Not exactly answering the question, but I recently used aliexpress.com and their captcha system is super easy: it shows a sliding button like the one to answer

Not exactly answering the question, but I recently used aliexpress.com and their captcha system is super easy: it shows a sliding button like the one to answer a call on a phone. The prompt just asks you to slide it to validate your input. Not sure how it works, but it sure is a much better UX than when I have to spend 3 minutes identifying for hydrants. Maybe we could make a FLOSS version of it?