Super Wide Field of View Head Mounted Display Using

Home , Field of view

Hajime Nagahara* Super Wide Field of View Graduate School of Engineering Science Head Mounted Display Osaka University Using Catadioptrical Optics Osaka, Japan

Yasushi Yagi The Institute of Scientific and Industrial Research Osaka University Abstract Osaka, Japan Many virtual reality, mixed reality, and telepresence applications use head mounted Masahiko Yachida displays (HMD). HMD systems are portable and can display stereoscopic images. Graduate School of Engineering However, the field of view (FOV) of commercial HMD systems is too narrow for Science conveying the feeling of immersion. The horizontal FOV is typically around 60°, Osaka University significantly narrower than that of the human eye. In this paper, we propose new Osaka, Japan display optics for a super wide FOV head mounted display. The proposed optics consists of an ellipsoidal and a hyperboloidal mirror that will display distortionless images by using the characteristics of the mirrors, even if the image has a large FOV. We constructed a prototype HMD system with a 180° horizontal ϫ 60° vertical FOV that includes the peripheral vision of the human eye. The FOV has a 60° ϫ 60° overlap area that can display stereoscopic images. We estimated the resolution, focus, and aberration of the prototype in an optical simulation and experimentally confirmed that the prototype displays distortionless wide FOV images.

1 Introduction

Many virtual reality, mixed reality, and telepresence applications use head mounted displays (HMD). HMD systems are portable and can display stereoscopic images. However, the field of view (FOV) of commercial HMD systems is too narrow for conveying a feeling of immersion. The horizontal FOV of many such systems is around 60°, significantly narrower than that of the human eye. On the other hand, immersive projection displays (IPD) such as CAVE (Neira, Sandin, & DeFanti, 1993) usually have been used for applications that display large FOV images surrounding users. Moreover, the IPD is not easy to use, because of its high cost and its huge size. Hence, an inexpen- sive and compact HMD with a wide FOV is being sought for such applications. In this paper, we describe a super wide field of view head mounted display consisting of an ellipsoidal and a hyperboloidal curved mirror. The horizontal FOV of the prototype HMD we constructed is 180° and so it includes human peripheral vision. It is well known that the lack of peripheral vision seriously influences postural control in humans (Dickinson & Leonard, 1967). Furthermore, Furness (1990) reported that a FOV over 80° was required for a feeling of immersion.

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Takahashi et al. (Takahashi, Arai, & Yamamoto, 1998) 140° FOV prototype HMD system using four liquid also measured the influence of a wide FOV on human crystal display (LCD) panels and fresnel lenses. The attitude control. They concluded that an HMD with a FOV was stitched over the panels and the displayed im- wide FOV, such as 140°, was better than a typical ages had seams. Inami et al. (Inami, Kawakami, Maeda, HMD. Caldwell et al. (Caldwell, Reddy, Kocak, & Yanagida, & Tachi, 1997) developed a prototype HMD Wardle, 1996) reported that a narrow FOV hindered that extended the horizontal FOV to 110° monocular task efficiency and some of the feeling of reality in tele- by using Maxwellian optics. The prototype realized a operations, even though they compared only 30° and large FOV with simple optics, but the size of the ob- 60° FOVs. These studies indicate that a wide FOV served pupil was extremely small. These examples indi- along with peripheral vision are important factors for cate that while there are a number of wide FOV HMDs, increasing the feeling of immersion. The sense of reality their FOVs are still smaller than that of the human a user gets from viewing a display is affected by factors vision system. such as image resolution, dynamic range, field of view, In this paper, we propose new HMD optics to realize and whether the view is stereoscopic (Bolas, 1994). We a super wide FOV. The optics use catadioptrical optics focus on the sense of immersion and vection (visually consisting of a hyperboloidal convex and an ellipsoidal induced perception of self motion) given by human pe- concave mirror. They achieve a distortionless wide FOV ripheral vision, and define these qualities as the sense of by using simple optics. Several previous HMD have also reality in this paper. used catadioptrical optics that have reflective devices In previous studies, various manufacturers and re- and refractive devices. However, none has exploited hy- searchers have developed HMDs (Robinett & Rolland, perboloidal and ellipsoidal characteristics. We con- 1992). Some of them have wide FOVs, such as for a structed a prototype HMD unit that for each eye covers flight simulator. Eyephone02 (VPL Inc.) has an 80° a 120° horizontal view and a 60° vertical view. Thus, a horizontal FOV. Gemini-Eye 3 (CAE Inc.) increased 180° horizontal FOV is achieved for both eyes and a the horizontal FOV to 100°. Sim Eye XL100A (Kaiser stereopsis view can be achieved within a 60° horizontal Electro-Optics Inc.) also has a horizontal FOV of 100°. and 60° vertical FOV. The prototype HMD covers the Datavisor 80 (n-Vision Inc.) has a 120° FOV. Most whole human FOV including peripheral vision. In the commercial HMD systems treat their optics simply as a next sections, we describe the concept of our HMD and magnifier and use eyepiece-based optics such as LEEP the omnidirectional video-based virtual reality system of optics (Howlett, 1983, 1992). As the FOV increases in our prototype HMD. We also describe simulation and such systems, the barrel-like distortion introduced by experimental results confirming the characteristics of the the optics is also enlarged. This distortion has usually prototype HMD. been ignored. Moreover, eyepiece-based optics have a trade-off between the FOV and eye relief (clearance between eye and lens). Hence, the eyepiece lenses have 2 Proposed HMD Optics restricted the FOV. Some HMDs were made with unique, possibly better, designs. Fiber-Optic HMD Figure 1 shows the proposed HMD optics consist- (CAE Inc.; Welch & LaRussa, 1998) has a 120° FOV. ing of planar, hyperboloidal, and ellipsoidal mirrors, a HMD 120/40 (SEOS Inc.; Coates & Huxford, 2004) lens, and an LCD. The lens is aligned on the focus of that uses mirrors has a 120° FOV. However, these de- the hyperboloidal mirror. The planar mirror between vices are used for shrinking the size of optics and not for the lens and the hyperboloidal mirror inclines the rays focusing or magnifying rays. Therefore, the problem of to avoid interference with the observer’s face. The axis conventional eyepiece-based magnifiers has remained an of the ellipsoidal mirror is inclined to avoid the hyper- issue with these optics. boloidal mirror obstructing the FOV of the observer’s Takahashi, Arai, and Yamamoto (1998) constructed a eye. The proposed optics does not require an eyepiece

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Figure 1. Optics of the proposed catadioptrical unit.

lens system, so the FOV is not limited by the lens size of the eyepiece lens system. Using this property, the prototype HMD can display an FOV virtual image of about 120°h ϫ 60°v to the observer. Figure 2 shows the hyperboloidal, ellipsoidal mirrors and their geometric relation. In this ﬁgure, bold lines indicate the hyperboloidal and ellipsoidal parts of the HMD optics. The hyperboloid and the ellipsoid are de- ﬁned by Equations (1–2), and each has two focal points, Ϫ Fhϩ(0, 0, ch), FhϪ(0, 0, ch) and Feϩ(0, 0, ce), Ϫ e e e T FeϪ(0, 0, ce). The ellipsoidal coordinate [ x, y, z] is rotated and shifted from the hyperboloidal coordinate [x, y, z]T as described in Equation (3). M is the rotation

and translation matrix. The focal point Fhϩ of the hyperboloidal mirror is aligned with the focal point Feϩ of Figure 2. Characteristics of (a) hyperboloid and (b) ellipsoid. the ellipsoidal mirror.

x2 ϩ y2 z2 Ϫ ϭϪ 2 2 1 (1) ah bh shown in Figure 2b. This relation is the same as Snell’s e ˜ e ˜ reflective law. Here, V1 and V2 are defined as ray vec- e 2 e 2 e 2 z e ˜ e ˜ x ϩ y tors along these lines, and V1 is reflected to V2 as de- ϩ b2 ϭ 2 e 1 (2) scribed in Equations 5 and 6, where ␭ in Equation (5) ae 1 is derived from Equations (2) and (5). where e ϭ V˜ 1 MV˜ 1 (4) 2 ϩ 2 ϭ 2 2 Ϫ 2 ϭ 2 ah bh ch, be ce ae e ϭ ␭ e ϩ e Pe 1 V˜ a FeϪ (5) ͓ex,ey,ez,1͔T ϭ M ͓x,y,z,1͔T (3) e ˜ ϭ e Ϫ e e V2 Feϩ Pe (6) The normal vector at an arbitrary point Pe on the

ellipsoid bisects the angle between lines passing through Because the focal point Fhϩ of the hyperboloidal mir- e e e Pe and Feϩ or Fe Ϫ in the ellipsoidal coordinates as ror is set on the focal point Feϩ of the ellipsoidal mirror,

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e ˜ e ˜ ˜ ˜ V1 and V2 are expressed by V1 and V2, respectively, as described in Equations (4–7).

The normal vector at an arbitrary point Ph on the hyperboloid divides the angle between lines passing

through Ph and Fhϩ or FhϪ as shown in Figure 2a. ˜ ˜ Here, V2 and V3 are deﬁned as ray vectors along these ˜ ˜ lines and V2 is reﬂected to V3 as described in Equations ␭ (8) and (9), where 2 in Equation (8) is derived from Equations (1) and (8).

ϭ Ϫ1e V˜ 2 M V˜ 2 (7)

ϭ ␭ ϩ Ph 2V˜ 2 Fhϩ (8)

ϭ Ϫ Figure 3. Prototype optical unit. (a) side view; (b) front view. V˜ 3 FhϪ Ph (9)

ϭ pi AV˜ 3 (10) Table 1. Mirror Parameters of the Prototype HMD

A projection lens is set on the focal point F Ϫ of the h Hyperboloidal mirror a 21.21 /[mm] hyperboloidal mirror. h bh 30.00 /[mm] The observer’s eye is set on the focal point F Ϫ of the e Ellipsoidal mirror a 42.43 /[mm] ellipsoidal mirror, where s/he can see the image from e b 45.00 /[mm] ˜ e the LCD. We can derive V3 from an arbitrary ray vector ˜ ˜ V1. The point on the LCD plane pi is projected from V3 as described in Equation (10) where A is the 2D projec-

tion matrix of the LCD plane and lens set on FhϪ. Therefore, we can completely compensate for the opti- optical unit is 200 g. Figure 4 shows a photograph of ˜ cal distortion by using the relation between pi and V1 as the prototype HMD, and Table 2 shows the speciﬁca- described in Equations (4–10). In summary it is these tions. Figure 5 shows the layout of the HMD optical hyperboloidal and ellipsoidal characteristics by which we units and the relationship of their FOVs. The units are can easily spread and gather rays with wide angles and rotated about the vertical axis by 60° with respect to display a distortionless wide ﬁeld of view images. each other (Figure 5). Each optical unit has 120° horizontal and 60° vertical FOV. Therefore, the HMD can cover a 180° horizontal ϫ 60° vertical FOV including a 3 Prototype HMD System 60° overlap area that gives a stereo capability. Usually, pupil position differs between individuals. Therefore, Figure 3 shows the prototype mirror unit. We each optical unit has an adjustment mechanism to carefully decided the parameters of the hyperboloidal match the pupil position of each observer. Each adjust- and ellipsoidal mirrors and lens of the prototype HMD ment mechanism is capable of three 19-mm adjustable by using commercial optical simulation software, ZE- linear movements. The movements are orthogonal, so MAX (Focal software Inc.). Table 1 shows the hyperbo- the optical unit is adjustable with three degrees of free- loidal and ellipsoidal mirror parameters for the proto- dom. The LCD module is a 0.5 in. device with 1.44 type HMD optical unit. The mirrors are made of million pixels that can project a 800 ϫ 600 pixel color aluminum, and they were fabricated with a computer image. The LCD and backlight modules are compo- numerical control (CNC) machine. The weight of the nents of a commercial HMD (Sony: Grasstron PLM-

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Figure 6. Experimental system.

Figure 4. Prototype HMD. balance weight is 3,750 g. The prototype HMD is not comfortable because of its heaviness. However it should be easy to make the optics lighter. For example, we Table 2. Specifications of Prototype HMD could use plastic mirrors. Such a modification will be Image resolution 800 ϫ 600 [pixels] addressed in the future. Binocular FOV 180° ϫ 60° We evaluated the HMD by showing subjects a re- Overlapped FOV 60° ϫ 60° corded video. Figure 6 shows the experimental video Total weight 3,750 [g] telepresence consisting of the HMD, an omnidirectional image sensor (Yamazawa, Yagi, & Yachida, 1993), and a graphic workstation with a SCSI160 hard disk unit. We captured the input video with the omnidirectional image sensor and recorded it on the hard disk (1296 ϫ 1026 pixels, 15 Hz). Figure 7 shows one of the omnidirectional input images. The workstation (Octain 2: SGI) transformed the input image into LCD images (800 ϫ 600 pixels for each display screen). Figure 8 shows the transformed LCD images. The lines in the figure indicate longitude and latitude in spherical coordinates. Note that the transformed LCD images are horizontally reversed due to the reflection in the mirrors and are deformed to compensate for the distortion induced by the HMD Figure 5. Binocular alignment and horizontal FOV. optics. Figure 9 shows the displayed image. The FOV is much wider than that of a typical HMD (Figure 10). The image presents a view including peripheral vision. S700). A magnetic motion tracker, The Flock of Birds The HMD displays a spherical image around a user. (Ascension Inc.), is attached to the top of the helmet to Note that Figure 9 is a Mercator projection, because a detect the observer’s head motion. The total weight of spherical image cannot be printed on a flat page. The the prototype including helmet, mount, and counter actual image of the HMD does not show the distortion

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Figure 10. Displayed image of typical HMD (60°h ϫ 40°v FOV).

their system used a commercial HMD with a normal FOV (about 60° horizontal FOV).

4 Simulation Figure 7. Omnidirectional image. We evaluated the prototype HMD’s resolution, intensity distribution, aberration, and virtual image position in simulations. Figure 11 shows the angular resolution. The resolution distributions were calculated using the specific pro- jective relations described in Section 2. The longitudinal resolution decreases as the depression angle increases. The resolution distribution is suitable for spherical image projection. The longitudinal and latitudinal resolutions are of the same angular specification as in the omnidirectional image sensor. Thus, the prototype HMD Figure 8. Binocular transformed image (180°h ϫ 60°v FOV). does not lose visual information as a result of the omnidirectional input image being transformed into the LCD image. Figure 12 plots the relative intensity distribution of the displayed image against latitude. The image intensity decreases with decreasing latitude. The HMD optics magnify the pixels of the LCD. The pixel magnification ratios correspond to the displayed image resolution as shown in Figure 11. Pixels on low-resolution parts of the image are more enlarged than pixels on high-resolution Figure 9. Displayed image of the prototype HMD (180°h ϫ 60°v FOV). parts. The magnification attenuates the displayed pixel intensity. Therefore, the intensity is similar to the resolution attribute. The intensity changes according to latitude (Figure 12). However, users said that they did not present in the figure. The system updates the image at care about this phenomenon when they wore the proto- 25 Hz for head motion and at 10 Hz for changes in the type HMD, because the change was continuous. environment. Onoe, Yamazawa, Takemura, and Yokoya There are aberrations that directly reduce image (1998) also constructed an analogous system. However, sharpness: spherical aberration, astigmatism, and coma.

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Figure 13. Spot radius vs. distance from observer’s pupil.

We estimated these aberrations by calculating the spot radius for each ray direction. Figure 13 shows a plot of the RMS (root mean square) spot radius against the distance from the observer’s pupil. In this ﬁgure, the spot size is indicated as a solid angle from the pupil. The spot size varied with distance. If the spot size is smaller than the apparent pixel size as shown in Figure 11, the Figure 11. Resolution of projected image on the HMD. (a) user would perceive that the displayed image is focused horizontal resolution attribute; (b) vertical resolution attribute. and a virtual image exists in the distance. We call this distance the virtual image position. The virtual image position is an important factor for stereoscopic displays such as HMDs. The eyes converge on a nearby object by rotating inward and the lenses of the eyes simulta- neously accommodate to the distance of the object to bring it into focus. Hence, we have to consider the difference between the convergence displayed by the image disparity and the accommodation displayed by the virtual image distance. Usually, for the difference to be ignorable the virtual image distance of commercially available HMD systems is set to more than 2 m. Figure 14 shows a plot of the virtual image position against latitude. The gray area indicates the range of distances in which the image is in focus. This ﬁgure shows that a Figure 12. Relative intensity distribution. virtual image is more than 2 m away from the user’s pupil position and that the displayed image is focused over the whole latitude of FOV. It also shows that the optics have a wide depth of focus that would bridge the

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Figure 16. Test charts for conﬁrmation of image quality.

the wavelength of the ray when the virtual image is 2 m from the observer’s pupil. The focal shift is small in comparison with the image distance. This means that Figure 14. Virtual image position vs. latitude. the chromatic aberration can be ignored in the prototype HMD optics. Basically, refraction devices such as lenses cause chromatic aberration, whereas reﬂection devices such as mirrors do not. The prototype HMD optics mainly use mirror devices. Therefore, the HMD has the advantage of decreased chromatic aberration.

5 Experiments

We conducted subjective experiments that evaluated the displayed image quality (focus and resolution) of the prototype HMD. The subjects were males in their twenties who had normal or corrected to normal vision. To evaluate the resolution, we showed the sub- Figure 15. Focal shift due to chromatic aberration. jects test charts. The subjects were asked to identify the size of chart they could recognize. We used three types of test charts, vertical, horizontal, and Landolt circles, as shown in Figure 16, and put on the chart in each direc- difference between convergence and accommodation. tion of view area. Figure 17 shows sample test charts in The horizontal and vertical FOV properties are the the LCD images. Five subjects participated in this ex- same. Therefore, we confirmed that the aberrations and periment in which the sizes of the chart corresponded the focus adaptation are not problems for the prototype to angular resolutions of 1.25, 2.5, 5.0, and 10 pixel/ HMD. deg. Figure 18a–c shows the average resolution for each Chromatic aberrations are phenomena whereby the chart as assessed by the five subjects. Figure 18d shows angle of refraction differs depending the wavelength. the ideal resolution attribute according to the LCD res- They appear as cyan/green and red fringing in display olution. The results include multiple effects of the reso- optics, because the lens does not focus different wave- lution attributes shown in Figure 18d and the optical lengths onto the exact same focal plane or position. Fig- focus. Figure 18 shows that the center of the view area ure 15 shows the simulated focal shift caused by chro- was well focused. On the other hand, the resolution of matic aberrations in the prototype HMD. This figure the peripheral area was not high because of optical blur, indicates that the focal position changes according to and this was especially evident for the region over 60° in

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gnetting is the optical phenomenon in which the displayed image drops out when the observer’s eye rotation is large. When the observer’s pupils move as the eyes rotate, the pupil obstructs the rays from the HMD. The position of the observer’s eye is also critical because the observational pupil is small on the prototype HMD. Therefore, we estimated the applicable area without vignetting by eye rotation. We employed ten subjects in this experiment. Table 4 shows the average and stan- dard deviation of eye rotation angle without vignetting, showing that the applicable angle without vignetting is about 30° in both the vertical and horizontal directions. We also estimated the influence of vignetting caused by the small observational pupil. In this experiment, we assumed a telepresence application and showed a movie recorded from a moving car. The prototype telepresence Figure 17. Sample of test chart image on LCD. system can display either constant view mode, that skins the same view regardless of head view direction, or dynamic view with that accomodates head motion by Figure 18b. However, as human peripheral vision is in- changing the view according to head view direction. sensitive to resolution, we think that the image quality is Table 5 shows the number of subjects out of ten who sufficiently good for this type of display. observed the vignetting phenomenon under uncon- The prototype HMD has a 60°h ϫ 60°v overlap area, scious eye motion. Over half of the subjects felt vignett- where the images can have a disparity to create the ste- ing in the constant view mode. However, when we en- reo effect. We estimated the stereo capability of the pro- abled the dynamic view mode, nine subjects were not totype to display stereo disparity images on the overlap concerned about the problem. In the constant view area. We showed subjects our own purpose built dispar- mode, the user’s eyes tracked the objects. In the dy- ity images showing a moving stick between two station- namic view mode, the users tracked objects by moving ary sticks (Figure 19). We set the stationary sticks 500 their heads. Thus, eye motion was decreased and the mm away from the user and moved one stick Ϯ200 mm vignetting problem was moderated. In this way, we back and forth in relation to the stationary ones. We showed that the current prototype HMD would be ap- measured the position when the subject thought the plicable for uses involving unconscious eye motion. Im- moving stick was at the same depth as the stationary proved optics to overcome the vignetting problem will sticks, and defined the position difference as the depth be addressed in future work. Next, we compared a wide perception error. Table 3 shows the average and stan- FOV (180°h ϫ 60°v) and a narrow FOV (60°h ϫ 40°v) dard deviation of depth perception error for ten sub- by giving subjects a questionnaire. Figure 10 shows the jects. The result shows the position error is about 20 narrow binocular FOV images. The narrow FOV images mm. On the other hand, the overlap region resolution is were displayed with the prototype wide FOV HMD in about 7 pixel/deg as shown in Figure 11. This resolu- order to compare only the difference of FOVs without tion corresponds to the disparity caused by a 20 mm other artifacts such as resolution and aberration. We depth difference. Therefore, the center area was suffi- used images of a moving car. Table 6 shows the ques- ciently focused and the disparity was correctly displayed tionnaire results. All subjects answered that the wide as distortionless images to the limit of LCD resolution. FOV improved immersion and vection. The prototype HMD has a vignetting problem. Vi- We have confirmed experimentally that the prototype

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Figure 18. Results of image quality test using the test, charts (a) vertical chart; (b) horizontal chart; (c) Landole chart; (d) LCD resolution limit.

Table 3. Estimation of Depth Perception by Displaying Stereo Images

Average [mm] SD [mm]

22.35 16.149

Table 4. Estimation of Eye Movement Limit Without Vignetting

Average [deg] SD [deg]

Horizontal Ϯ 16.6 5.13 Vertical Ϯ 15.5 4.33

Figure 19. Depth perception test. 6 Conclusions

We have developed new HMD optics giving a super wide FOV. The optics used a catadioptric composed HMD could display 180°h ϫ 60°v wide FOV images. It of hyperboloidal and ellipsoidal mirrors. The prototype appears that our wide FOV HMD would be effective for HMD was evaluated in simulations and experiments, virtual reality and robotics applications. A detailed eval- and it was conﬁrmed that the wide FOV displayed by uation of human factors involved with wide FOVs will the HMD has practical advantages. The prototype can be presented in a future work. display a 180° horizontal view and a 60° stereo view.

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Table 5. Influence of Vignetting presence controlled robots. Proceedings of the IEEE Interna- tional Conference on Robotics and Automation, 1375–1380. Accommodation Occurrence of Coates, N. R., & Huxford, R. B. (2004). Image display appa- with head motion vignetting ratus. U.S. Patent Application US2004/0001183-A1. Dickinson, J. D., & Leonard, J. A. (1967). The role of periph- Off 6/10 eral vision in static balancing. Ergonomics, 10, 421–429. On 1/10 Furness, T. A. (1990). Creating better virtual worlds. Proceed- ings of the Human-Machine Interfaces for Teleoperators and Virtual Environments, 48–51. Table 6. Comparison of Wide and Narrow FOV Howlett, E. M. (1983). Wide angle color photography method and system. U.S. Patent 4,406,532. Subjects who were more Howlett, E. M. (1992). High-resolution insets in wide-angle influenced by wide FOV head-mounted stereoscopic displays. Proceedings of SPIE Stereo Display and Applications III, 193–293. Extension of FOV 10/10 Inami, M., Kawakami, N., Maeda, T., Yanagida, Y., & Tachi, Immersion 10/10 S. (1997). A stereoscopic display with large field of view Vection 10/10 using Maxwellian optics. Proceedings of the International Conference on Artificial Reality and Tele-Existence, 71–76. Neira, C. C., Sandin, D. J., & DeFanti, T. A. (1993). A room with a view: Surround-screen projection-based virtual real- The optical simulation estimated the resolution, focus, ity. Proceedings of the ACM SIGGRAPH’93, 135–142. and aberration of the prototype, and the experiment Onoe, Y., Yamazawa, K., Takemura, H., & Yokoya, N. confirmed that the prototype HMD displays distortion- (1998). Telepresence by real-time view-dependent image less wide-FOV images. generation from omnidirectional video stream. Computer The current prototype system is too heavy, because its Vision and Image Understanding, 71(2) 154–165. mirrors and adjusters are made of aluminum. However, Robinett, W., & Rolland, J. P. (1992). A computational the optics are easy to make lighter, for example, by us- model for the stereoscopic optics of a head-mounted dis- ing plastic mirrors in a future work. We are also plan- play. Presence, 1(1) 45–62. ning to evaluate human factors affected by wide FOVs. Takahashi, M., Arai, K., & Yamamoto, K. (1998). Wide field of view using a 4LCD HMD is effective for postural control. Second International Conference on Psychophysiology in References Ergonomics. Welch, B. L., & LaRussa, J. A. (1998). Fiber optic coupled Bolas, M. T. (1994). Human factors in the design of an im- helmet mounted display system. U.S. Patent 4,743,200. mersive display. IEEE Computer Graphics and Applications, Yamazawa, K., Yagi, Y., & Yachida, M. (1993). New real-time 55–59. omnidirectional image sensor with hyperboloidal mirror. Caldwell, D. G., Reddy, K., Kocak, O., & Wardle, A. (1996). Proceedings of the 8th Scandinavian Conference Image Pro- Sensory requirements and performance assessment of tele- cessing, 1381–1387.

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