These interfaces cover both the input and display aspects of human-computer interface. On the input side, a variety of tracking technologies have become available that provide position and orientation information in three-dimensional space about various parts of the user's body. It turns out that a delay of a tenth of a second is close enough to real time for users to feel that the computer is keeping up in many classes of applications, though applications that require interaction with objects with high frequencies of motion will require higher frame rates and smaller delays. This degree of responsiveness contrasts sharply with the transactional model of human-computer interaction in which the user makes a discrete request, such as a button push, text input or menu selection, that typically produces a discrete response. One critical aspect of such transactional interfaces is that the computer receive the inputs in the order in which they were received.
Event queues are well suited for maintaining such order.
These commands can then be removed from the event queue and acted on in a way that does not have strong performance requirements. The new interfaces discussed in this paper are continuous in nature, providing a stream of data about the user's current physical state. This vision has implications for the design of the user interface. This observation strongly suggests that the interface to a task be designed specifically for that task, blurring the distinction between interface batches and application objects. Blurring the distinction between interface and application objects, however, makes the design of standard interfaces, with a common look and feel, especially challenging.