GDI往画布上写个字，怎么总是带个黑边框呢！

   crFont = new Font("幼圆", 48, FontStyle.Regular);
            crSize = picture.MeasureString(_waterText, crFont);
            // 生成水印图片（将文字写到图片中）
            Bitmap floatBmp = new Bitmap((int)crSize.Width + 3,(int)crSize.Height + 3,PixelFormat.Format32bppArgb);
            Graphics fg = Graphics.FromImage(floatBmp);
            PointF pt = new PointF(0, 0);
            Brush shadowBrush = Brushes.Gray;
            fg.DrawString(_waterText, crFont, shadowBrush, pt.X , pt.Y );如题！会的帮我看看！谢谢！