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Generate an Image with a Random Number

, 29 Nov 2006
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Generate an image with a random code number for user identification.

Sample Generated Code Image


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:

Private Sub Page_Load(ByVal sender As System.Object, _
        ByVal e As System.EventArgs) Handles MyBase.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 = _

    ' 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, FontStyle.Italic)
    Dim drawBrush As New _
    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, _

    ' Dispose Objects used

End Sub

Build the Solution (F7), and run the application (F5).

Create the Check form.

  1. Create a New Default.aspx Web Form (Right click CodeImage project -> Add -> Add Web Form).

  2. Right click Default.aspx -> Set As Start Page. Open it.

  3. From the IDE menu, click View -> Toolbox. From the HTML tab, drag an Image control to the form. Right click Image -> Properties. Set the image source to "CodeImage.aspx". Set the alternate text to "Hidden Code". Click Apply. Click OK.

  4. From the HTML tab, drag a Label control to the form. Click it twice. Change the text to "Enter the code in the image:".

  5. From the Web Forms tab, drag a TextBox control to the form.

  6. From the Web Forms tab, drag a Button control to the form.

  7. From the Web Forms tab, drag a Label control to the form. Right click -> Properties. Set the Text property to Nothing.

Double click the Button1 button control, and change the Button1_Click code to:

Private Sub Button1_Click(ByVal sender As System.Object, _
        ByVal e As System.EventArgs) Handles Button1.Click
    If TextBox1.Text <> Session("hiddenCode") Then
        Label1.Text = "Wrong Code!"
        Label1.Text = "Correct Code!"
    End If
End Sub

Build and run the Web application (F7) (F5).

Points of Interest

That's it!


Just posted.


This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here


About the Author

Software Developer CIMEX S.A.
Cuba Cuba
Rodolfo Ortega is a Cuban Computer Scientist. He works as IT Auditor for the CIMEX S.A. subsidiary in Holguin, Cuba. He lives and works in Holguin, in the eastern part of the island of Cuba.

You can contact him at rodolfom[] for any personal message: Ideas on new articles, bibliography about new APIs, questions, are wellcome.

Submit questions related with current article to the article forum.

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Comments and Discussions

Generalin C# please... Pin
pradeep kumarappagari6-Dec-06 22:59
memberpradeep kumarappagari6-Dec-06 22:59 
GeneralRe: in C# please... Pin
Jasmine25017-Dec-06 6:39
memberJasmine25017-Dec-06 6:39 
GeneralRe: in C# please... Pin
pradeep kumarappagari7-Dec-06 18:38
memberpradeep kumarappagari7-Dec-06 18:38 
GeneralRe: in C# please... Pin
rmortega777-Dec-06 9:05
memberrmortega777-Dec-06 9:05 
GeneralRe: in C# please... Pin
pradeep kumarappagari7-Dec-06 18:36
memberpradeep kumarappagari7-Dec-06 18:36 
GeneralHard to read Pin
Jasmine25016-Dec-06 10:45
memberJasmine25016-Dec-06 10:45 
Great article but what struck me when I looked at the image at the top of the page is "that's kinda hard to read". I use a solid background (medium blue), on which I draw a few lines (light blue), then I make the letters in black, using a limited set of letters and symbols which are carefully chosen to be easy to recognize. Then I output the whole thing as a GIF. Mine is also done as a user control, so once it's all built, you can simply insert a server tag into the page, and it's all encapsulated nicely with it's own Validator that is capable of invalidating the whole form. I also use an encrypted value which is a HiddenField, so you don't have to use session values, and never have to worry about the plaintext of the verification code being output to the client, which a bot could theoretically read fairly simply. I will be posting an article about that eventually. You can see it in action on this page:[^]

I use C#, so my article will be substantially different from this one, but you could implement the better drawing for your version.

"Quality Software since 1983!" - see the "Programming" section for (freeware) JazzySiteMaps, a simple application to generate .Net and Google-style sitemaps!

GeneralRe: Hard to read Pin
M_Johnson6-Dec-06 21:36
memberM_Johnson6-Dec-06 21:36 
GeneralRe: Hard to read Pin
Jasmine25017-Dec-06 6:37
memberJasmine25017-Dec-06 6:37 
AnswerRe: Hard to read Pin
mdekok30003-Nov-07 9:59
membermdekok30003-Nov-07 9:59 
GeneralPerformance Fix... Pin
Jon Rista29-Nov-06 10:46
memberJon Rista29-Nov-06 10:46 
GeneralRe: Performance Fix... Pin
rmortega774-Dec-06 3:21
memberrmortega774-Dec-06 3:21 
GeneralRe: Performance Fix... Pin
Member 418550221-Sep-08 18:16
memberMember 418550221-Sep-08 18:16 

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