OCR CAPTCHA: Generate images to test whether the user is human

This class is meant to generate CAPTCHA (Completely Automated Public Turing to tell Computers from Humans Apart) images to test whether the user accessing a site is a real human or an automatic script.

The class generate an image with a random key text string printed on it mixed with some noise made of random text characters or line grid.

The image is meant to be used in a form so the user reads the key text in the image and must enter it to prove that is a real human.

The form also passes an hidden field that contains a public key that is used when the form is submitted to the server to verify whether the key text on the image matches what the user entered.