The Incredible Machine - Rope Drawing Code

void do_conic(color,n,x1,x2,x3,y1,y2,y3)
short color,n;
long x1,x2,x3,y1,y2,y3;
{
   short cur_x, cur_y;
   short old_x, old_y;
   long px, py, rx, ry, nx, ny;
   long t, c;
   long temp;

   vm_lcolor = color;

//   y2 = y2 << 1;

   px = x3 + x1 - (x2<<1);
   py = y3 + y1 - (y2<<1);

   rx = (x2 - x1) << (n + 1);
   ry = (y2 - y1) << (n + 1);

   temp = n << 1;

   nx = x1 << temp;
   ny = y1 << temp;

   c = 1 << n;

   old_x = (short) (nx>>temp);
   old_y = (short) (ny>>temp);

   for ( t = 0; t <= c; t++)
      {
         cur_x = (short) (nx>>temp);
         cur_y = (short) (ny>>temp);
         if (old_x != cur_x || old_y != cur_y)
            {
               vm_line (old_x, old_y, cur_x, cur_y);
               old_x = cur_x;
               old_y = cur_y;
            }
         nx += (rx+(t*2+1)*px);
         ny += (ry+(t*2+1)*py);
      }
}


void draw_curved_line(x1,y1,x2,y2,len_diff)
short x1,y1,x2,y2,len_diff;
{
   short x_cp,y_cp;

   if (len_diff > 4)
   {
      x_cp = (x1+x2)>>1;
      y_cp = ((y1+y2)>>1) + len_diff;
      do_conic(vm_lcolor,4,(long)x1,(long)x_cp,(long)x2,(long)y1,(long)y_cp,(long)y2);
   }
   else
      vm_line(x1,y1,x2,y2);
}