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- 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.
-
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