求下面代码的解释和改进!!
下面这一段是我在网上找的在Web页面上绘制图形的代码!是通过在服务器端把图形绘制好后再返回给客户端。可是绘制出来的图形背景色是黑色的,怎么使图形的背景色变为白色,然后怎么改变图形显示的位置啊??它默认是显示在左上角的,怎么让它在我指定的位置显示啊??谢谢!!
private void Page_Load(object sender, System.EventArgs e)
{
Response.ContentType="image/gif";
System.Drawing.Bitmap b=new System.Drawing.Bitmap(400,400);
System.Drawing.Graphics g=System.Drawing.Graphics.FromImage(b);
g.Clear(Color.White);
SolidBrush brush = new SolidBrush(Color.Yellow);
g.FillRectangle(brush, 200,50,400,300);
System.Drawing.Pen p=new System.Drawing.Pen(System.Drawing.Color.Blue,2);
g.DrawRectangle(p,50,50,100,100);
g.Flush();
b.Save(Response.OutputStream,System.Drawing.Imaging.ImageFormat.Gif);
Response.End();
}