E-Mail: MiMe AT EUnet DOT cz