  Virtual reality (VR) allows users to see and manipulate virtual scenes and
items through input devices, like head-mounted displays. In this study, the
effects of button size, spacing, and depth on the usability of virtual buttons
in VR environments were investigated. Task completion time, number of errors,
and subjective preferences were collected to test different levels of the
button size, spacing, and depth. The experiment was conducted in a desktop
setting with Oculus Rift and Leap motion. A total of 18 subjects performed a
button selection task. The optimal levels of button size and spacing within the
experimental conditions are 25 mm and between 5 mm and 9 mm, respectively.
Button sizes of 15 mm with 1-mm spacing were too small to be used in VR
environments. A trend of decreasing task completion time and the number of
errors was observed as button size and spacing increased. However, large size
and spacing may cause fatigue, due to continuous extension of the arms. For
depth effects, the touch method took a shorter task completion time. However,
the push method recorded a smaller number of errors, owing to the visual
push-feedback. In this paper, we discuss advantages and disadvantages in
detail. The results can be applied to many different application areas with VR
HMD.
