bresenham_ellipse

下载来的一个看图软件的源代码

void 
gdImageEllipse (gdImagePtr im, int cx, int cy, int w, int h, int color)
{
  long d, b_sq, b_sq_4, b_sq_6;
  long a_sq, a_sq_4, a_sq_6;
  int x, y, switchem;
  long lsqrt (long);
  int pix, half, pstart;
  int thick = im->thick;

  half = thick / 2;
  w /= 2;			/* ImageArc uses diameter, not radius */
  h /= 2;

  d = 2 * (long) h *h + (long) w *w - 2 * (long) w *w * h;
  b_sq = (long) h *h;
  b_sq_4 = 4 * (long) h *h;
  b_sq_6 = 6 * (long) h *h;
  a_sq = (long) w *w;
  a_sq_4 = 4 * (long) w *w;
  a_sq_6 = 6 * (long) w *w;

  x = 0;
  y = -h;
  switchem = a_sq / lsqrt (a_sq + b_sq);

  while (x <= switchem)
    {
      pstart = y - half;
      for (pix = pstart; pix < pstart + thick; pix++)
	{
	  gdImageSetPixel (im, cx + x, cy + pix, color);
	  gdImageSetPixel (im, cx - x, cy + pix, color);
	  gdImageSetPixel (im, cx + x, cy - pix, color);
	  gdImageSetPixel (im, cx - x, cy - pix, color);
	}
      if (d < 0)
	d += b_sq_4 * x++ + b_sq_6;
      else
	d += b_sq_4 * x++ + b_sq_6 + a_sq_4 * (++y);
    }

  /* Middlesplat!
     ** Go a little further if the thickness is not nominal...
   */
  if (thick > 1)
    {
      int xp = x;
      int yp = y;
      int dp = d;
      int thick2 = thick + 2;
      int half2 = half + 1;

      while (xp <= switchem + half)
	{
	  pstart = yp - half2;
	  for (pix = pstart; pix < pstart + thick2; pix++)
	    {
	      gdImageSetPixel (im, cx + xp, cy + pix, color);
	      gdImageSetPixel (im, cx - xp, cy + pix, color);
	      gdImageSetPixel (im, cx + xp, cy - pix, color);
	      gdImageSetPixel (im, cx - xp, cy - pix, color);
	    }
	  if (dp < 0)
	    dp += b_sq_4 * xp++ + b_sq_6;
	  else
	    dp += b_sq_4 * xp++ + b_sq_6 + a_sq_4 * (++yp);
	}
    }

  d += -2 * (long) b_sq + 2 * (long) a_sq - 2 * (long) b_sq *(x - 1) + 2 * (long) a_sq *(y - 1);

  while (y <= 0)
    {
      pstart = x - half;
      for (pix = pstart; pix < pstart + thick; pix++)
	{
	  gdImageSetPixel (im, cx + pix, cy + y, color);
	  gdImageSetPixel (im, cx - pix, cy + y, color);
	  gdImageSetPixel (im, cx + pix, cy - y, color);
	  gdImageSetPixel (im, cx - pix, cy - y, color);
	}

      if (d < 0)
	d += a_sq_4 * y++ + a_sq_6 + b_sq_4 * (++x);
      else
	d += a_sq_4 * y++ + a_sq_6;
    }
}