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Hand Tracking for Immersive Virtual Reality: Opportunities and Challenges

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Abstract

Hand tracking has become an integral feature of recent generations of immersive virtual reality head-mounted displays. With the widespread adoption of this feature, hardware engineers and software developers are faced with an exciting array of opportunities and a number of challenges, mostly in relation to the human user. In this article, I outline what I see as the main possibilities for hand tracking to add value to immersive virtual reality as well as some of the potential challenges in the context of the psychology and neuroscience of the human user. It is hoped that this paper serves as a roadmap for the development of best practices in the field for the development of subsequent generations of hand tracking and virtual reality technologies.
Hand Tracking for Immersive Virtual
Reality: Opportunities and Challenges
Gavin Buckingham*
Department of Sport and Health Sciences, University of Exeter, Exeter, United Kingdom
Hand tracking has become an integral feature of recent generations of immersive virtual
reality head-mounted displays. With the widespread adoption of this feature, hardware
engineers and software developers are faced with an exciting array of opportunities and a
number of challenges, mostly in relation to the human user. In this article, I outline what I
see as the main possibilities for hand tracking to add value to immersive virtual reality as
well as some of the potential challenges in the context of the psychology and neuroscience
of the human user. It is hoped that this paper serves as a roadmap for the development of
best practices in the eld for the development of subsequent generations of hand tracking
and virtual reality technologies.
Keywords: VR, embodiment, psychology, communcation, inclusivity
INTRODUCTION
Immersive virtual reality (iVR) systems have recently seen a huge growth due to reductions in
hardware costs and a wealth of software use cases. In early consumer models of the Oculus Rift Head-
Mounted Display (HMD), interactions with the environment (a key hallmark of iVR) were usually
performed with hand-held controllers. Hands were visualized in games and applications
(infrequently) in a limited array of poses based on nger position, assumed from contact with
triggers and buttons on these controllers. Although the ability to visualize the positions of individual
digits was possible with external motion tracking and/or datagloveperipherals which measured
nger joint angles and rotations, these technologies were prohibitively expensive and were unreliable
without careful calibration. A step change in hand tracking occurred with the Leap Motion Tracker, a
small encapsulated infra-red emitter and optical camera developed with the goal of having people
interacting with desktop machines by gesturing at the screen. This device was very small, required no
external power source, and was able to track the movements of individual digits in three dimensions
using a stereo camera system with reasonable precision (Guna et al., 2014). Signicant improvements
in software, presumably through a clever use of inverse kinematics, along with a free software-
development kit and a strong user base in the Unity and Unreal Game Engine communities led to a
proliferation of accessible hand tracking addons and experiences tailor-made for iVR. Since then,
hand tracking has become embedded into the hardware of recent generations of iVR HMDs (e.g., the
rst and second iterations of the Oculus Quest) through so-called inside outtracking, and looks set
to continue to evolve with emerging technologies such as wrist-worn electromyography (Inside
Facebook Reality Labs, 2021). This paper will briey outline the main use-cases of hand tracking in
VR, and then discuss in some detail the outstanding issues and challenges which developers need to
keep in mind when developing such experiences.
OpportunitiesWhy Hand Tracking?
Our hands, with the dexterity afforded by our opposable thumbs, are one of the canonical features
which separates us from non-human primates. We use our hands to gesture, feel, and interact with
Edited by:
Nadia Magnenat Thalmann,
Université de Genève, Switzerland
Reviewed by:
Antonella Maselli,
Italian National Research Council, Italy
Richard Skarbez,
La Trobe University, Australia
*Correspondence:
Gavin Buckingham
g.buckingham@exeter.ac.uk
Specialty section:
This article was submitted to
Virtual Reality and Human Behaviour,
a section of the journal
Frontiers in Virtual Reality
Received: 21 June 2021
Accepted: 24 September 2021
Published: 20 October 2021
Citation:
Buckingham G (2021) Hand Tracking
for Immersive Virtual Reality:
Opportunities and Challenges.
Front. Virtual Real. 2:728461.
doi: 10.3389/frvir.2021.728461
Frontiers in Virtual Reality | www.frontiersin.org October 2021 | Volume 2 | Article 7284611
PERSPECTIVE
published: 20 October 2021
doi: 10.3389/frvir.2021.728461
our environment almost every minute of our waking lives. When
we are prevented from, or limited in, using our hands, we are
profoundly impaired, with a range of once-mundane tasks
becoming frustratingly awkward. Below, I briey outline three
signicant potential benets of having tracked hands in a virtual
environment.
Opportunity 1Increased Immersion and
Presence
The degree to which a user can to perceive a virtual environment
through the sensorimotor contingencies they would encounter in
the physical environment is termed immersion(Slater and
Sanchez-Vives, 2016). The subjective experience of being in a
highly-immersive virtual environment is known as presence,
and recent empirical evidence suggests that being able to see ones
tracked hands animated in real time in a virtual environment is an
extremely compelling method of engagement (Voigt-Antons
et al., 2020). Research has shown that we have an almost
preternatural sense of our hands positions and shape when
they are obscured (Dieter et al., 2014), and when our hands
are removed from our visual worlds it is a stark reminder of our
disembodiment. Indeed, we spend the majority of our time
during various mundane tasks foveating our hands (Land,
2009), so removing them from the visual scene presumably
has a range of consequences for our visuomotor behaviour.
Opportunity 2More Effective Interaction
The next point to raise is that of interaction. A key goal of virtual
reality is to allow the user to interact with the computer-generated
environment in a natural fashion. This interaction can be
achieved in its simplest form by the user by moving their head
to experience the wide visual world. More modern VR
experiences, however, usually involve some form of manual
interaction, from opening doors to wielding weapons.
Accurate tracking of the hands potentially allows for far more
precise interactions that would be possible with controllers,
adding not only to the users immersion (Argelaguet et al.,
2016;Pyasik et al., 2020), but even the accuracy of their
movements (Vosinakis and Koutsabasis, 2018), which seems
particularly key in the context of training (Harris et al., 2020).
Opportunity 3More Effective
Communication
The nal point to discuss is that of communication, and in
particular manual gesticulationthe use of ones hands to
emphasize words and punctuate sentences through a series of
gestures. Gesturesin the context of HCI has come to mean the
swipes and pinching motions uses to perform commands.
However, the involuntary movements of hands during natural
communication appear to play a signicant role not just for the
listener, but also the communicator to such an extent that
conversations between two congenitally blind individuals
contain as many gestures as conversations between sighted
individuals (Iverson and Goldin-Meadow, 1998;Özçalışkan
et al., 2016). Indeed, recent research has shown that
individuals are impaired in recognizing a number of key
emotions in the images of bodies which have the hands
removed (Ross and Flack, 2020), highlighting how important
hand form information is in communicative experiences. The
value of manual gestures for communication in virtual
environments is compounded given that veridical real-time
face tracking and visualization is technically very difcult due
to the extremely high temporal and spatial resolution required to
detect and track microexpressions. Furthermore, computer-
generated faces are particularly prone to large uncanny-valley
like effects whereby faces which fall just short of being realistic
elicit a strong sense of unease (MacDorman et al., 2009;
McDonnell and Breidt, 2010). Signicant recent strides have
been made in tracking and rendering photorealistic faces
(Schwartz et al., 2020), but the hardware costs are likely to be
prohibitive for the current generation of consumer-based VR
technologies. Tracking and rendering of the hands, with their
large and expressive kinematics, should thus be strong a focus for
communicative avatars in the short term.
Challenge 1Object Interaction
Our hands are one of our main ways to effect change in the
environment around us. Thus, one of the main reasons to
visualise hands in VR is to facilitate and encourage
interactions with the virtual environment. From opening doors
to wielding weapons, computer-generated hands are an integral
part of many game experiences across many platforms. As
outlined above, these manual interactions are typically
generated by reverse-engineering interactions with a held
controller. For example, on the Oculus Quest 2 controller, if
the buttons underneath the index and middle ngers are lightly
depressed, the hand appears to close slightly; if the buttons are
fully depressed, the hand closes into a st. Not only does this
method of interacting with the world feel quite engaging, it elicits
a greater sense of ownership over the seen hand than a
visualization of the held controller itself (Lavoie and
Chapman, 2021). But despite the compelling nature of this
experience, hand tracking offers the promise of a real-time
veridical representation of the hands true actions, requiring
no mapping of physical to seen actions and untethered from
any extraneous hardware. Anecdotally, however, interacting with
virtual objects using hand tracking feels imprecise and difcult to
use, which is supported by recent ndings showing that during a
block moving task hands tracked with a Leap Motion tracker
score lower on the System Usability Scale than hands tracked with
a hand-held controller (Masurovsky et al., 2020). Furthermore,
subjective Likert ratings on a number of descriptive metrics
suggested that the controller-free interaction felt signicantly
less comfortable and less precise than the controller-based
interactions. Even more worryingly, this same article noted
that participants performed worse on a number of
performance metrics when their hands were tracked with the
Leap than with the controller.
It is likely that the main reason that controller-free hand
tracking is problematic during object interaction is the lack of
tactile and haptic cues in this context. Tactile cues are a key part to
successful manual actions, and their removal impairs the
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Buckingham VR Hand Tracking
accuracy of manual localization (Rao and Gordon, 2001), alters
grasping kinematics (Whitwell et al., 2015;Furmanek et al., 2019;
Ozana et al., 2020;Mangalam et al., 2021), and affects the normal
application of ngertip forces (Buckingham et al., 2016). While
controller-based interactions with virtual objects do not deliver
the same tactile and haptic sensations experienced when
interacting with objects in the physical environment, the
vibro-tactile pulses and the mass of the controllers do seem to
aid in scaffolding a compelling percept of touching something. A
range of solutions to replace tactile feedback in the context of VR
have been developed in recent years. From a hardware
perspective, solutions range from glove-like devices which
provide tactile feedback and force feedback to the digits
(Carlton, 2021) to stimuli which precisely deform the
ngertips to create a sensation of the mechanics of interaction
(Schorr and Okamura, 2017) to devices which deliver contactless
ultrasonic pulses aimed at the hands to simulate tactile cues
(Rakkolainen et al., 2019). Researchers have also used a lower-
cost mixed reality solution known as haptic retargetingwhere
an individual interacts with a single physical peripheral and the
apparent position and orientation of the hands are subtly
manipulated to create the illusion of interacting with a range
of different objects (Azmandian et al., 2016;Clarence et al., 2021).
It is currently unclear which of these solutions (or one hitherto
unforeseen) will solve this issue, but it clearly a major challenge
for the broad uptake immersive virtual reality.
Challenge 2Tracking Location
With inside-outcameras in current consumer models (e.g., the
Oculus Quest 2), hand tracking is at its most reliable when the
hands are roughly in front ofthe face, presumably to maximise the
overlap of the elds of view of the individual cameras which track
the hands. In these headsets, the orientation of these cameras is
xed, presumably due to the assumption that participants will be
looking at what they are doing in VR. This assumption is probably
appropriate for discrete game-style events”–it is well-established
that individuals foveate the hands and the action endpoint during
goal-directed tasks (Desmurget et al., 1998;Johansson et al., 2001;
Lavoie et al., 2018). In more natural sequences of tasks (e.g.,
preparing food), however, the hands are likely to spend
signicant proportion of time in the lower visual eld due to
their physical location below the head. This asymmetry in the
common locations of the hand during many tasks was discussed in
the context of a lower visual eld specialization for manual action
by Previc (1990) and has received support parallels from a range of
studies showing that humans are more efcient utilizing visual
feedback to guide effective reaching toward targets in their lower
visual eld than their upper visual eld (Danckert and Goodale,
2001;Khan and Lawrence, 2005;Krigolson and Heath, 2006). This
behavioural work is supported by evidence from the visual system
for a lower visual eld speciality for factors related to action
(Schmidtmann et al., 2015;Zhou et al., 2017), as well as
neuroimaging evidence that grasping objects in the lower visual
eld preferentially activates a network of dorsal brain regions
specialised for planning and controlling visually-guided actions
(Rossit et al., 2013). As the range of tasks undertaken in VR widens
to include more natural everyday experiences where the hands
might be engaged in tasks in the lower visual elds, limitations of
tracking and visualization in this region of space will likely become
more apparent. Indeed, this issue is not only one of tracking, but
hardware eld of view. Currently the main focus on eld of view is
concerned with increasing the lateral extent, with little
consideration given to the fact that the letterboxshape of
most VR HMDs reduce the vertical eld of view in the lower
visual eld by more than 10% compared to that which the eye
affords in the physical environment (Kreylos, 2016;Kreylos, 2019).
Together, these issues of tracking limitations and physical
occlusion are likely to result in unnatural head movements in
manual tasks to ensure the hands are kept in view which could limit
the transfer of training from virtual to physical environments, or
signicant impacts on immersion as the hands disappear from
peripheral view at an unexpected or inconsistent point.
Challenge 3Uncanny Phenomenon and
Embodiment
The uncanny phenomenon (sometimes referred to as the
uncanny valley) refers to the lack of afnity yielding feelings
of unease or disgust when looking at, or interacting with,
something articial which falls just short of appearing natural
(Mori, 1970;Wang et al., 2015). The cause of this effect is still
undetermined, but recent studies have suggested that this effect
might be driven by mismatches between the apparently-biological
appearance of the offending stimuli and non-biological
kinematics and/or inappropriate features such as temperature
and surface textures (Saygin et al., 2012;Kätsyri et al., 2015). The
main triggers for uncanny valley seem to be in the realms of
computer-generated avatars (MacDorman et al., 2009;
McDonnell and Breidt, 2010) and interactive humanoid robots
(Destephe et al., 2015;Strait et al., 2017) and, as such, much of
research into this topic has focussed on faces. Recent studies have
suggested that this effect is amplied when experienced through
an HMD (Hepperle et al., 2020), highlighting the importance of
this factor in the context of tracked VR experiences.
Little work has, by contrast, examined such responses toward
hands. In the context of prosthetic hands, Poliakoff et al. (2013,
2018) demonstrated that images of life-like prosthetic hands were
rated as more eerie than anatomical or robotic hands in
equivalent poses. This effect appears to be eliminated in some
groups with extensive experience (e.g., in observers who
themselves have a limb absence), but is still strongly
experienced by prosthetists and non-amputees trained to use a
prosthetic hand simulator (Buckingham et al., 2019). Given the
strong possibility of inducing a presence-hindering effect if
virtual hands are sufciently disconcerting (Brenton et al.,
2005), it seems prudent to recommend outline or cartoon
hands as the norm for even strongly-embodied VR
experiences. This suggestion is particularly important for
untetheredHMDs, due to the fact that rendering
photorealistic images of hands tracked at the high frequencies
required to visualize the full range of dextrous actions will require
signicant computing power. A nal point in this regard which
also bears mention is that the uncanny valley is not a solely visual
experience, but a multisensory one. For example, it has been
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Buckingham VR Hand Tracking
shown that users experience of their presence in VR rapidly
declines when the visual cues in a VR scenario do not match with
the degree of haptic feedback (Berger et al., 2018). Furthermore it
has recently been shown that when the articiality of tactile cues
and visual cues are mismatched, this can also generate a reduction
in feelings of ownership (DAlonzo et al., 2019). Thus if tactile
cues are to become a feature of hand tracking and visualization,
care must be taken to avoid features of this so-called haptic
uncanny valley(Berger et al., 2018).
A more general issue which developers must grapple with than
hedonic perception is so-called embodiment”–the feeling of
ownership that one feels toward an effector that they are
controlling. This term is usually discussed in the context of a
body part or a tool, so has clear implications in the context of
hand tracking in VR (Kiltenietal.,2012) and is usually measured
either through subjective questionnaires or ostensibly objective
measures of felt body position and physiological responses to
threat. Anecdotally the dynamic and precise experience of viewing
computer-generated hands which are being tracked yields an
extremely strong sense of embodiment which does not require a
lengthy period of training or induction. In the context of virtual hands
presented through an HMD, the literature suggests that embodiment
happens naturally with realistic and veridical stimuli. Pyasik et al.
(2020) have shown that participants feel stronger levels of ownership
toward 3-D scans of their own hand than they did toward an
articially-smoothed and whitened hand. Furthermore, it has been
shown that feelings of embodiment are enhanced when the virtual
hands appear to be connected to the body rather than disembodied
(Seinfeld and Müller, 2020).Atthetimeofwriting,however,much
work remains to be done to build up a comprehensive picture of what
visual factors are required to balance embodiment, enjoyment, and
effective interaction with virtual environments.
Challenge 4Inclusivity
Inclusivity is an increasingly important ethical issue in technology
(Birhane, 2021), and the development of hand tracking and
visualization in iVR throws up a series of unique challenges in
this regard. A fundamental part of marker-free hand tracking is to
segment the skin from the surrounding background to build, and
ultimately visualize, the dynamics of the hand. One potential
issue which has not received explicit consideration is that of skin
pigmentation. There are a number of recent anecdotal examples
(Fussell, 2017) of examples framed around hardware limitations
where items from automatic soap dispensers to heart-rate
monitors fail to function as effectively for individuals with
darker skin tones (which are less reective) than lighter skin
tones (which are more reective). It is critical that, as iVR is more
widely adopted, the cameras which track the hands are able to
adequately image all levels of skin pigmentation.
A related issue comes from the software which is used to turn the
images captured by the cameras into dynamic models of the hands,
using models of possible hand congurations (inverse kinematics).
These models, assuming they are built from training sets, are likely to
suffer from the same algorithmic bias which has been problematic in
face classication research (Buolamwini and Gebru, 2018), with
datasets largely derived from Caucasian males yielding startling
disparities in levels of misclassication across skin type and
gender. This issue becomes one not just of skin pigmentation, but
of gender, age, disability, and skin texture and presumably will be
exacerbated at these intersections. Any hardware and software which
aims to cater for the average userrisks leaving hand tracking
functionally unavailable to large portions of society. One possible
solution to this could be to have users generate their own
personalised training sets, akin personalized voice prolesused
in some speech recognition software and home assistant devices.
The nal issue on this topic relates to the visualization of the
hands, related to the discussion of embodiment in the section
above. Although the current norm for hand visualization is for
outline or cartoon-style hands which lack distinguishing features,
presumably there will be a drive for the visualization of more
realistic-looking hands. As is becoming standard for facial avatars
in CG environment, it is important for individuals to be able to
develop a model in the virtual environment steps away from the
defaultof an able-bodied Caucasian male or female toward one
which accurately represents their bodily characteristics (or,
indeed, that of another). This can be jarringfor example it
has been shown that the appearance of opposite-gender hands
reduces womens experience of presence in virtual environments
(Schwind et al., 2017). With hands, this is also likely to be
particularly important from an embodiment perspective, with
an emerging body of literature suggesting that individuals are less
able to embody hands which appear to be from a visibly different
skin tone than their own (Farmer et al., 2012;Lira et al., 2017).
CONCLUSION
In summary, hand tracking is probably here to stay as a cardinal
(but probably still optional) feature of immersive virtual reality.
The opportunities for facilitating effective and engaging
interpersonal communication and more formal presentations
in a remote context is particularly exciting for many aspects of
our social, teaching, and learning worlds. Being cognisant of the
challenges which come with these opportunities is a rst step
toward developing a clear series of best practices to aid in the
development of the next generation of VR hardware and
immersive experiences.
AUTHOR CONTRIBUTIONS
GB conceived and wrote the manuscript.
ACKNOWLEDGMENTS
The author would like to thank João Mineiro for his comments on
an earlier draft of this manuscript.
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Buckingham VR Hand Tracking
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... Hence, a VR system that ensures the necessary conditions for presence [54]-whereas PI is more directly related to the immersive features of a VR system with adequate immersive properties, embodiment and plausible and believable scenarios-can elicit behavioral and psychophysiological responses [55][56][57] consistent with real-world counterparts. Modern VR setups allow for relatively precise recordings of human-hand motion capture [58] with the feedback of the user's hand enabling a more or less embodied experience. The possibility of the inclusion of virtual models of anthropomorphic hands mimicking a user's own, as well as a variety of other end effectors (including different tools), allows for testing different levels of embodiment and their impact on collaborative situations, not limited to user's own body, like in real-life testing. ...
... As pointed out by Bauer et al. [9], robot touch may also serve other communication purposes, important for establishing communication (such as a handshake); therefore, including it in VR scenes with cobots seems to be an important issue to be solved. While the use of haptic technologies significantly improves the embodiment of virtual scenes [58], haptics is not currently widespread due to the limited number of commercially naturalistic haptic interfaces. ...
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