在线图片制作是怎样实现的？

public void made_img(string checkCode)
{
if (checkCode == null || checkCode.Trim() == String.Empty)
return ;

System.Drawing.Bitmap image = new System.Drawing.Bitmap((int)Math.Ceiling((checkCode.Length * 15.0)), 15);
Graphics g = Graphics.FromImage(image);

try
{
//清空图片背景色
g.Clear(Color.White);
Font font = new System.Drawing.Font("", 9, System.Drawing.FontStyle.Regular );
System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(new Rectangle(0, 0, image.Width, image.Height), Color.Black, Color.Black, 1.2f, true);
g.DrawString(checkCode, font, brush, 1, 1);
System.IO.MemoryStream ms = new System.IO.MemoryStream();
image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
Response.ClearContent();
Response.ContentType = "image/Gif";
Response.BinaryWrite(ms.ToArray());
}
finally
{
g.Dispose(