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Image Verification Helper Class

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19 Jan 20051 min read 71K   912   32   7
To generate random alphanumeric numbers in image format for image verification.

Sample image


This article will help the intermediate ASP.NET developers who may be sometimes in need of implementing a feature like Image Verification when getting inputs from the users. Basically, this is a new technique which will stop the automated programs to process the input forms. But, here I'm not explaining you about the whole technique of Image Verification. This article will explain two things, which we really need :

  • How to generate alpha-numeric characters randomly?
  • How to make it as an image?

Generating Random Alpha-Numerics

Microsoft .NET framework provides System.Security.Cryptography namespace which contains RandomNumberGenerator to produce random numbers.

The class CImgVerify contains the code to produce the random alphabets/numbers in the function getRandomAlphaNumeric.

RandomNumberGenerator rm; 
rm = RandomNumberGenerator.Create();

The above code will create a RandomNumberGenerator object.

byte[] data = new byte[3]; 

The byte type "data" will hold three elements of array to have the randomly generated numbers by GetNonZeroBytes. The GetNonZeroBytes fills an array of bytes with a cryptographically strong random sequence of nonzero values.

The generated random numbers will be stored in a byte variable as an array. Each and every elements will be an integer variable. So, make a proper alphanumeric string. The following code will use a loop to identify whether the randomly generated number falls into alphabets.

for(int nCnt=0;nCnt<=data.Length-1;nCnt++)
  //First convert it into a integer
  int nVal = Convert.ToInt32(data.GetValue(nCnt)); 
  // Check whether the converted int falls in between alphabets,symbols
  if(nVal > 32 && nVal < 127) 
     sTmp = Convert.ToChar(nVal).ToString(); //Convert to character
     sTmp = nVal.ToString(); //Remain as integer
  sRand += sTmp.ToString(); //Append it to a string

Generating Image in the Sky

So, now we got the random alphanumeric to display as a image. The image can be created by the System.Drawing namespace. The following is the code which itself self explanatory.

public Bitmap generateImage(string sTextToImg)
{ // 
  //Here, i haven't used any try..catch 
  PixelFormat pxImagePattern = PixelFormat.Format32bppArgb; 
  Bitmap bmpImage = new Bitmap(1,1,pxImagePattern); 
  Font fntImageFont = new Font("Trebuchets",14); 
  Graphics gdImageGrp = Graphics.FromImage(bmpImage); 
  float iWidth = gdImageGrp.MeasureString(sTextToImg,fntImageFont).Width; 
  float iHeight = gdImageGrp.MeasureString(sTextToImg,fntImageFont).Height; 
  bmpImage = new Bitmap((int)iWidth,(int)iHeight,pxImagePattern ); 
  gdImageGrp = Graphics.FromImage(bmpImage); 
  gdImageGrp.TextRenderingHint = TextRenderingHint.AntiAlias; 
  gdImageGrp.DrawString(sTextToImg,fntImageFont, new SolidBrush(Color.Red),0,0); 
  return bmpImage;

If you have further queries/suggestions, post here.


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Written By
Web Developer
India India
Vadivel Kumar is from India and he works in C#/ASP.NET platforms.

Comments and Discussions

GeneralImage Verification Pin
Jani0323-Jan-09 0:30
Jani0323-Jan-09 0:30 
Generaldear vadivelkumar Pin
phuonglk4-Dec-08 1:52
phuonglk4-Dec-08 1:52 
QuestionCreating An Image While Making An Email Signup Form. Pin
mshariq29-Jun-06 21:26
mshariq29-Jun-06 21:26 
GeneralRe: Creating An Image While Making An Email Signup Form. Pin
siravicb5-Mar-08 11:40
siravicb5-Mar-08 11:40 
GeneralRe: Creating An Image While Making An Email Signup Form. Pin
Mahmoud Aydeh18-Feb-09 6:05
Mahmoud Aydeh18-Feb-09 6:05 
QuestionCan anybody provide me the function 'generateImage' in VB??? Pin
xelios5-Dec-05 18:27
xelios5-Dec-05 18:27 
AnswerRe: Can anybody provide me the function 'generateImage' in VB??? Pin
zubairy29-Sep-11 11:54
zubairy29-Sep-11 11:54 
Dim pxImagePattern As System.Drawing.Imaging.PixelFormat
pxImagePattern = System.Drawing.Imaging.PixelFormat.Format32bppArgb

Good Luck

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