From VRML 2 Basic Concepts and Definitions CPS616 -- Information Track of CPS -- Spring Semester 98. byZeynep Ozdemir, Rob Baker, Meryem Ispirli, Geoffrey C. Fox (Definitions Only)
Visibility sensors sense whether a box-shaped region in the VRML world is visible from the viewer's position and orientation so that it is possible to control the animation or actions that are only necessary when a sensed region can be seen.