To reduce the incidence of spam and reduce the likelihood of denial of service attacks, many web pages include an image block with distorted text, known as a Captcha.

Captcha is an image with a code written on it. The website visitor is required to read the code on the image and enter the value in a text field. If the word entered is wrong, the form submission is not processed. As CAPTCHA is a smartly blurred image, the spam bot can't read it. So the form cannot be auto-submitted by a 'bot'.

This article will give step by step tutorial how to create Capcta in ASP.Net using c#.

STEP 1

Create Dynamic Images as per the Image size.

 private void GenerateImage()
{
// Create a new 32-bit bitmap image.
 Bitmap bitmap = new Bitmap(this.m_width, this.m_height,
PixelFormat.Format32bppArgb);

// Create a graphics object for drawing.
 Graphics g = Graphics.FromImage(bitmap);
g.SmoothingMode = SmoothingMode.AntiAlias;
Rectangle rect = new Rectangle(0, 0, this.m_width, this.m_height);

// Fill in the background.
 HatchBrush hatchBrush = new HatchBrush(HatchStyle.SmallConfetti,
Color.LightGray, Color.White);
g.FillRectangle(hatchBrush, rect);

// Set up the text font.
 SizeF size = default(SizeF);
float fontSize = rect.Height + 1;
Font font = default(Font);
// Adjust the font size until the text fits within the image.
 do
 {
fontSize -= 1;
font = new Font(this.familyName, fontSize, FontStyle.Bold);
size = g.MeasureString(this.m_text, font);
} while (size.Width > rect.Width);

// Set up the text format.
 StringFormat format = new StringFormat();
format.Alignment = StringAlignment.Center;
format.LineAlignment = StringAlignment.Center;

// Create a path using the text and warp it randomly.
 GraphicsPath path = new GraphicsPath();
path.AddString(this.m_text, font.FontFamily, Convert.ToInt32(font.Style), font.Size, rect, format);
float v = 4f;
PointF[] points = {
new PointF(this.random.Next(rect.Width) / v, this.random.Next(rect.Height) / v),
new PointF(rect.Width - this.random.Next(rect.Width) / v, this.random.Next(rect.Height) / v),
new PointF(this.random.Next(rect.Width) / v, rect.Height - this.random.Next(rect.Height) / v),
new PointF(rect.Width - this.random.Next(rect.Width) / v, rect.Height - this.random.Next(rect.Height) / v)
};
Matrix matrix = new Matrix();
matrix.Translate(0f, 0f);
path.Warp(points, rect, matrix, WarpMode.Perspective, 0f);

// Draw the text.
 hatchBrush = new HatchBrush(HatchStyle.LargeConfetti, Color.LightGray, Color.DarkGray);
g.FillPath(hatchBrush, path);

// Add some random noise.
 int m = Math.Max(rect.Width, rect.Height);
for (int i = 0; i <= Convert.ToInt32(Math.Truncate(rect.Width * rect.Height / 30f)) - 1; i++)
{
int x = this.random.Next(rect.Width);
int y = this.random.Next(rect.Height);
int w = this.random.Next(m / 50);
int h = this.random.Next(m / 50);
g.FillEllipse(hatchBrush, x, y, w, h);
}

// Clean up.
 font.Dispose();
hatchBrush.Dispose();
g.Dispose();

// Set the image.
 this.m_image = bitmap;
}

Step 2

Create a ASP.Net webform Page to generate CapctaImage and this page will be refered as Image src in calling page

 CaptchaImage.CaptchaImage ci = new CaptchaImage.CaptchaImage(this.Session("CaptchaImageText").ToString(), 200, 50, "Arial black");

// Change the response headers to output a JPEG image.
 this.Response.Clear();
this.Response.ContentType = "image/jpeg";

// Write the image to the response stream in JPEG format.
 ci.Image.Save(this.Response.OutputStream, ImageFormat.Jpeg);

// Dispose of the CAPTCHA image object.
 ci.Dispose();
}

Many of these properties have to do with the inherent tradeoff between human readability and machine readability. The harder a CAPTCHA is for OCR software to read, the harder it will be for us human beings, too! For illustration, compare these two CAPTCHA images:

Capcta in ASP.Net

Download the source code for more details.