Checking the correctness of captcha on the fly

Hello!

Recently, filling in the captcha once again when registering on the site, the thought came, why not check the entered characters on the fly (for example, like checking the login, etc.)? Because sometimes it is so distorted letters and numbers that even a person is not clear what is drawn there. Plus, such characters as lowercase I and uppercase l are often confused. As a result, the entered result is not correct and you have to repeat the process again.

Unfortunately, I am not strong in programming, but I think that it is possible to find a way to implement such a test without losing the security of the site.