From: Tony Healy Subject: Re: Definitions for Virtual Reality Systems Date: January 2, 1995 From: Tony Healy Hi John, thanks for your comments and I apologise for being a bit confusing. Active vs Passive Worlds ------------------------ What I had in mind is that we attribute activity separately to both the world and to the way we view the world. Perhaps we could use different words to describe the states of the worlds, such as changing and still. * Changing World - (eg Model of a bear growling around the woods) things change in the world without input from us, and under the control of either internal animation logic or the logic of actual people and things. * Still World - (eg Model of a building) things do not change without input from us. Our ability to modify a world depends on the way we're viewing the world, not on whether the world is changing or still. If we are viewing the world using Active Presence Emulation, then we can do things in and to the world. If we are viewing by Passive Presence Emulation, then we have not been given the ability by the system to do anything: we can only sit back and watch, as in film. Snapshot World -------------- A Snapshot World is a fixed representation of a world, captured at some point in time, or over some period of time, and can't be modified by the user. Since a snapshot world, such as the world shown in a movie, will usually change over time, it is different from a Still Virtual World, which just sits there for all time. Similarly, the world in a precomputed animation will usually change over time. The important distinction is that we view movies and precomputed animations passively, and thus cannot change the worlds we are watching, even though they undergo their own changes while we're watching. Rotating Head Presence Emulation -------------------------------- >>Presence Emulation where we are able to rotate our head and see appropriate >>imagery, but not to move freely and look in any direction. While systems >>that implement Rotating Head Presence Emulation typically allow presence at >>more than one point, these points are pre-selected and relatively few, and >>thus do not meet the criterion for Active Presence Emulation. Rotating Head >>Presence Emulation characterises Apple's Quicktime VR. >I don't think that this is a new type. It is a method for providing passive >presence emulation but is not qualitatively different from other methods. Quicktime VR poses a tricky challenge to a set of definitions such as this. Quicktime VR creates the illusion of a world by using sets of 2D images or photographs representing the views in different directions from a specified point. This gives photorealistic imagery, but means worlds can be viewed from only specified points. The user is not free to move freely, and usually cannot tilt his or head up and down either. Viewing is severely constrained. Quicktime VR does not meet the criteria for Active Presence Emulation, but it does allow for some interaction by the user, in that we can rotate our head and see appropriate imagery. Thus it doesn't fall into the category of Passive Presence Emulation either. After thinking about this for a bit, it seemed to me that the essential characteristic of Quicktime VR is that it accommodates head rotation well, but nothing else. Thus the new viewing type. Tony Healy Silicon CHiC ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Tony Healy Silicon CHiC Online Interactives Phone (61)2 746 1054 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~