From: "PSOTKA, JOSEPH" Subject: PAPER: Virtual Egocenters as a Function of Display Field.... Date: 23 Oct 92 14:29:00 EDT Virtual Egocenters as a Function of Display Field of View and Viewing Station Point Joseph Psotka, Ph.D. U. S. Army Research Institute ATTN: PERI-ICC 5001 Eisenhower Avenue Alexandria, VA 22333-5600 (703)274-5540/5545/5569 Psotka@alexandria-emh2.army.mil or psotka@26.1.0.50 FAX: 274-5461 Abstract The location of one's virtual egocenter in a geometric space is of critical importance for immersion technologies. Fourteen Ss viewed an animated 3D model of the room in which they sat from either .3 or .8 meters. The display was on a 190 by 245 mm monitor. They saw four models of the room designed with four geometric field of view (FOVg) conditions of 18, 48, 86, and 140 degrees. They drew the apparent paths of the camera in the room on a bitmap of the room as seen from infinity above. Large differences in the paths of the camera were seen as a function of both FOVg and viewing station point. Ten Ss were then asked to find the position for each display that minimized camera motion. The results fit well with predictions from an equation that took the ratio of human FOV (roughly 180 degrees) to FOVg times the Projection Point (PP) of the image: Zero Station Point = (180/FOVg)*PP Introduction The location of one's virtual egocenter in a geometric space is of critical importance for immersion. Furness (1992) and Howlett (1990) report that immersion is only experienced when the field of view (FOV) is greater than 60 degrees, or at least in the 60 to 90 degrees of FOV range. Why this should be is not understood, nor are there theoretical frameworks for beginning to understand this phenomenon. As a start this research begins to explore how egocenters are determined from perceptual arrays. Some work exists that may be helpful to understand the psychology of egocenters (Howard, 1982; Ono, 1981). Kubovy (1986) provides an insightful description of the use of techniques by Renaissance artists to manipulate the location of virtual egocenters, and thus manipulate attitudes and emotions. A series of experiments by Ellis (McGreevy and Ellis, 1986; Nemire and Ellis,1991) may indirectly reflect on virtual egocenters. Ellis and McGreevy discovered a systematic error in pointing the direction of objects in a virtual display. The error was a function of the geometric FOV of the display. They developed a complex model that accurately predicted these errors on the basis of memory for the size and shape of objects and geometric distortion based on linear projections. The regular shape of the error (see Figure 1) led me to think that it could also be produced by an altered location of the virtual egocenter in the display such that for small FOV the observer located the virtual egocenter too near to the objects; and for wide FOV the observer located the virtual egocenter too far from the objects. Ellis and Nemire added some evidence for this hypothesis by demonstrating that the enhanced structure of a pitched optic array does bias the perception of gravity-referenced eye level. This finding is a direct replication of Kubovy's arguments about egocenters and Renaissance artists. These experiments are an extension of Ellis' work to confirm his findings and extend his interpretation of their source. ------------------------ Place Figure 1 About Here ------------------------ Stimuli An accurate model of an office was constructed using 3D Studio on a 386 PC with VGA graphics. The model contained walls, floor, and ceiling, three tables with computers and displays, two bookshelves with empty shelves, and two wastebaskets in the room. It was rendered with Phong shading and looked like a reasonable cartoon of the actual office holding the equipment (see Figure 2). ------------------------ Place Figure 2 About Here ------------------------ Animations of this model were then created showing a stationary camera located at the geometric center of the room panning slowly 360 degrees around the room. Four animations were created with four different lenses for the scene: 17, 28, 50, and 135 mm. The geometric field of view for each of these lenses was: 140, 86, 48, and 18 degrees, respectively, where 140 degrees is similar to a fish-eye lens and 18 degrees is a telephoto view. The animations were viewed on a flat screen Zenith monitor whose screen dimensions were 190 by 245 mm. Subjects viewed the animations from two locations 800 and 300 mm from the screen. At those sites the screen subtended a FOV of 17 and 45 degrees, approximately. Although their heads were not restrained mechanically, Ss held their positions reasonably well. The projection point of each of these lenses was 40, 140, 290, and 800 mm in the room. These projection points are independent of the viewerUs location. They are dependent on the actual size of the viewing screen. Thus the two viewing sites for the subjects corresponded approximately to the projection points for the lenses of 135 and 50 mm. Procedure Subjects were asked to view the animations and determine the location and path of the camera in each animation. They were told that the animation was of the very same room where they sat. They were shown a bitmap hardcopy of the room from an overhead view and asked to trace the path of the camera on it. They were not specifically told that the geometric "camera" was mathematically or "theoretically" stationary in the animations. Fourteen students and colleagues with a variety of psychological training served as experimental subjects without pay. Ten of these Ss were asked at the end of the experiment to select for each animation the viewing station that produced the least camera motion. Results In general, the subjects had no difficulty describing the apparent paths of the camera as they saw it as oval paths of varying eccentricity centered on the geometric center of the room. The diameters of the ovals varied with the focal length of the lens. The width of these ovals in room coordinate system for each animation a nd station point are given in Table 1. A positive number indicates that the camera view was across the center of the room; and a negative number indicates the camera was stationed closer to the scene than the center of the room. A zero would indicate the geometric center of the room. Table 1 Width of Camera Path as a function of FOVg and Station Point Geometric Field of View of Room Station Point 18 48 86 140 Group 1 - .3m -1082.5 -557.5 167.5 1825.0 Group 2 - .8m -1570.0 -155.0 832.5 1077.5 Both viewing sites yielded similar relationships between the diameters of the axes and the geometric FOV of the animations (see Figure 3), but the viewing site of 800 mm produced concave functions, whereas the viewing site of 300 mm produced convex functions. By interpolating these points, one can determine where Ss would have seen no camera motion. For the 800 mm view site, the paths had 0 diameter with 60 degree FOV or a projection point of approximately 250 mm. For the 300 mm view site, the paths had 0 diameter with 80 degree FOV or a projection point of approximately 150 mm. The mean locations for the station points with least camera motion were 9112, 1092, 291, and 53 mm from the monitor for the four geometric fields of view of 18, 48, 86, and 140 degrees, respectively, whose projection points were 800, 290, 140, and 40 mm. Discussion It appears that the egocentric station point is affected by the geometric FOV of the displayed image; the relationship between the viewing site and the geometric projection point, and the actual FOV of the image. The location of the egocenter is NOT experienced as the same as the geometric station point of the camera under any of the conditions of these experiments. It appears that the least egocenter motion was produced in these experiments with a FOV that varied between 48 and 86 degrees, curiously close to the required limits in order to experience satisfactory immersion (Furness, 1992). However, this appears to be an accident of the stimulus conditions in this experiment. egocenter motion was almost completely nullified for the 17 mm lens ( and 140 FOV) at a viewing site of 50 mm ; and for the 28 mm lens (and 86 FOV) at a viewing site of 290 mm. The other two animations did not appear to have a station point that yielded 0 camera path; although the station points selected by subjects did appear to reduce the absolute value of the camera path substantially. This finding needs to be explored further. It may be related to the finding that immersion is not satisfactory with displays that are less than 40 degreees because no satisfactory compromise exists between the conflicting cues of linear perspective and the visual systemUs need for a visual field of 180 degreees to find a stationary egocenter. Ss repeatedly remarked that they appeared to be using the frame of the monitor as the frame of reference of their retinal field. When asked to describe what was happening, they said they appeared to be contracting their field of attention to the frame of the monitor, and then treating that as if ti were their entire 180 degree visual field. If they were in fact doing this at a processing level, then the projection point of the animation would not be determined by the size of the monitor, but by the virtual size of their expanded attentional field, roughly 180 degrees. The projection point would then be expanded by a similar ratio, yielding the enlarged path of the camera with smaller FOVg. In fact, if one proposed that the zero station point is determined by the product of the animationUs projection point (PP) times the ratio of 180/FOVg, one could calculate the predicted station points for zero camera motion. Zero Station Point = (180/FOVg)*PP For this experiment these predictions are: 8000, 1100, 287, and 50 mm. quite close to the empirical values of : 9112, 1092, 291, and 53 mm. This seems to indicate that when the FOVg is 180 degrees, the egocenter is located correctly, but when the FOVg is less than 180 degrees, the egocenter is displaced proportionately. What might happen with a field of view greater than 180 degrees? Clearly much work remains to be done if we wish to specify exactly how people interpret constructed geometric displays to select their egocentric viewing spot. Yet this work is very necessary if we wish to be able to create three-dimensional models that have the power to generate a truly satisfying and natural immersion experience. For psychological theory, this research opens the possibility of dealing quantitatively with very abstract constructs, like virtual egocenters, in ways that were either impossible or very difficult without the new VR technologies. Clearly parametric studies need to be carried out in detail to create a nomograph of functions relating egocenter to FOVg and viewing station points. This pilot work suggests that even very close viewing station points such as those with head mounted displays (HMDs) are not immune to illusions caused by FOV that are smaller than 180 degrees. Their possible implication in more severe phenomena like simulator sickness, or less severe discomfort and dislike of HMDs is only one further direction that needs exploration. It is clear, for instance, that these sorts of egocenter illusions adapt out very quickly in a VR environment. However, after adaptation is more or less complete, are there still physiological conflicts that can be detected in response to the conflicting cues of linear perspective and reduced FOV? Are there aftereffects that return to the real visual world? Other, broader theoretical issues that need exploration are higher order cognitive implications of these new relations between multiple realities. When we view the animation apparently rotating on the monitor, somehow we build up a model of the room. That model is also somehow projected into the same space as the real room that we occupy. While viewing the animation, we have both an egocenter in real space, and a virtual egocenter in the space of the animation. It appears from these experiments that those egocenters interact with each other so that we feel some conflict as we rotate and move in one and remain stationary in the other. What are the long term effects of this conflict? For instance, if parts of the visual field, or even half or more of it were blocked out and replaced with active noise, would observers begin experiencing something like lateral neglect? What would happen if we decorrelated color patches from objects? We know for instance, that color is processed in separate pathways from form (Livingstone and Hubel, 1987). Using VR technologies, could these separate pathways be made explicit and what would its effects be? What are the memory implications for conflicts between one reality and another? What are the physiological processing correlates of immersion? These are only some of the interesting psychological questions that need a firm base of experimental data to rest the initial creation of exploratory theoretical frameworks. References Ellis, S. R. (Ed.), (1991). Pictorial Communication in Virtual and Real Environments. London: Taylor and Francis. Furness, T. (1992) Personal communication. Howard, I. P. (1982). Human Visual Orientation. New York: Wiley. Howlet, E. M. (1990). Wide angle orthostereo. In Merritt, J. O. and Fisher, S. S. (Eds.) Stereoscopic displays and Applications. Bellingham, WA: The International Society for Optical Engineering. Kubovy, M. (1986) The psychology of perspective and Renaissance art. Cambridge: Cambridge University Press. Livingstone, M. s. and Hubel, D. H. (1987). Psychophysical evidence for separate channels for the perception of form, color, movement, and depth. Journal of Neuroscience, 7, 3416 - 3468. McGreevy, M. W. and Ellis, S. R. (1986). The effect of perspective geometry on judged direction in spatial information instruments. Human Factors, 28, 439 - 456. Nemire, and Ellis, S. R. (1991) Optic bias of perceived eye level depends on structure of the pitched optic array. Presented at the Psychonomic Society, San Francisco, CA. Ono, H. (1981). On Well's (1792) law of visual direction. Perception and Psychophysics, 30, 403-406.