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kane020397

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Bresenhams Circle Algorithm

This is problem I understand his line algorithm but not his circle.  This is the one I have got can some one explain what he does with these variables and why he does it.  I mean actually explain line by line what he does and why?

//THE CODE

typedef long           fixed16_16;
fixed16_16 SIN_ACOS[1024];


/**************************************************************************
 *  circle_fast                                                           *
 *    Draws a circle by using fixed point numbers and a trigonometry      *
 *    table.                                                              *
 **************************************************************************/

void circle(int x,int y, int radius, byte color)
{
  fixed16_16 n=0,invradius=(1/(float)radius)*0x10000L;
  int dx=0,dy=radius-1;
  word dxoffset,dyoffset,offset = (y<<8)+(y<<6)+x;

  while (dx<=dy)
  {
    dxoffset = (dx<<8) + (dx<<6);
    dyoffset = (dy<<8) + (dy<<6);
    VGA[offset+dy-dxoffset] = color;  /* octant 0 */
    VGA[offset+dx-dyoffset] = color;  /* octant 1 */
    VGA[offset-dx-dyoffset] = color;  /* octant 2 */
    VGA[offset-dy-dxoffset] = color;  /* octant 3 */
    VGA[offset-dy+dxoffset] = color;  /* octant 4 */
    VGA[offset-dx+dyoffset] = color;  /* octant 5 */
    VGA[offset+dx+dyoffset] = color;  /* octant 6 */
    VGA[offset+dy+dxoffset] = color;  /* octant 7 */
    dx++;
    n+=invradius;
    dy = (radius * SIN_ACOS[n>>6]) >>16;
  }
}

void main()
{
  int i;
  setvmode(0x13);

  for(i=0;i<1024;i++)                 /* create the sin(arccos(x)) table. */
    SIN_ACOS[i]=sin(acos((float)i/1024)) * 0x10000L;

  circle(160, 100, 50, 7);

  setvmode(0x03);
}
//

THANK YOU VERY MUCH... THIS IS VERY IMPORTANT!!!
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Avatar of jos010697
jos010697

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