By asking visitors to correctly say which direction of say, four (up,
down, left or right) a part of a captcha appears to be "moving", it
filters out the bots while allowing humans with optical illusion
fooled eyeballs to pass the test. Given 16 of those captcha requests
in a row and given that there are 2^2 choices per captcha, there is a
2^16*2 - 1 possible ways to enter 16 captchas wrong, but only 1 way to
enter them right, leading to a filtration failure of around 1 in 4
billion.
Also, multiple captchas can be done in a row for each failure, but once say, 16 captchas have been solved in a row, access is granted for say, a month, until the captcha has to be done again.
It just feels like Hacker News is full of bots and I have a solution to this.
And would it really be effective against bots? It isn't obvious to me why it would be any more effective than other schemes.