Effective CAPTCHA with Amodal Completion and Aftereffects by Complementary Colors and Difference of Luminance

We propose an effective CAPTCHA with amodal completion and aftereffects and improve it by applying complementary colors and difference of luminance. CAPTCHA is a method which distinguishes human beings from artificial intelligence which is usually called bots on Internet. Bots acquire accounts of web services for unfair uses such as stealth marketing or SPAM attacks. That is, CAPTCHA is a countermeasure against those attacks. The most popular kind of CAPTCHA methods is text-based CAPTCHA in which distorted alphabets and numbers appear with obstacles or noise. However, it is known that all of text-based CAPTCHA algorithms can be analyzed by computers. In addition, too much distortion or noise prevents human beings from alphabets or numbers. There are other kinds of CAPTCHA methods such as image CAPTCHA and audio CAPTCHA. However, they also have problems in use. As a related work, an effective text-based CAPTCHA algorithm with amodal completion was proposed. The CAPTCHA provides computers a large amount of calculation cost while amodal completion helps human beings to recognize characters momentarily. On the other hand, extreme concentration is required for momentary recognition. The CAPTCHA is once improved in our laboratory. Aftereffects are additionally combined with the CAPTCHA algorithm. Moreover, in this paper, we add colors to the algorithm with considering combinations of complementary colors and those of luminance. The aftereffects extend time for recognition of characters from a moment to several seconds.