Eye direction control and reduction of discomfort
by vection in HMD viewing of panoramic images
Seitaro Inagaki*

Kenji Funahashi†

Nagoya Institute of Technology

Nagoya Institute of TechnologyA BSTRACT
We have previously proposed an “eye direction exaggeration method.”That facilitates rearward visibility by exaggerating the angle of theeye direction when viewing panoramic images with an HMD in aseated position. In this study, we improved this exaggeration method.
However, the exaggeration sometimes increased discomfort suchas VR sickness. We also tried to reduce discomfort by presenting
horizontally moving particles and inducing vection stably.
Index Terms: Human-centered computing—Interaction design—
Empirical studies in interaction design
1I NTRODUCTION

Most VR services that allow users to view 360-degree images areused in a seated position. However, it is difficult for users to lookrearward while rotating the necks and hips in a seated position [2].
In fact, it is clear that the viewer’s gaze is directed to a 90-degree
range centered on the frontal direction of the video for 75% of the360-degree video playback time on Youtube [5]. Sitting in a swivel
chair makes it easier to look rearward, but the user’s body motion
is amplified by the rotation of the seat surface and the sensation
is greatly displaced, when viewing a 360-degree image with an
HMD [1]. Then, we have previously proposed an “eye direction
exaggeration method,” that facilitates rearward visibility in viewing
panoramic images with an HMD in a seated position [3]. The
direction in which the user looks is called the eye direction. By
presenting a rearward view on the HMD that is different from theactual viewing direction, the user can have a rearward view without
having to sit up. This method is intended for use in a posture inwhich the user cannot easily see rearward, and does not take into
account the fact that the eye direction may go around horizontally.
However, it is possible to go around the eye direction by changing
posture or by sitting in a swivel chair. In this study, we propose a new
exaggeration method that can cope with the case where the viewer’s
eye direction goes around a circle. We also propose a method toreduce discomfort by intentionally inducing vection, which meansthe sensation of illusory self-motion [4], by displaying horizontally
moving particles in the HMD during exaggerated the angle of eyedirection.
2

real eye direction is called the “change point.” Assuming that theangle in the real eye direction is r degrees (−90 ≤ r ≤ 90), the angle
in the virtual eye direction is v degrees (−180 ≤ v ≤ 180), and theangle of the change point is a degree (0 ≤ a ≤ 90), then consider thefollowing equation (Figure 1-(b)). The derivation process is omitted
for reasons of space limitation.
v = h(r) = e(r) = r (|r| ≤ a)(1)
v = h(r) = f (r) = r ± b|r − a|c (|r| > a)Parameter b is “amount of change” and is determined so that v = 180
when r = 90. Parameter c (c ⊆ N) is “variation type.” The additionand subtraction signs in the Equation (1) are calculated as addition
for right-handed rotation and subtraction for left-handed rotation. We
prepared some exaggeration patterns by changing parameter c anda (“Exag(c, a),” Figure 2). In the experiment, the most satisfactory
pattern was the Exag(1,0) for 12 different exaggeration patterns (3patterns for parameter c and 4 patterns for parameter a).
3 S IGMOIDAL E XAGGERATIONThe eye direction exaggeration method described in the previous
section assumes a seated position at home and does not expect theeye direction to circle horizontally. We try a new type of exaggeration using a sigmoidal function whose slope is close to a straightline near the front and whose slope becomes smaller near the back.
In this exaggeration, assuming that the angle in the real eye direction
is r degrees, consider the angle in the virtual eye direction v = h(r)degrees as follows.
v = h(r) =(2)The gain g is set, for example, so that the slope of h(r) near theorigin is equal to the slope of Exag(1,0) as g = 0.025 (Figure 3).
In a simple experiment, we confirmed that the angles of the virtualeye direction were almost smoothly connected when the angle ofthe real eye direction is r = ±180. On the other hand, it was also
found that, compared to the exaggeration method described in the#180°

E YE D IRECTION E XAGGERATION M ETHOD

The direction in which the viewer is actually looking, regardless ofthe scenery displayed on the HMD, is called the “real eye direction”
and the direction of the scenery displayed on the HMD by the program is called the “virtual eye direction.” The virtual eye direction
is basically the same as the real eye direction, but the angle of thereal eye direction is exaggerated to display the scenery on the HMD
in a virtual eye direction different from the real eye direction (Figure1-(a)). This angle at which the virtual eye direction differs from the* e-mail: s.inagaki.706@stn.nitech.ac.jp† e-mail: kenji@nitech.ac.jp

360
− 180
1 + egr#′%(")*(")( " ="90°"!

!

0(b)
(a)

!

90°"

(b)

Figure 1: (a) Real and virtual eye direction of this system, (b) Conceptual diagram

𝒗

"

𝒉(𝒓)

180

h !

165
150
135
120
10590!

75
60
4530
15𝒓

0
0

1530

45

60

75

90

Figure 2: Example ofexaggerated
pattern:Exag(2,30)

Figure 3: Sigmoidal exaggeration

Table 1: Discomfort with or without particles
Figure 4: Scene of the experiment
backgroundparticles
average point(a) slow
none
equal
4.3
3.3

fast
3.6

none
3.6

(b) fast
equal
3.3

slow
3.9

45°

20°

previous section, the subjects felt stronger discomfort. In the case of
sigmoidal exaggeration, the change of the angle in the virtual eye
direction relative to the change of the angle in the real eye direction
becomes very small at around |r| > 150. In other words, the sceneryin the HMD hardly changes in spite of the neck movement.
4D ISCOMFORT R EDUCTION T HROUGH V ECTION

Particles are displayed with white opaque and translucent tails moving in a horizontal circle around the subject in the HMD, in order toinduce vection to properly perceive changes the real eye direction.
To avoid disturbing the viewing of the contents, parameters of particles are set as follows (the angular velocity in the real eye directionis ω, and the angular velocity of the landscape in the HMD is −ω,or −h′ (r)ω when exaggerated).

Peripheral Vision

0°20°

Particle display area

45°

Figure 5: Viewing area and particle display (ω =3.7 [rad/s],−ω/h′ (r)ε = −3.7 [rad/s])

3. 10 cm diameter for a distance of about 5.5 m

When the angular velocity of the virtual eye direction is muchsmaller than that of the real eye direction, the presentation of particles with the angular velocity of the real eye direction, i.e., the samelevel as that of the real neck rotation, was found to be effective in
reducing the discomfort (Table 1).
In another experiment as the sigmoidal exaggeration with ε = 0,
the subjects were asked to look around freely, especially by intentionally lifting their hips, and to circle around beyond the back. Easy
rearward visibility and a rotational motion with little discomfort
were confirmed. As a future work, we would like to examine theparameters of the particles and consider dynamic modification.

4. The number of β objects around the subject (The horizontal
field of view of the HMD is approximately 90 degrees, andapproximately β /4 exist in the field of view.)

ACKNOWLEDGMENTS
This work was supported in part by JSPS KAKENHI Grant Number
JP20K11918, JP23K11182 and JP22K12337.

1. Right or left gyration in the region where the elevation (depression) angle is θ1 ≤ |θ | ≤ θ2 in the HMD coordinate system
2. Movement with an angular velocity equivalent to −ω/h′ (r)ε(ε is a constant) in the HMD coordinate system

5. With transparency α, the tail of the trajectory for δ seconds
5Central Vision

E XPERIMENT AND C ONCLUSION

An experiment was to investigate whether the presence or absenceof particles and their apparent velocity (parameter ε) contribute to
the reduction of discomfort, i.e., the angular velocity in the virtualeye direction is (a) smaller: b′ = 1/3, and (b) larger: b′ = 3 than
the real eye direction for the virtual eye direction v = b′ r with realeye direction r. The constant ε was investigated for two cases: theparticle motion is equal (ε = 0), and opposite (ε = 1) to the change
in the virtual eye direction. The subjects were asked to experience
without particles, with particles (equal magnification), and withparticles (slow or fast inversion). They looked around freely and rate
their discomfort from 0 to 9 (Figure 4). The particles are empirically
set to α = 0.005, β = 100, δ = 0.15, and are displayed in the areaof 25 ≤ θ ≤ 45 degrees with the peripheral vision (Figure 5).

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