/// <summary>
        /// 获取含验证码字符串的图片。
        /// </summary>
        /// <param name="validateCode">验证码字符串。</param>
        /// <returns>图片对象。</returns>
        public static Image CreateCaptchaSimpleImage(string validateCode)
        {
            Bitmap image = new Bitmap((int)Math.Ceiling(validateCode.Length * 16.0), 27);
            using (Graphics g = Graphics.FromImage(image))
            {
                var font = new Font(FontFamily.GenericMonospace, 13);
                var brush = new LinearGradientBrush(new Rectangle(0, 0, image.Width, image.Height), Color.Blue,
                    Color.DarkRed, 1.2f, true);
                g.DrawString(validateCode, font, brush, 3, 2);
                return image;
            }
        }

        /// <summary>
        /// 识别字符串。
        /// </summary>
        /// <param name="bitmap">bitmap图片。</param>
        /// <returns>识别结果。</returns>
        public string GetStringFromImage(Bitmap bitmap)
        {
            using (var ocrApi = OcrApi.Create())
            {
                ocrApi.Init(Languages.English);
                return ocrApi.GetTextFromImage(bitmap);
            }
        }