C#.net 动态生成GIF图背景无法透明？？？？(代码如下)

为什么无法消除黑背景？？
求助

<%@ Page Language="C#" ContentType = "image/Gif"  %>
<<%@ Import  Namespace = "System" %>
<%@  Import Namespace = "System.Drawing" %>
<%@ Import Namespace = "System.Drawing.Drawing2D" %>
<%@ Import Namespace = "System.Drawing.Imaging" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" >
<head > </head >
<script language ="c#" runat  = "server" >
protected void Page_Load(object sender, EventArgs e)
    {

        Bitmap currentBitmap = new Bitmap(891, 556);
        ImageAttributes ia = new ImageAttributes();
        Graphics g = Graphics.FromImage(currentBitmap);
        float[][] colorMatrix ={
        new float[] {1, 0, 0, 0, 0},
        new float[] {0, 1, 0, 0, 0},
        new float[] {0, 0, 1, 0, 0},
        new float[] {0, 0, 0, 0, 0}, //注意：此处为0，图像为全透明
        new float[] {0, 0, 0, 0, 1}};
         Font axesFont = new Font("Arial", 10);
         Brush blackBrush = new SolidBrush(Color.Blue);
         g.DrawString(" -22.3 ", axesFont, blackBrush, 300, 400);

        ColorMatrix cm = new ColorMatrix(colorMatrix);
        ia.SetColorMatrix(cm, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);


        g.DrawImage(currentBitmap, new Rectangle(0, 0, currentBitmap.Width, currentBitmap.Height), 0, 0, currentBitmap.Width, currentBitmap.Height, GraphicsUnit.Pixel, ia);
        //  picBox.Image = currentBitmap;
        // g.Dispose();
        //currentBitmap.Save(Response.OutputStream, ImageFormat.gif);
        //currentBitmap.Save(response.outputstream,ImageFormat .Gif);

      //  g.Clear(Color.Transparent);
      //  g.Clear(colorMatrix.;

        currentBitmap.Save(Response.OutputStream, ImageFormat.Gif);



    }
</script >

<body>

</body>
</html>

补充：.NET技术 ,  C#