Source (converted from C++) -> http://www.dailyfreecode.com/Code/draw-ellipse-midpoint-ellipse-algorithm-714.aspx

/// <summary>
        /// Draws a Ellipse.
        /// </summary>
        /// <param name="image">
        /// The destination image.
        /// </param>
        /// <param name="centerX">
        /// The x center position of the circle.
        /// </param>
        /// <param name="centerY">
        /// The y center position of the circle.
        /// </param>
        /// <param name="width">
        /// The width of the Ellipse.
        /// </param>
        /// <param name="height">
        /// The height of the Ellipse.
        /// </param>
        /// <param name="color">
        /// The color to use.
        /// </param>
        public static void DrawEllipse<T>(this GenericImage<T> image, int centerX, int centerY, int width, int height, T color)
        {
            // source (converted from C++) -> http://www.dailyfreecode.com/Code/draw-ellipse-midpoint-ellipse-algorithm-714.aspx
            float aa = (width * width);
            float bb = (height * height);
            float aa2 = (aa * 2);
            float bb2 = (bb * 2);

            float x = 0;
            float y = height;

            float fx = 0;
            float fy = (aa2 * height);

            float p = (int)(bb - (aa * height) + (0.25 * aa) + 0.5);

            image[(int)(centerX + x), (int)(centerY + y)] = color;
            image[(int)(centerX + x), (int)(centerY - y)] = color;
            image[(int)(centerX - x), (int)(centerY - y)] = color;
            image[(int)(centerX - x), (int)(centerY + y)] = color;

            while (fx < fy)
            {
                x++;
                fx += bb2;

                if (p < 0) p += (fx + bb);

                else
                {
                    y--;
                    fy -= aa2;
                    p += (fx + bb - fy);
                }

                image[(int)(centerX + x), (int)(centerY + y)] = color;
                image[(int)(centerX + x), (int)(centerY - y)] = color;
                image[(int)(centerX - x), (int)(centerY - y)] = color;
                image[(int)(centerX - x), (int)(centerY + y)] = color;
            }

            p = (int)((bb * (x + 0.5) * (x + 0.5)) + (aa * (y - 1) * (y - 1)) - (aa * bb) + 0.5);

            while (y > 0)
            {
                y--;
                fy -= aa2;

                if (p >= 0) p += (aa - fy);

                else
                {
                    x++;
                    fx += bb2;
                    p += (fx + aa - fy);
                }

                image[(int)(centerX + x), (int)(centerY + y)] = color;
                image[(int)(centerX + x), (int)(centerY - y)] = color;
                image[(int)(centerX - x), (int)(centerY - y)] = color;
                image[(int)(centerX - x), (int)(centerY + y)] = color;
            }
        }