HIP stands for Human Interaction Proof and provides a mechanism that allows any developer to control bot submissions (spams) to their webforms. This technique has been used for some time by Yahoo, Hotmail and others to avoid the creation of fake email accounts. This kind of human proof is also known as CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart). Most web developers have experienced in some way the effects of unwanted subscriptions or submissions to their applications. One of the most recent intrussions of this kind of spam is Blog Comment Spam that is sent using a form in a webpage.
This CAPTCHA of smwm obscures its message from computer interpretation by twisting the letters and adding a background color gradient.
A CAPTCHA Server Control Example for ASP.NET