Self Bird's Eye View with Omnidirectional Camera on HMD
Kenji Funahashi *Naoki Sumida

Shinji Mizuno

Nagoya Institute ofNagoya Institute of

Aichi Institute ofTechnology, Japan

Technology, Japan

Technology, Japan

Figure 1: (left) System configuration, (center) Subjective view, (right) Bird's eye view.

ABSTRACT

2 MAPPING TO BIRD'S-EYE DOME

The experience to see oneself from behind in real-time, that is like
out-of-body experience, is refreshing, it is also expected as a trigger
to improve oneself. However selfie-stick, drone, or something isnecessary to take a bird's-eye view. We propose the method to makevirtual self bird's-eye view using an omnidirectional camera on anHMD.

An object position is point P, that polar coordinate is (r, q, f), on a
subjective coordinate O (Fig. 4). This 𝑃 is projected to P’ on a bird's
eye coordinate O’. The radius of bird's eye dome is the same as a
subjective one, and vector OO’ is defined as (qx, qy, qz). The
projected point P’ is shown as:

Keywords: Bird's eye view, omnidirectional camera, HMD.
Index Terms: Human-centered computing—Human computerinteraction (HCI)—Interaction paradigms—Virtual reality
1 INTRODUCTION
Head Mounted Display (HMD) has become increasingly popular inrecent years. Many systems display a first-person view through an
HMD, instead of third-person view which is like bird's eye view.
The experience to see oneself from behind in real time is refreshing,
it is also expected as a trigger to improve oneself. There areresearches about survey view in the sport psychology field [1].
However a selfie stick, a drone, or something is necessary to takethe bird's eye view. In this paper, we propose the method to makevirtual bird's eye views of oneself by using an omnidirectional
camera on an HMD (Fig. 1 (left)). Objects a and b are projected toa' and b' on a dome screen respectively (Fig. 2). Here, the distanceto talk with a friend can be assumed as, for example, about one
meter. There are of course other objects that are not one meter fromthe subject, but these, especially distant objects are not payedattention. Our method is based on the assumption that objects areon a subjective dome screen (Fig. 3), and it means the distance isequal to the radius of the dome. Then we make a bird's eye
omnidirectional image (Fig. 1 (right)) from the subjective
omnidirectional camera image on an HMD (Fig. 1 (center)). The
subject's back image is simply drawn using Computer Graphics.

Figure 2: Dome screen and objects.

* kenji@nitech.ac.jp

Figure 3: Two Dome screens and objects in 2D.

Fig. 5 shows a mapping example from a subjective dome to a bird'seye one. The upper illustrations are subjective dome observed fromthe outside below, point a, b, and c are on a front hemisphere. The
vertical and horizontal lines are longitude and latitude lines drawn
at regular intervals. The lower illustrations are bird's eye dome, theline just beside of the subjective dome is drawn ahead on the bird'seye dome.

Figure 6: System overview.

3 EXPERIMENT
We used Oculus Rift as an HMD and Richo Theta as anomnidirectional camera to develop our self bird's eye viewexperience system (Fig. 6). The position and direction wereobtained only through one sensor of an HMD. When the direction
was changed from the initial position (Fig. 7 (left)), system judgedthat the subject changed it keeping the relation between the head

Figure 7: Top view of Head and body motions.

Figure 4: Two Dome screens and objects in 3D.

and the body (Fig. 7 (center)). Three types of parameters weretested; dome radius (assumed object distance) and the offsets to the
bird's eye view point are; (1) 1m, (-0.5m, 0m, 0.5m), (2) 3m, (-0.5m,0m, 0.5m), (3) 3m, (-1.5m, 0m, 1.5m). Fifteen subjects rated eachtrial on a scale of 1 to 3 points, the average scores were (1) 31, (2)15, and (3) 34 respectively. This result suggests the ratio between aradius and view point offset is more important than the absolutevalues. We also obtained a positive impression, especially throughthe type (3) and (1).
4CONCLUSION

In this paper, we have proposed a virtual self bird's eye viewexperience system with binocular parallax that is relatively
appropriate. It needs only an omnidirectional camera on an HMD
without a selfie stick. Although you can see yourself from behindas bird's eye view, the image is slightly distorted because the UV
map is made manually by using the 3D Computer Graphics tool,
Blender. In the future, the UV map should be calculated
appropriately. The head motion and body one also should be
separated by using two motion sensors (Fig. 7 (right)). It is
desirable to reflect the actual subject's back image.
REFERENCES[1]Figure 5: Dome mapping example.

N. Fujii, H. Nakamoto, S. Ikudome, S. Unenaka, and S. Mori, “The
Relationship Between Survey View and Mental Rotation in FootballPlayers”, Japanese Society of Sport Psychology, Vol.41, No.2, pp.93103, 2014. https://doi.org/10.4146/jjspopsy.2014-1313