Thread:Talk:CAPTCHA/Possible solution to Multilingual, usable and effective captchas/reply

Captchas need to be hard to solve by machines. If machine recognition can be used to infer the categories when creating the captcha, it may be also used to solve it.

The captcha system does not need to pick from all possible images on Commons, it is fine to use just a subset of those that works, and even make the system to improve over time (discarding the images for which users had asked to reload the captcha or users fail to resolve).