  Image CAPTCHA, aiming at effectively distinguishing human users from
malicious script attacks, has been an important mechanism to protect online
systems from spams and abuses. Despite the increasing interests in developing
and deploying image CAPTCHAs, the usability aspect of those CAPTCHAs has hardly
been explored systematically. In this paper, the universal design factors of
image CAPTCHAs, such as image layouts, quantities, sizes, tilting angles and
colors were experimentally evaluated through the following four dimensions:
eye-tracking, efficiency, effectiveness and satisfaction. The cognitive
processes revealed by eye-tracking indicate that the distribution of eye gaze
is equally assigned to each candidate image and irrelevant to the variation of
image contents. In addition, the gazing plot suggests that more than 70% of the
participants inspected CAPTCHA images row-by-row, which is more efficient than
scanning randomly. Those four-dimensional evaluations essentially suggest that
square and horizontal rectangle are the preferred layout; image quantities may
not exceed 16 while the image color is insignificant. Meanwhile, the image size
and tilting angle are suggested to be larger than 55 pixels x 55 pixels and
within -45~45 degrees, respectively. Basing on those usability experiment
results, we proposed a design guideline that is expected to be useful for
developing more usable image CAPTCHAs.
