You see letters, which are distorted somehow. These letters are a password, that you should enter. These approaches are meant to prevent automated form filling. Automated form filling is a pressing issue in the internet, because there are a whole lot of spammers, who would fill any web form just to have their pages linked. If you have forms filled automatically, you definitely leave the field of good sports and even worse you destroy channels of communication between people for the sake of google ranking. Well, since automats do have hard times to filter letters from pixel data CAPTCHA appears to be a good approach. Well, actually it isn't, because you prevent not only automats from filling the form but you prevent the seeing impaired as well. Screenreaders cannot read the pixelized passphrase. So, there are humans that cannot recognize the passphrase, because they cannot see it at all. Actually there are robots already that achieve better results in filtering the passphrases than some of the seeing impaired could ever achieve.
... and thats the whole problem. There are human beings, who have certain short commings that an automat might not have. Those human beings are excluded from certain services, which is bad.
What to do about it? What else could we ask requesters to find out whether they are flesh and blood or semi conductor? Jon suggests logical questions like '5 + 7'. Oh, oh, this should be very easy for a machine that was invented to calculate. What task distinguishes the set of human beings from the set of robots? Must be something related to pattern recognition ... I really don't know.
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