What it is and where it is used. Perceptual Vision Interfaces (PVI) are the systems that use visual cues of the user, such as the motion of the face, to control a program. The main application of PVI  is seen in designing hands-free computer input devices to supplement (or to replace) the conventional hand-operated input devices such as mouse or joystick.

Problem. For PVI to be operational, it has to be able to track a user robustly - with respect to user's motion (i.e. regardless of facial orientation or expression), and precisely - with subpixel precision, and smoothly, so that user position position computed from  low-resolution video image can be converted to the cursor position on a high-resolution computer screen. Mathematically stated, this means designing a continuous monotonically increasing mapping function which maps a six-degree of freedom face position (x,y,z,a,b,c) to the two-degree position of the cursor (i,j). Conventionally used at the time visually distinctive facial features such brows, eyes, mouths could not provide a solution to this problem.