Virtual and augmented reality systems that immerse users in a 3D environment could benefit from haptic (force and/or tactile) feedback to increase realism and task performance. A canonical task that cannot be achieved compellingly without haptic feedback is the grasp and lift of an object with mass. Traditional kinesthetic (force) feedback devices provide realistic physical interactions with virtual objects, but typically require large actuators, making them bulky and encumbering. In contrast, tactile feedback devices can use smaller actuators and may enable freer movement of a user in a virtual world. One form of tactile feedback, skin deformation feedback, has been previously shown to provide effective haptic feedback of force, stiffness, and friction. Here we compare human perception of mass via kinesthetic feedback and skin deformation feedback in a virtual environment. Participants grasp and lift two virtual blocks and attempt to equalize their masses by the method of adjustments. From the accuracy of this equalization, we determined that the Weber Fraction (the just noticeable difference in proportion to the original stimulus value) of virtual mass during a grasp and lift task was 0.11 for kinesthetic force feedback and 0.35 for skin deformation feedback. In addition, participants exhibited differences in exploratory procedures between the two types of feedback.

A new variation of high order multiway tree structures, the P-tree is presented. P-trees have average access costs that are significantly better than those of B-trees and are no worse (and often better) in storage utilization. Unlike compact B-trees, they can be maintained dynamically, and unlike dense multiway trees and B-trees, their associated insertion algorithm, which is also presented, is cheap and involves (at most) a very localized rearrangement of keys.