提示信息，生成为图像

/// <summary>
/// 清空保密区域，图像信息
/// </summary>
/// <param name="msg">提示信息</param>
/// <param name="width">图片宽度</param>
/// <param name="height">图片高度</param>
/// <returns></returns>
private Bitmap TipPic(string msg, int width = 400, int height = 200)
{
    Bitmap tmp = new Bitmap(width, height);
    Graphics g = Graphics.FromImage(tmp);

    g.FillRectangle(new SolidBrush(Color.White), new Rectangle(0, 0, tmp.Width, tmp.Height));   // 白色背景清空保密区域内容

    Font font = new Font("宋体", 12f);
    SizeF size = g.MeasureString(msg, font);
    float x = (tmp.Width - size.Width) / 2;
    float y = (tmp.Height - size.Height) / 2;
    g.DrawString(msg, font, new SolidBrush(Color.Black), x, y);

    return tmp;
}