Implementation of flying, scaling and grabbing in virtual worlds

Abstract

In a virtual world viewed with a head-mounted display, the user may wish to perform certain actions under the control of a manual input device. The most important of these actions are flying through the world, scaling the world, and grabbing objects. This paper shows how these actions can be precisely specified with Frame-to-frame invariants, and how the code to implement the actions can bc dcrivcd from the invariants by algebraic manipulation. INTRODUCTION Wearing a Head-Mounted Display (HMD) gives a human user the scnsalion of being inside a three-dimensional. computersimulated world. Because the HMD rcplaccs the sights and sounds of the real world with a computer-gencratcd virtual world, this synthesizccl world is called virtual reality. The virtual world surrounding the user is dcfincd by a graphics database called a model, which gives the colors and coordinates for each of the polygons making up the virtual world. The polygons making up the virtual world arc normally grouped into cntitics called obj,jccfs, each of which has its own location and orientation. The human being wearing the HMD is called the user, and also has a location and orientation within the virtual world. To turn the data in the model into the illusion of a surrounding virtual world, the HMD system requires certain hardware components. The tracker measures the position and orientation of the user’s head and hand. The graphics engine gcncratcs the images seen by the user, which arc then displayed on the HMD. The manual input device allows the user to use gcslurcs of the hand to cause things to happen in the virtual world. BASIC ACTIONS An aclinn changes ~hc state of the virtual world or the user’s viewpoint wiLhin it under control of a gcsturc of the hand, as mcasurcd by the manual input d&cc. The hand gesture initiates and tcrminatcs the action, and the changing position Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the ACM copyright notice and the title of the publication and its date appear, and notice is given that copying is by permission of the Association for Computing Machinery. To copy otherwise, or to republish, requires a fee and/or specific permission. Q 1992 ACM 0-89791-471-6/92/000310189...$1.50 and orientation of the hand during the gesture is also used to control what happens as the action progresses. The manual input device may be a hand-held manipulandum with pushbuttons on it, or it may be an instrumented glove. In either case, the position and orientation of the input device must be measured by the tracker to enable manual control of actions. The input device must also allow the user to signal to the system to start and stop actions, and to select among alternative actions. Certain fundamental manually-controlled actions may be implemented for any virtual world. These actions involve changing the location, orientation or scale of either an object or a user, as shown in Table 1.