can touch each other, but for wide area networks, or any environment where the demands above cannot be met, Buttolo et al propose their "one-user-at-a-time" architecture. Mark and his colleagues (Mark, Randolph, Finch, van Verth, and Taylor, 1996) have proposed a number of solutions to recurring problems in haptics, such as improving the update rate for forces communicated back to the user. They propose the use of intermediate representation of force through a "plane and probe" method: a local planar approximation to the user's hand location is computed when the probe or haptic tool penetrates the plane, and the force is updated at approximately 1 kHz by the force server, while the application recomputes the position of the plane and updates it at approximately 20 kHz. Mark et al. also propose solutions to add surface texture and friction to what otherwise would be the slick surface produced under their model, using a parameterized "snag" distribution on the object surface. They also present a method for specifying torques as well as force, and a "recovery-time algorithm" for preventing force discontinuity artifacts, such as occur when the haptic probe's sideways movement is too fast relative to the computation of the new intermediate representation. Mark et al. have developed a device-independent library of routines for haptic interfaces, Armlib, which supports multi-user and multi-hand applications. Armlib works with a number of different haptic display devices, including the PHANToM. Psychophysical studies: perceptions of shape and texture in multimodal virtual environmentsThe behavior of the human haptic system has been the subject of far more systematic study than has touching with robotic masters. Texture, apprehended by most subjects through lateral, side-to-side hand movement or exploratory procedure, is only one of several haptically important dimensions of object recognition, including hardness, shape, and thermal conductivity...
