Captcha in ASP.NET

// 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:

Download the source code for more details.