The authors also tested software designed to crack CAPTCHAs against images created using reCAPTCHA, and found that they failed completely. The authors ascribe this to the fact that the letters in scanned images contain distortions that are not the result of a clean mathematical transformation.
http://arstechnica.com/news.ars/post/20070525-anew-twist-on-anti-spam-tech-can-help-digitize-books.html
http://arstechnica.com/news.ars/post/20080814-captchas-workfor-digitizing-old-damaged-texts-manuscripts.html
The authors also tested software designed to crack CAPTCHAs against images created using reCAPTCHA, and found that they failed completely. The authors ascribe this to the fact that the letters in scanned images contain distortions that are not the result of a clean mathematical transformation.
http://recaptcha.net/whyrecaptcha.html
I considered using that and it’s definitely a worthy cause, but the images are often so hard to read that I decided to pass.
Can you choose words / strings from other languages than just English? For many of my clients this would be a potential pitfall.