The Captcha function by skewed characters can roughly be divided into 4 types

What is capture?
"Completely Automated Public Turing test to tell Computers and Humans Apart"
It seems that it is an abbreviation for. It is a general way to distinguish whether a partner is a human, that is, manual or ... machine, that is, fully automated.

There are roughly 4 types in all,

1.Gimpy
2.Bongo
3.Pix
Four.Sounds

there is.

The CAPTCHA Project.
http://www.captcha.net/

Let's look through each method in order.
1.Gimpy
http://www.captcha.net/cgi-bin/gimpy

It is a form in which characters are superimposed and rocked to make mechanical reading impossible and the background color is also changed. It is very common.

2. Bongo
http://www.captcha.net/captchas/bongo/

How to distinguish by shape. Please choose a big circle and a triangle. In some online banking, the same mechanism as in the case is used as authentication in transfer.

3. Pix
http://gs264.sp.cs.cmu.edu/cgi-bin/esp-pix

This is amazing, you choose what is reminiscent of the displayed picture. If you see it, it is hard to break through this, is not it? Because the displayed image itself should be something like Gimpy method to generate every time. It is funny ....

4. Sounds
http://www.captcha.net/captchas/sounds/

Although there is no practical example, it is similar to Gimpy method, typing words that flowed as sound, that is, converting it into a character string and inputting it. It feels like it will be overwhelmingly burdensome, but it's pretty unique.

By the way, although it is the most popular "Gimpy", there are also horrible strong men in the world, there are actually automatic programs that break through this. An old script "Emi watch" and such feeling. Though it can not break through complicated things, though ...