Create your Own/Custom Image Captcha in simple steps.

Introduction

Captchas are used in attempts to prevent automated software from performing actions ensures that its human generated.In this arctle we are creating a Custom image, with random texts printed at random location in random colors and to add difficulty for the character recognition softwates we are drawing lines in the background. The following describes how to do it. Please note that you can make it more complex by editing the image.

Class Diagram

Class Description

WebCaptcha class contains private field rndm which is used to generate random numbers. Other properties as their name suggest are self explanatory. CaptchaImg holds the image (System.Drawing.Image) which we will create and our objective.

GetRandomCArr is a function which returns a random character array of length passed to this function as a parameter.

Class Description

Please find the class description bellow.

public class WebCaptcha
{
public string Value { get; set; }
public Image CaptchaImg { get; set; }
public bool CaseSensitive { get; set; }
private Random rndm;

public bool IsValid(string input)
{
return CaseSensitive?this.Value.Equals(input):this.Value.Equals(input, System.StringComparison.OrdinalIgnoreCase);
}

public WebCaptcha()
{
this.rndm = new Random();
char[] rmcarr = this.GetRandomCArr(6);
this.Value = new string(rmcarr);
this.CreateImage(rmcarr);
}

private void CreateImage(char[] rmcarr)
{
int FontSize = 20;
Bitmap bmp = new Bitmap(140, 55);
Graphics gImage = Graphics.FromImage(bmp);
gImage.FillRectangle(Brushes.WhiteSmoke, 0, 0, bmp.Width, bmp.Height);
//Draw lines
for (int i = 0; i < 200; i = i + 5)
{
gImage.DrawLine(Pens.LightGray, new Point(0, i + 5), new Point(i + 5, 0));
gImage.DrawLine(Pens.LightGray, new Point(0, 60 - (i + 5)), new Point(i + 5, 60));
}
//Draw random text
for (int i = 0; i < rmcarr.Length; i++)
{
gImage.DrawString(rmcarr[i].ToString(), new Font(FontFamily.GenericSansSerif, FontSize,FontStyle.Bold), GetRandomBrushes(), new PointF(5 + i * FontSize, 5 + (rndm.Next() % 2 == 0 ? 0 : 15)));
}
this.CaptchaImg = bmp;
}

//Get random char array
private char[] GetRandomCArr(int length)
{
var mch = Enumerable.Range(35, 2).Select(c => (char)c).Union(
Enumerable.Range(49, 9).Select(c => (char)c)).Union(
Enumerable.Range(64, 27).Select(c => (char)c)).Union(
Enumerable.Range(97, 26).Select(c => (char)c)).ToArray();
var mcstr = Enumerable.Range(1, length).Select(a => mch[rndm.Next() % mch.Length]).ToArray();
return mcstr;
}

//Get a random brush
private Brush GetRandomBrushes()
{
var mbrsharr = new Brush[] { Brushes.Blue,Brushes.BlueViolet, Brushes.Green, Brushes.Red,Brushes.Purple, Brushes.DarkGreen, Brushes.Brown, Brushes.Black };
return mbrsharr[rndm.Next() % mbrsharr.Length];
}
}

Now Image is generated, you can use it wherever you want.

Please note that, you can make it more complex always if you want. 

Thanks,

Debata