Source -> http://www.dailyfreecode.com/code/fill-circle-scan-line-circle-fill-675.aspx

/// <summary>
        /// Fills a circle.
        /// </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="radius">
        /// The radius of the circle.
        /// </param>  
        /// <param name="color">
        /// The color to use.
        /// </param>    
        public static void FillCircle<T>(this GenericImage<T> image, int centerX, int centerY, int radius, T color)
        {
            // source -> http://www.dailyfreecode.com/code/fill-circle-scan-line-circle-fill-675.aspx
            int x1;
            int x2;

            var counter = (centerY + radius);

            for (var count = (centerY - radius); count <= counter; count++)
            {
                x1 = (int)(centerX + Math.Sqrt((radius * radius) - ((count - centerY) * (count - centerY))) + 0.5);
                x2 = (int)(centerX - Math.Sqrt((radius * radius) - ((count - centerY) * (count - centerY))) + 0.5);

                image.DrawLine(x1, count, x2, count, color);
            }
        }