Bostik, Ondrej, Karel Horak, and Jan Klecka. “Bubble Captcha - A Start of the New Direction of Text Captcha Scheme Development”. MENDEL 23, no. 1 (June 1, 2017): 57-64. Accessed November 29, 2024. http://flames.test.infv.eu/index.php/mendel/article/view/53.