PORTAL Portal Widget for Remote Target Acquisition and Control in Immersive Virtual Environments

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PORTAL: Portal Widget for Remote Target Acquisition and

Control in Immersive Virtual Environments

Dongyun Han

dongyun.han@usu.edu

Utah State University

Logan, USA

Donghoon Kim

donghoon.kim@usu.edu

Utah State University

Logan, USA

Isaac Cho

isaac.cho@usu.edu

Utah State University

Logan, USA

Figure 1: PORTAL is designed to leverage the secondary view interaction to allow the user to directly select and manipulate

remote objects using simple virtual hands. (B) PORTAL uses ray-casting as its selection tool. (C) Once the user creates PORTAL

by targeting a remote object, the user can see and interact with the remote object at a within-reach distance through PORTAL.

ABSTRACT

This paper introduces PORTAL (

rtal widget for

emote

arget

cquisition and contro

) that allows the user to interact with out-

of-reach objects in a virtual environment. We describe the PORTAL

interaction technique for placing a portal widget and interacting

with target objects through the portal. We conduct two formal

user studies to evaluate PORTAL for selection and manipulation

functionalities. The results show PORTAL supports participants to

interact with remote objects successfully and precisely. Following

that, we discuss its potential and limitations, and future works.

CCS CONCEPTS

•Human-centered computing →Interaction techniques.

KEYWORDS

Remote Object Interaction, Empirical studies in HCI

1 INTRODUCTION

3D user interactions for out-of-reach objects in Immersive Vir-

tual Environments (IVEs) are challenging [

]. Many techniques

including direct HOMER [

], Worlds-in-Miniature (WIM) [

], Go-

Go [

], and Linear Oset [

], have been studied for remote

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for prot or commercial advantage and that copies bear this notice and the full citation

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must be honored. Abstracting with credit is permitted. To copy otherwise, or republish,

to post on servers or to redistribute to lists, requires prior specic permission and/or a

fee. Request permissions from permissions@acm.org.

Conference acronym ’XX, June 03–05, 2018, Woodstock, NY

ACM ISBN 978-1-4503-XXXX-X/18/06. . . $15.00

https://doi.org/XXXXXXX.XXXXXXX

object interactions. However, they have limitations in terms of ne-

grained object controls and precise depth perception for distant

objects.

To provide better depth perception for distant objects or loca-

tions, some techniques introduce a secondary view that is impossi-

ble to see through the user’s rst point of view in IVEs. For example,

Kiyokawa and Takemura [

] proposed Tunnel Window allow-

ing the user to select and manipulate the remote objects through

the window. More recently, Li et al. [

] introduced vMirror sup-

porting the user to select occluded objects by locating a virtual

mirror to show a dierent angle of the user’s view. Both adopt

ray-casting to select or manipulate objects with the proposed tech-

nique. However, the usability of these techniques for distant object

selection and manipulation tasks is questionable because previous

research [

] shows that direct selection with no oset out-

performs varied distant object interaction techniques in terms of

task completion time.

This paper introduces PORTAL, allowing the user to directly

interact with remote objects using the simple hand metaphor by

taking advantage of the secondary view interaction. It presents an

illusion that the remote objects are within-arm reach position and

allows the user to interact with them by putting the hands into the

portal. We conducted two formal user studies. Study 1 evaluates its

usability and eciency in selecting and manipulating out-of-reach

objects by comparing PORTAL with existing techniques. In Study 2,

PORTAL is compared with the direct interaction after teleporting

to distant objects. The results show that PORTAL outperforms the

direct HOMER and Linear Oset techniques and have a similar

performance with Teleportation. Overall, the participants reported

that they prefer PORTAL most as well as it is easy and enjoyable

to use. The contributions of this paper are following:

arXiv:2210.00171v1 [cs.HC] 1 Oct 2022

Conference acronym ’XX, June 03–05, 2018, Woodstock, NY Han et al.

(1)

We introduce PORTAL, designed for the user to select and

manipulate distant objects with no oset and a better depth

perception of remote locations.

(2)

We conduct two user studies and report our ndings to

evaluate the eciency and usability of PORTAL.

(3)

We provide a thorough discussion on our ndings, the po-

tential and limitations of PORTAL.

2 RELATED WORKS

2.1 Remote Object Interaction in IVE

To interact with objects in IVEs, the control and motor spaces

should be considered [

]. The control space refers to the

spatial range in which the user is aordable to control objects,

while the motor space is the physical space available for the user

to operate the objects (i.e., arm-reach distance). When the control

space corresponds to the motor space (Fig. 2 A), the user cannot

interact with an out-of-reach object. To interact with it, the user

has to move to the remote position rst [

] using a navigation

technique (e.g., Teleportation [

].) To avoid this, some interaction

techniques provide a bigger control space than the user’s arm-

reach to select out-of-reach objects. An example is ray-casting, a

widely adopted selection technique in many commercial VR and

AR devices [

]. It shoots a ray from a controller or the

user’s head to select a remote object that the ray hits. Another

example is an image-plane pointing technique, also known as Sticky

Finger [

]. It allows the user to select a distant object that is

occluded by using one of the index ngers. However, they provide

limited support for object manipulation.

There are two categorical approaches to fully support remote 6

degree-of-freedom (DOF) object manipulation (xyz + yaw, pitch,

roll) in IVEs. The rst approach is oset techniques (Fig. 2 B), ex-

panding the control space by multiplying the motor space by a scale

factor that impacts on the control-display (CD) ratio. The CD ratio

determines how the input device’s movements (

x) are mapped to

the virtual cursor’s movement (

X). It is dened as (

x) / (

X). For

example, if the CD ratio is 1, the control space is equivalent to the

motor space (i.e., simple virtual hand as Fig. 2 A). If the it is smaller

than 1 (

x <

X), the user can reach remote objects as the control

space is expanded by 1 / CD ratio. Examples of this oset technique

are Direct HOMER [

],Go-Go [

], Linear Oset [

], Voodoo

Dolls [

], and World in Miniature (WIM) [

]. These techniques

have two limitations. 1) precise control of out-of-reach objects is

dicult due to the sensitive CD ratios. 2) for some techniques, one

hand must be tied to the widgets (e.g., Voodoo Dolls and WIM).

The second approach is the clutching mechanism [

] that relo-

cates the control space to nearby target objects (Fig. 2 C). It keeps 1

CD ratio for the user to interact with out-of-reach objects directly

within a limited space. PORTAL adopts the clutching mechanism

to allow the user to precisely interact with remote objects while

keeping both hands free in order to overcome the limitations of the

aforementioned techniques.

2.2 Secondary View Interaction in IVE

The secondary view is an additional view that displays a dierent

perspective than the user’s primary view to provide additional

interactions for the user. The Magic Lens technique [

], for

Figure 2: (A) Simple hand metaphor can not reach remote

objects because they are located out of reach. (B) Oset tech-

niques extends the control space by multiplying the motor

space by a scale factor. (C) PORTAL relocates a portion of

the control space near the remote objects.

example, reveals hidden information when it is overlaid on part of

the primary view. Its applicable scenario is the medical training [

] to allow the user to explore anatomy (e.g., bones and organs)

by overlaying the lens on a virtual human body.

Various types of secondary view interaction have been intro-

duced. Photoportals [

] promotes telepresence between remote

users in a virtual environment projected on 3D powerwalls to take

photos and videos together. Nam et al. [

] introduce Worlds-in-

Wedges for visual comparison by rendering dierent VR scenes in

multiple widgets.

Secondary views are also studied for 3D object. Li et al. [

]

proposed vMirror, an interactive widget leveraging reection of

mirrors to select remote and occluded objects. But it lacks object

manipulation. Martin et al. [

] also adopted a mirror metaphor

providing a new perspective around an object for accurate object

alignment. Stoev and Schmalstieg [

] introduced Through-The-

Lens (TTL) to allow the user to interact with remote objects through

TTL with the limited DOF. To our best knowledge, only Tunnel

Window [

] fully supports remote 6-DOF manipulation. It

uses ray-casting as its selection tool. Compared to Tunnel Window,

PORTAL uses a simple virtual hand metaphor allowing the user to

interact with the target directly [

] through the secondary view.

In addition we conducted two user studies to evaluate its eciency

and usability on the remote target selection and manipulation tasks

while no evaluation was provided for Tunnel Window.

3 PORTAL: A REMOTE INTERACTION WIDGET

PORTAL is an interactive widget leveraging a secondary view. It

is comprised of the primary portal and the secondary portal as

shown in Fig. 3. The portals work as a spatial tunnel to connect

dierent locations in IVEs to gives the user an impression that

remote objects are right in front of him or her. The source codes

and data are available at our GitHub repository 1.

3.1 Design Considerations

We design PORTAL with two considerations: 1) the user’s control

space should reach remote objects and support direct selection and

manipulation of them; 2) it should provide precise depth perception

to the user to facilitate the ne-grained controls to the remote

objects. We satisfy the rst design consideration by adopting the

1https://github.com/VIZ-US/PORTAL

PORTAL: Portal Widget for Remote Target Acquisition and Control in Immersive Virtual Environments Conference acronym ’XX, June 03–05, 2018, Woodstock, NY

Figure 3: PORTAL contains the primary and the secondary

portals. The portal camera in the secondary portal renders

its view on the primary portal. Through PORTAL, the user

perceive remote objects are in within-reach positions.

clutching mechanism [

]. It relocates a portion of the user’s control

space to nearby target objects (Fig. 2 C) and results in the user

reaching out-of-reach objects in IVEs. The boundary of this control

space is decided by the primary portal. When the user’s virtual

hands go beyond the primary portal, their copies are located at the

distant position as shown in Fig. 1 C which follow the user’s hand

movement. The second consideration is achieved by projecting

the secondary view from the portal camera on the primary portal.

Finally, the user gets the impression that the distant objects and

their surroundings are right in front of him or her with precise

depth perception.

3.2 PORTAL Placement

To place PORTAL, the user must select a remote target object by

ray-casting. When the user presses the trackpad on the controller

(Fig. 4 B), a ray emerges from the virtual hand as shown in Fig. 1 B.

When the user clicks the trigger button (Fig. 4 A) while the ray is

pointing to the remote target, the primary and secondary portals

are created. The primary portal is located in front of the user with

the distance in the length of the user’s arm reach (R)

0.5 in the

ray’s direction (Fig. 3 D). It is a circular shape, and its radius is

0.6m. The secondary portal is created in front of the remote target

by R

0.25 in the opposite direction of the ray (Fig. 3 A). Their

orientations are perpendicular to the ray. The secondary portal is

invisible to the user.

The view from the secondary portal is rendered on the primary

portal. The portal camera is positioned in front of the target object

by R

0.75 to the user direction (Fig. 3 C). Once the portal camera is

placed in the position, it begins rendering its view on the primary

portal by taking the following steps. We rst nd the projection

matrices for the left- and right-views of the main VR camera in the

scene. We apply the projection matrices to the portal camera and

nd the left- and right-eye projections which the portal camera

sees. Each projection is mapped to the left or right texture on

the primary portal object via Unity Shader. The distance between

the user’s head and the illusion of the target object through the

Figure 4: Vive Pro Eye and its controllers are used. HMD is

connected to Vive Wireless Adopter. Participants are asked

to wear the eye mask to follow the COVID-19 procedures.

primary portal (Fig. 3 E) is identical to the distance between the

portal camera and the target object (Fig. 3 C). The portal camera

moves following the user’s head movement. It enables the user

to see the target object and its surroundings with precise depth

perception through the primary portal.

3.3 PORTAL Interaction

The user can interact with a remote object by putting the virtual

hands into the primary portal and grabbing the object like the one

in front of the user. A part of the hands that entered PORTAL is

colored in red (Fig.1 C). The red hands work the same as the simple

virtual hands do. Through PORTAL, the user can bring the object

to the user’s side and send an object to the remote side.

The user can reposition PORTAL like other 3D objects in IVEs.

When the virtual hand is halfway through the primary portal, a

blue spherical marker appears on the hand (Fig.1 C), and it allows

the user to grab the primary portal and relocate and rotate it. The

primary portal’s movement is linked to that of the secondary portal.

As the portal camera belongs to the secondary portal object, when

the user moves the primary portal, it changes the portal camera’s

position and orientation accordingly. As a result, the projected view

on the primary portal is changed. This interaction allows the user

to navigate the remote location around the target object.

The user can close PORTAL and update the view on the primary

portal. The same controller operation for creating PORTAL is used.

The user can close PORTAL by performing the operation on the out

of PORTAL. Conversely, when the user performs the operation tar-

geting a new object within PORTAL, the position of the secondary

portal is only changed. As a result, the view on the primary portal

is updated, and it displays the new target.

4 STUDY 1: PORTAL VS. EXISTING TECHNIQUES

Study 1 (IRB #12168) includes two tasks to evaluate PORTAL com-

pared to the remote interaction techniques including direct HOMER

(HOMER) and Linear Oset (LO). Please note that the objective of

Study 1 is to evaluate the usability and eciency of PORTAL on

remote object selection and manipulation tasks. So, the functions

relocating PORTAL and sending and receiving an object through

PORTAL are deactivated in Study 1. Study 1 takes approximately

one hour.

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PORTAL:PortalWidgetforRemoteTargetAcquisitionandControlinImmersiveVirtualEnvironmentsDongyunHandongyun.han@usu.eduUtahStateUniversityLogan,USADonghoonKimdonghoon.kim@usu.eduUtahStateUniversityLogan,USAIsaacChoisaac.cho@usu.eduUtahStateUniversityLogan,USAFigure1:PORTALisdesignedtoleveragethesecondary...

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PORTAL Portal Widget for Remote Target Acquisition and Control in Immersive Virtual Environments

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