How do you Draw Transparent Image using System.Drawing?

Question

I'm trying to return a transparent GIF from an .aspx page for display within a web page. I am trying to get the image to have transparency, but I just keep getting Black being where the image should be Transparent.

Does anyone know what I'm doing wrong?

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) _
  Handles Me.Load
    '' Change the response headers to output a GIF image.
    Response.Clear()
    Response.ContentType = "image/gif"

    Dim width = 110
    Dim height = width

    '' Create a new 32-bit bitmap image
    Dim b = New Bitmap(width, height)

    '' Create Grahpics object for drawing
    Dim g = Graphics.FromImage(b)

    Dim rect = New Rectangle(0, 0, width - 1, height - 1)

    '' Fill in with Transparent
    Dim tbrush = New System.Drawing.SolidBrush(Color.Transparent)
    g.FillRectangle(tbrush, rect)

    '' Draw Circle Border
    Dim bPen = Pens.Red
    g.DrawPie(bPen, rect, 0, 365)

    '' Fill in Circle
    Dim cbrush = New SolidBrush(Color.LightBlue)
    g.FillPie(cbrush, rect, 0, 365)


    '' Clean up
    g.Flush()
    g.Dispose()

    '' Make Transparent
    b.MakeTransparent()

    b.Save(Response.OutputStream, Imaging.ImageFormat.Gif)
    Response.Flush()
    Response.End()
End Sub

Answer 1

Unfortunately, there is no easy way to create a transparent Gif using a Bitmap object. (See this KB article)

You can alternatively use the PNG format that supports transparency with the code you are using.