Generate an Image with a Random Number

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Introduction

We need to check if it is a real person who is registering on our Web site. So we decided to generate an image with a random code, hidden for robots, and feed it to the browser instead of a page response. We show it in our registration form, and let the user enter the code to check it.

Create the Web Project

  1. File -> New -> New project

Create a new CodeImage Visual Basic ASP.NET Web Application.

Change the Web Form

  1. Rename WebForm1.aspx to CodeImage.aspx. Open it.
  2. Switch to HTML View (Right click -> View HTML source).
  3. Remove all HTML code. Leave only the <%@ Page ... %> header.
  4. Switch to Code View (Right click -> View Code).

Change the Page_Load method with the code to generate the random code image. Also set Session("hiddenCode") to allow code checks later:

CodeImage.aspx

<%@ Page Language="VB" AutoEventWireup="false" CodeFile="CodeImage.aspx.vb" Inherits="CodeImage" %>

CodeImage.aspx.vb

Partial Class CodeImage

    Inherits System.Web.UI.Page

 

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

        'Put user code to initialize the page here

 

        ' Create a Bitmap image

        Dim ImageSrc As System.Drawing.Bitmap = New System.Drawing.Bitmap(155, 85)

 

        ' Fill it randomly with white pixels

        For iX As Integer = 0 To ImageSrc.Width - 1

            For iY As Integer = 0 To ImageSrc.Height - 1

                If Rnd() > 0.5 Then

                    ImageSrc.SetPixel(iX, iY, System.Drawing.Color.White)

                End If

            Next iY

        Next iX

 

        ' Create an ImageGraphics Graphics object from bitmap Image

        Dim ImageGraphics As System.Drawing.Graphics = System.Drawing.Graphics.FromImage(ImageSrc)

 

        ' Generate random code.

        Dim hiddenCode As String = (Fix(Rnd() * 10000000)).ToString

        ' Set Session variable

        Session("hiddenCode") = hiddenCode

 

        ' Draw random code within Image

        Dim drawFont As New System.Drawing.Font("Arial", 20, Drawing.FontStyle.Italic)

        Dim drawBrush As New _

           System.Drawing.SolidBrush(System.Drawing.Color.Black)

        Dim x As Single = 5.0 + (Rnd() / 1) * (ImageSrc.Width - 120)

        Dim y As Single = 5.0 + (Rnd() / 1) * (ImageSrc.Height - 30)

        Dim drawFormat As New System.Drawing.StringFormat

        ImageGraphics.DrawString(hiddenCode, drawFont, drawBrush, _

            x, y, drawFormat)

 

        ' Change reponse content MIME type

        Response.ContentType = "image/jpeg"

 

        ' Sent Image using Response OutputStream

        ImageSrc.Save(Response.OutputStream, System.Drawing.Imaging.ImageFormat.Jpeg)

 

        ' Dispose used Objects

        drawFont.Dispose()

        drawBrush.Dispose()

        ImageGraphics.Dispose()

    End Sub

End Class

 

CodeImageDefault.aspx

<%@ Page Language="VB" AutoEventWireup="false" CodeFile="CodeImageDefault.aspx.vb" Inherits="CodeImageDefault" %>

 

<!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 runat="server">

    <title>Generate an Image with a Random Number</title>

</head>

<body>

    <form id="form1" runat="server">

    <div>

    <img style="Z-INDEX: 101; LEFT: 152px; POSITION: absolute; TOP: 24px" height="85" alt="Hidden Code"

                   src="CodeImage.aspx" width="155"/>

              <div style="DISPLAY: inline; Z-INDEX: 102; LEFT: 88px; WIDTH: 304px; POSITION: absolute; TOP: 120px; HEIGHT: 19px"

                   ms_positioning="FlowLayout">Enter the Code in image:</div>

              <asp:TextBox id="TextBox1" style="Z-INDEX: 103; LEFT: 160px; POSITION: absolute; TOP: 144px"

                   runat="server"></asp:TextBox>

              <asp:Button id="Button1" style="Z-INDEX: 104; LEFT: 168px; POSITION: absolute; TOP: 176px" runat="server"

                   Text="Button" Width="128px" Height="32px"></asp:Button>

              <asp:Label id="Label1" style="Z-INDEX: 105; LEFT: 128px; POSITION: absolute; TOP: 224px" runat="server"

                   Width="232px" Height="32px"></asp:Label>

    </div>

    </form>

</body>

</html>

 

CodeImageDefault.aspx.vb

 

Partial Class CodeImageDefault

    Inherits System.Web.UI.Page

 

    Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click

        If TextBox1.Text <> Session("hiddenCode") Then

            Label1.Text = "Wrong Code!"

        Else

            Label1.Text = "Correct Code!"

        End If

    End Sub

End Class

 
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