reCAPTCHA

This is a nice idea that solves two problems at once. The first problem is that spammers regularly use computers to automate the process of posting spam messages to websites. The second problem is that old books need to be digitised from scanned copies of their words.

One way of preventing computers from being able to post to a site is to used a so-called CAPTCHA, which is simply an image of some text, suitably distorted or coloured to make it very difficult for a computer to work out what the letter are but still very obvious to a human.

reCAPTCHA simply displays two such words from scanned copies of text. It knows the letters which make up one word but not the other so when you enter the two words, if you get the one it knows correct it assumes you have entered the correct text for the one it doesn't know. Computers used by spammers will not be able to guess either word so are stopped but human users will enter both words correctly and so help digitise the scanned words. Very neat! (via thoughtplay).

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