Conference PaperPDF Available

Holoportation: Virtual 3D Teleportation in Real-time

Authors:
  • perceptiveIO, Inc

Abstract and Figures

We present an end-to-end system for augmented and virtual reality telepresence, called Holoportation. Our system demonstrates high-quality, real-time 3D reconstructions of an entire space, including people, furniture and objects, using a set of new depth cameras. These 3D models can also be transmitted in real-time to remote users. This allows users wearing virtual or augmented reality displays to see, hear and interact with remote participants in 3D ,almost as if they were present in the same physical space. From an audio-visual perspective, communicating and interacting with remote users edges closer to face-to-face communication. This paper describes the Holoportation technical system in full, its key interactive capabilities, the application scenarios it enables, and an initial qualitative study of using this new communication medium.
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Holoportation: Virtual 3D Teleportation in Real-time
Sergio Orts-Escolano Christoph RhemannSean FanelloWayne ChangAdarsh KowdleYury Degtyarev
David KimPhilip DavidsonSameh KhamisMingsong DouVladimir TankovichCharles Loop
Qin CaiPhilip ChouSarah Mennicken Julien Valentin Vivek Pradeep Shenlong Wang
Sing Bing Kang Pushmeet Kohli Yuliya Lutchyn Cem Keskin Shahram Izadi∗†
Microsoft Research
Figure 1. Holoportation is a new immersive telepresence system that combines the ability to capture high quality 3D models of people, objects and
environments in real-time, with the ability to transmit these and allow remote participants wearing virtual or augmented reality displays to see, hear
and interact almost as if they were co-present.
ABSTRACT
We present an end-to-end system for augmented and vir-
tual reality telepresence, called Holoportation. Our system
demonstrates high-quality, real-time 3D reconstructions of an
entire space, including people, furniture and objects, using a
set of new depth cameras. These 3D models can also be trans-
mitted in real-time to remote users. This allows users wearing
virtual or augmented reality displays to see, hear and interact
with remote participants in 3D, almost as if they were present
in the same physical space. From an audio-visual perspec-
tive, communicating and interacting with remote users edges
closer to face-to-face communication. This paper describes
the Holoportation technical system in full, its key interactive
capabilities, the application scenarios it enables, and an initial
qualitative study of using this new communication medium.
Author Keywords
Depth Cameras; 3D capture; Telepresence; Non-rigid
reconstruction; Real-time; Mixed Reality; GPU
ACM Classification Keywords
I.4.5 Image Processing and Computer Vision: Reconstruc-
tion; I.3.7 Computer Graphics: Three-Dimensional Graphics
and Realism,
Authors contributed equally to this work
Corresponding author: shahrami@microsoft.com
Permission to make digital or hard copies of all or part of this work
for personal or classroom use is granted without fee provided that copies
are not made or distributed for profit or commercial advantage and that
copies bear this notice and the full citation on the first page. Copyrights
for components of this work owned by others than the author(s) must be
honored. Abstracting with credit is permitted. UIST 2016, October 16-19,
2016, Tokyo, Japan.
Copyright is held by the owner/author(s). ACM 978-1-4503-4189-9/16/10
DOI: http://dx.doi.org/10.1145/2984511.2984517
INTRODUCTION
Despite huge innovations in the telecommunication industry
over the past decades, from the rise of mobile phones to the
emergence of video conferencing, these technologies are far
from delivering an experience close to physical co-presence.
For example, despite a myriad of telecommunication tech-
nologies, we spend over a trillion dollars per year globally on
business travel, with over 482 million flights per year in the
US alone1. This does not count the cost on the environment.
Indeed telepresence has been cited as key in battling carbon
emissions in the future2.
However, despite the promise of telepresence, clearly we are
still spending a great deal of time, money, and CO2get-
ting on planes to meet face-to-face. Somehow much of the
subtleties of face-to-face co-located communication — eye
contact, body language, physical presence — are still lost in
even high-end audio and video conferencing. There is still a
clear gap between even the highest fidelity telecommunica-
tion tools and physically being there.
In this paper, we describe Holoportation, a step towards ad-
dressing these issues of telepresence. Holoportation is a new
immersive communication system that leverages consumer
augmented reality (AR) and virtual reality (VR) display tech-
nologies, and combines these with a state-of-the-art, real-time
3D capture system. This system is capable of capturing in full
360othe people, objects and motions within a room, using a
set of custom depth cameras. This 3D content is captured and
transmitted to remote participants in real-time.
Any person or object entering the instrumented room will be
captured in full 3D, and virtually teleported into the remote
participants space. Each participant can now see and hear
these remote users within their physical space when they wear
1http://www.forbes.com/sites/kenrapoza/2013/08/06/
business-travel- market-to- surpass- 1-trillion- this-year/
2http://www.scientificamerican.com/article/
can-videoconferencing- replace-travel/
their AR or VR headsets. From an audio-visual perspective,
this gives users an impression that they are co-present in the
same physical space as the remote participants.
Our main contribution is a new end-to-end immersive system
for high-quality, real-time capture, transmission and render-
ing of people, spaces, and objects in full 3D. Apart from the
system as a whole, another set of technical contributions are:
A new active stereo depth camera technology for real-time
high-quality capture.
A novel two GPU pipeline for higher speed temporally
consistent reconstruction.
A real-time texturing pipeline that creates consistent col-
ored reconstructions.
Low latency pipeline for high quality remote rendering.
Spatial audio that captures user position and orientation.
A prototype for headset removal using wearable cameras.
Furthermore, this paper also contributes key interactive capa-
bilities, new application scenarios, and an initial qualitative
study of using this new communication medium.
RELATED WORK
There has been a huge amount of work on immersive 3D
telepresence systems (see literature reviews in [16, 4, 32]).
Given the many challenges around real-time capture, trans-
mission and display, many systems have focused on one spe-
cific aspect. Others have constrained the scenarios, for exam-
ple limiting user motion, focusing on upper body reconstruc-
tions, or trading quality for real-time performance.
Early seminal work on telepresence focused on capturing dy-
namic scenes using an array of cameras [17, 27]. Given both
the hardware and computational limitations of these early sys-
tems, only low resolution 3D models could be captured and
transmitted to remote viewers. Since this early work, we have
seen improvements in the real-time capture of 3D models us-
ing multiple cameras [57, 55, 31]. Whilst results are impres-
sive, given the real-time constraint and hardware limits of the
time, the 3D models are still far from high quality. [48, 36]
demonstrate compelling real-time multi-view capture, but fo-
cus on visual hull-based reconstruction —only modeling sil-
houette boundaries. With the advent of Kinect and other con-
sumer depth cameras, other real-time room-scale capture sys-
tems emerged [39, 43, 25, 47]. However, these systems again
lacked visual quality due to lack of temporal consistency of
captured 3D models, sensor noise, interference, and lack of
camera synchronization. Conversely, high quality offline re-
construction of human models has been demonstrated using
both consumer, e.g. [13], and custom depth cameras, e.g.
[11]. Our aim with Holoportation is to achieve a level of vi-
sual 3D quality that steps closer to these offline systems. By
doing so, we aim to achieve a level of presence that has yet to
be achieved in previous real-time systems.
Beyond high quality real-time capture, another key differen-
tiator of our work is our ability to support true motion within
the scene. This is broken down into two parts – remote and
local motion. Many telepresence systems limit the local user
to a constrained seating/viewing position, including the sem-
inal TELEPORT system [19], and more recent systems [47,
10, 38, 5, 62], as part of the constraint of the capture setup.
Others constrain the motion of the remote participant, typi-
cally as a by-product of the display technology used whether
it is a situated stereo display [10] or a more exotic display
technology. Despite this, very compelling telepresence sce-
narios have emerged using situated autostereo [42, 45], cylin-
drical [28], volumetric [24], lightfield [1] or even true holo-
graphic [7] displays. However, we feel that free-form mo-
tion within the scene is an important factor in creating a more
faithful reconstruction of actual co-located presence. This im-
portance of mobility has been studied greatly in the context
of co-located collaboration [37], and has motivated certain
tele-robotics systems such as [26, 33], and products such as
the Beam-PRO. However, these lack human embodiment, are
costly and only support a limited range of motion.
There have been more recent examples of telepresence sys-
tems that allow fluid user motion within the space, such as
blue-c [21] or [4]. However, these systems use stereoscopic
projection which ultimately limits the ability for remote and
local users to actually inhabit the exact same space. Instead
interaction occurs through a ‘window’ from one space into
the other. To resolve this issue, we utilize full volumetric 3D
capture of a space, which is overlaid with the remote environ-
ment, and leverage off-the-shelf augmented and virtual reality
displays for rendering, which allow spaces to be shared and
co-habited. [40] demonstrate the closest system to ours from
the perspective of utilizing multiple depth cameras and opti-
cal see-through displays. Whilst extremely compelling, par-
ticularly given the lack of high quality, off-the-shelf, optical
see-through displays at the time, the reconstructions do lack
visual quality due to the use of off-the-shelf depth cameras.
End-to-end rendering latency is also high, and the remote user
is limited to remaining seated during collaboration.
Another important factor in co-located collaboration is the
use of physical props [37]. However, many capture systems
are limited to or have a strong prior on human bodies [9], and
cannot be extended easily to reconstruct other objects. Fur-
ther, even with extremely rich offline shape and pose models
[9], reconstructions can suffer from the effect of “uncanny
valley” [44]; and clothing or hair can prove problematic [9].
Therefore another requirement of our work is to support arbi-
trary objects including people, furniture, props, animals, and
to create faithful reconstructions that attempt to avoid the un-
canny valley.
SYSTEM OVERVIEW
In our work, we demonstrate the first end-to-end, real-time,
immersive 3D teleconferencing system that allows both lo-
cal and remote users to move freely within an entire space,
and interact with each other and with objects. Our system
shows unprecedented quality for real-time capture, allows for
low end-to-end communication latency, and further mitigates
remote rendering latency to avoid discomfort.
To build Holoportation we designed a new pipeline as shown
Figure 2. Overview of the Holoportation pipeline. Our capture units compute RGBD streams plus a segmentation mask. The data is then fused into a
volume and transmitted to the other site. Dedicated rendering PCs perform the projective texturing and stream the live data to AR and VR displays.
.
in Fig. 2. We describe this pipeline in full in the next sections,
beginning with the physical setup, our algorithm for high
quality depth computation and segmentation, realistic tempo-
ral geometry reconstruction, color texturing, image compres-
sion and network, and remote rendering.
Physical Setup and Capture Pods
For full 360capture of the scene, we employ N= 8 camera
pods placed on the periphery of the room, pointing inwards
to capture a unique viewpoint of the subject/scene. Each pod
(Fig. 3, middle) consists of 2Near Infra-Red cameras (NIR)
and a color camera mounted on top of an optical bench to
ensure its rigidity. Additionally a diffractive optical element
(DOE) and laser is used to produce a pseudo-random pattern
(for our system we use the same design as the Kinect V1).
We also mount NIR filters on top of each NIR camera to filter
out the visible light spectrum. Each trinocular pod generates
a color-aligned RGB and depth stream using a state-of-the-art
stereo matching technique described below. Current baseline
used in the active stereo cameras is 15 centimeters, giving
an average error of 3 millimeters at 1 meter distance and 6
millimeters at 1.5 meter distance.
In total, our camera rig uses 24 4MP resolution Grasshopper
PointGrey cameras. All the pods are synchronized using an
external trigger running at 30fps. Fig. 3, right, shows an
example of images acquired using one of the camera pods.
The first step of this module is to generate depth streams,
which require full intrinsics and extrinsics calibration. In
this work we use [63] for computing the camera parameters.
Another standard calibration step ensures homogeneous and
consistent color information among the RGB cameras. We
perform individual white balancing using a color calibration
chart. Once colors have been individually tuned, we use one
RGB camera as a reference and warp the other cameras to this
reference using a linear mapping. This makes the signal con-
sistent across all the RGB cameras. This process is performed
offline and then linear mapping across cameras is applied at
runtime.
Depth Estimation
Computing 3D geometry information of the scene from mul-
tiple view points is the key building block of our system. In
our case, the two key constraints are estimating consistent
depth information from multiple viewpoints, and doing so in
real-time.
Depth estimation is a well studied problem where a number
of diverse and effective solutions have been proposed. We
considered popular depth estimation techniques such as struc-
tured light, time-of-flight and depth from stereo for this task.
Structured light approaches allow for very fast and precise
depth estimation [59, 49, 6, 3, 15]. However, in our setup
of multi-view capture structured light-based depth estima-
tion suffers from interference across devices. Another alter-
native strategy is time-of-flight based depth estimation [22],
which has grown to be very popular, however, multi-path is-
sues make this technology unusable in our application. Depth
from RGB stereo is another possible solution that has seen
several advances in real-time depth estimation [51, 61, 56,
52]. Passive stereo uses a pair of rectified images and esti-
mates depth by matching every patch of pixels in one image
with the other image. Common matching functions include
sum of absolute differences (SAD), sum of squared differ-
ences (SSD), and normalized cross-correlation (NCC). This
is a well understood approach to estimating depth and has
been demonstrated to provide very accurate depth estimates
[8]. However, the main issue with this approach is the inabil-
ity to estimate depth in case of texture-less surfaces.
Motivated by these observations, we circumvent all these
problems by using active stereo for depth estimation. In an
active stereo setup, each camera rig is composed of two NIR
cameras plus one or more random IR dot pattern projector
(composed of laser plus DOE). Each serves as a texture in the
scene to help estimate depth even in case of texture-less sur-
faces. Since we have multiple IR projectors illuminating the
scene we are guaranteed to have textured patches to match
and estimate depth.
Additionally, and as importantly, we do not need to know
the pattern that is being projected (unlike standard structured
light systems such as Kinect V1). Instead, each projector sim-
ply adds more texture to the scene to aid depth estimation.
This circumvents the issues of interference which commonly
occur when two structured light cameras overlap. Here, over-
lapping projectors actually result in more texture for our
matching algorithm to disambiguate patches across cameras,
Figure 3. Left: Holoportation rig. Middle: Trinocular Pod: Right: NIR and RGB images acquired from a pod.
so the net result is an improvement rather than degradation in
depth quality.
With the type of depth estimation technique narrowed down
to active stereo, our next constraint is real-time depth estima-
tion. There are several approaches that have been proposed
for depth estimation [53]. We base our work on PatchMatch
stereo [8], which has been shown to achieve high quality
dense depth maps with a runtime independent of the num-
ber of depth values under consideration. PatchMatch stereo
alternates between random depth generation and propagation
of depth based on the algorithm by [2], and has recently been
extended to real-time performance by assuming fronto par-
allel windows and reducing the number of iterations of depth
propagation [65]. In this work, we develop a real-time CUDA
implementation of PatchMatch stereo that performs depth es-
timation at as high as 50fps on a high-end GPU such as an
NVIDIA Titan X in our case. Details of the GPU implemen-
tation are provided later in the paper. Examples of depthmaps
are shown in Fig. 4.
Foreground Segmentation
The depth estimation algorithm is followed by a segmentation
step, which provides 2D silhouettes of the regions of interest.
These silhouettes play a crucial role – not only do they help in
achieving temporally consistent 3D reconstructions [14] but
they also allow for compressing the data sent over the net-
work. We do not use green screen setups as in [11], to ensure
that the system can be used in natural environments that ap-
pear in realistic scenarios.
The input of the segmentation module consists of the current
RGB image It, and depth image Dt. The systems also main-
tains background appearance models for the scene from the
perspective of each camera pod in a manner similar to [29].
These background models are based on depth and RGB ap-
pearance of the empty scene (where the objects of interest
are absent). They are represented by an RGB and depth im-
age pair for each camera pod {Ibg, Dbg }, which are estimated
by averaging over multiple frames to make the system robust
against noise (e.g. depth holes, light conditions etc.).
We model the foreground/background segmentation labeling
problem using a fully-connected Conditional Random Field
(CRF) [30] whose energy function is defined as:
E(p) = X
i
ψu(pi) + X
i
X
j6=i
ψp(pi, pj)(1)
where pare all the pixels in the image. The pairwise terms
ψp(pi, pj)are Gaussian edge potentials defined on image gra-
dients in the RGB space, as used in [30]. The unary potential
is defined as the sum of RGB ψrgb (pi)and depth ψdepth(pi)
based terms: ψu(pi) = ψrgb (pi)+ψdepth(pi). The RGB term
is defined as
ψrgb (pi) = |Hbg (pi)Ht(pi)|+|Sbg (pi)St(pi)|,(2)
where Hand Sare the HSV components of the color im-
ages. Empirically we found that the HSV color space leads to
better results which are robust to illumination changes. The
depth based unary term is a logistic function of the form of
ψdepth(pi)=11/(1 + exp((Dt(pi)Dbg(pi)))),
where σcontrols the scale of the resolution and is fixed to
σ= 5cm.
Obtaining the Maximum a Posterior (MAP) solution under
the model defined in Eq. 1 is computationally expensive.
To achieve real-time performance, we use the efficient algo-
rithm for performing mean field inference that was proposed
in [58], which we implemented efficiently on the GPU. In
Fig. 4 we depict some results obtained using the proposed
segmentation algorithm.
Temporally Consistent 3D Reconstruction
Given the N= 8 depth maps generated as described previ-
ously, we want to generate a 3D model of the region of in-
terest. There are mainly 3 different strategies to achieve this.
The simplest approach consists of visualizing the 3D model
in the form of point clouds, however this will lead to multiple
problems such as temporal flickering, holes and flying pixels
(see Fig. 5).
A better strategy consists of fusing the data from all the cam-
eras [23]. The fused depth maps generate a mesh per frame,
with a reduced noise level and flying pixels compared to a
simple point cloud visualization. However the main draw-
back of this solution is the absence of any temporal consis-
tency: due to noise and holes in the depth maps, meshes gen-
erated at each frame could suffer from flickering effects, es-
pecially in difficult regions such as hair. This would lead to
3D models with high variation over time, making the overall
experience less pleasant.
Motivated by these findings, we employ a state of the art
method for generating temporally consistent 3D models in
real-time [14]. This method tracks the mesh and fuses the
data across cameras and frames. We summarize here the key
steps, for additional details see [14].
In order to fuse the data temporally, we have to estimate the
nonrigid motion field between frames. Following [14], we
parameterize the nonrigid deformation using the embedded
deformation (ED) model of [54]. We sample a set of K“ED
Figure 4. Top: RGB stream. Bottom: depth stream. Images are masked with the segmentation output.
Figure 5. Temporal consistency pipeline. Top: Point cloud visualization,
notice flying pixels and holes. Bottom: temporally reconstructed meshes.
nodes” uniformly, at locations {gk}K
k=1 R3throughout the
mesh V. The local deformation around an ED node gkis
defined via an affine transformation AkR3×3and a trans-
lation tkR3. In addition, a global rotation RSO(3) and
translation TR3are added. The full parameters to estimate
the nonrigid motion field are G={R, T }∪{Ak,tk}K
k=1. The
energy function we minimize is:
E(G) =λdataEdata (G) + λhullEhull (G) + λcorr Ecor r (G) +
λrotErot (G) + λreg Ereg (G),(3)
which is the weighted sum of multiple terms that take into
account the data fidelity, visual hull constraints, sparse cor-
respondences, a global rotation and a smoothness regularizer.
All the details regarding these terms, including the implemen-
tation for solving this nonlinear problem, are in [14].
Once we retrieve the deformation model between two frames,
we fuse the data of the tracked nonrigid parts and we reset
the volume to the current observed 3D data for those regions
where the tracking fails.
Color Texturing
After fusing depth data, we extract a polygonal 3D model
from its implicit volumetric representation (TSDF) with
marching cubes, and then texture this model using the 8 in-
put RGB images (Fig. 6). A naive texturing approach would
compute the color of each pixel by blending the RGB images
Figure 6. Projective Texturing: segmented-out RGB images and recon-
structed color.
according to its surface normal ˆ
nand the camera viewpoint
direction ˆ
vi. In this approach, the color weights are computed
as wi=vis·max(0,ˆ
n·ˆ
vi)α, favoring frontal views with fac-
tor αand vis is a visibility test. These weights are non-zero
if the textured point is visible in a particular view (i.e., not
occluded by another portion of the model). The visibility test
is needed to avoid back-projection and is done by rasterizing
the model from each view, producing rasterized depth maps
(to distinguish from input depth maps), and using those for
depth comparison. Given imperfect geometry, this may re-
sult in so-called “ghosting” artifacts (Fig. 7a), when missing
parts of geometry might cause wrongly projected color to the
surfaces behind it. Many existing approaches use global op-
timization to stitch [18] or blend [64, 46] the incoming color
maps to tackle this problem. However, these implementations
are not real-time and in some cases use more RGB images.
Instead we assume that the surface reconstruction error is
bounded by some value e. First, we extend the visibility test
with an additional depth discontinuity test. At each textured
point on a surface, for each input view we search for depth
discontinuity in its projected 2D neighborhood in a rasterized
depth map with radius determined by e. If such a discontinu-
ity is found, we avoid using this view in a normal-weighted
blending (the associated weight is 0). This causes dilation of
edges in the rasterized depth map for each view (Fig. 7bc).
This can eliminate most ghosting artifacts, however it can
classify more points as unobserved. In this case, we use reg-
ular visibility tests and a majority voting scheme for colors:
Figure 7. (a) Ghosting artifacts, (b) Dilated depth discontinuities, (c) Di-
lated depth discontinuities from pod camera view, (d) Number of agree-
ing color (white = 4), (e) Proposed algorithm, (f) Voting failure case.
for a given point, to classify each view as trusted, the color
candidates for this view must agree with a number of colors
(Fig. 7d) from other views that can see this point, and this
number of agreeing views should be maximum. The agree-
ment threshold is multi-level: we start from as small value
and increase the threshold if no previous views agreed. This
leads to a more accurate classification of trusted views as it
helps to avoid false-positives near color discontinuities. Fi-
nally, if none of the views agree, we pick the view with the
minimal RGB variance for a projected 2D patch in the in-
put RGB image, since larger color discontinuity for a patch
is more likely to correspond to a depth discontinuity of a per-
fectly reconstructed surface, and thus that patch should not be
used.
While this approach works in real time and eliminates most
of the artifacts (Fig. 7e), there are some failure cases when
two ghosting color candidates can outvote one true color (Fig.
7f). This is a limitation of the algorithm, and a trade-off be-
tween quality and performance, but we empirically derived
that this occurs only in highly occluded regions and does not
reduce the fidelity of the color reconstruction in the region of
interest, like faces. We also demonstrate that the algorithm
can handle complex, multi-user scenarios with many objects
(Fig. 8) using only 8 RGB cameras.
Spatial Audio
Audio is the foundational medium of human communication.
Without proper audio, visual communication, however im-
mersive, is usually ineffective. To enhance the sense of im-
mersion, auditory and visual cues must match. If a user sees a
person to the left, the user should also hear that person to the
left. Audio emanating from a spatial source outside the user’s
field of view also helps to establish a sense of immersion, and
can help to compensate for the limited field of view in some
HMDs.
In Holoportation we synthesize each remote audio source,
namely the audio captured from a remote user, as coming
from the position and orientation of the remote user in the
local user’s space. This ensures that the audio and visual
cues are matched. Even without visual cues, users can use
the spatial audio cues to locate each other. For example, in
Holoportation, it is possible for the first time for users to play
the game “Marco Polo” while all players are geographically
distributed. To the best of our knowledge, Holoportation is
the first example of auditory augmented or virtual reality to
enable communication with such freedom of movement.
In our system, each user is captured with a monaural mi-
crophone. Audio samples are chunked into 20ms frames,
and audio frames are interleaved with the user’s current head
pose information in the user’s local room coordinate system
(specifically, x, y, z, yaw, pitch, and roll). The user’s au-
dio frame and the head pose information associated with that
frame are transmitted to all remote users through multiple
unicast connections, although multicast connections would
also make sense where available.
Upon receiving an audio frame and its associated head pose
information from a remote user, the local user’s system trans-
forms the head pose information from the remote user’s room
coordinate system to the local user’s room coordinate sys-
tem, and then spatializes the audio source at the proper lo-
cation and orientation. Spatialization is accomplished by fil-
tering the audio signal using a head related transfer function
(HRTF).
HRTF is a mapping from each ear, each audio frequency, and
each spatial location of the source relative to the ear, into a
complex number representing the magnitude and phase of the
attenuation of the audio frequency as the signal travels from
the source location to the ear. Thus, sources far away will be
attenuated more than sources that are close. Sources towards
the left will be attenuated less (and delayed less) to the left
ear than to the right ear, reproducing a sense of directionality
in azimuth as well as distance. Sources above the horizon-
tal head plane will attenuate frequencies differently, giving a
sense of elevation. Details may be found in [20].
We also modify the amplitude of a source by its orientation
relative to the listener, assuming a cardioid radiation pattern.
Thus a remote user facing away from the local user will sound
relatively muffled compared to a remote user facing toward
the local user. To implement audio spatialization, we rely on
the HRTF audio processing object in the XAUDIO2 frame-
work in Windows 10.
Compression and Transmission
The previous steps generate an enormous amount of data per
frame. Compression is therefore critical for transmitting that
data over the wide area network to a remote location for ren-
dering. Recent work in mesh compression [11] as well as
point cloud compression [12] suggest that bit rates on the or-
der of low tens of megabits per second per transmitted per-
son, depending on resolution, are possible, although real-time
compression at the lowest bit rates is still challenging.
For our current work, we wanted the rendering to be real time,
and also of the highest quality. Hence we perform only a very
lightweight real time compression and straightforward wire
Figure 8. Examples of texturing multiple users and objects.
format over TCP, which is enough to support 5-6 viewing
clients between capture locations over a single 10 Gbps link
in our point to point teleconferencing scenarios. For this pur-
pose it suffices to transmit a standard triangle mesh with addi-
tional color information from the various camera viewpoints.
To reduce the raw size of our data for real-time transmission,
we perform several transformations on per-frame results from
the capture and fusion stages, as we now describe.
Mesh Geometry
As described previously, we use a marching cubes polygonal-
ization of the volumetric data generated in the fusion stage
as our rendered mesh. The GPU implementation of march-
ing cubes uses 5 millimeters voxels. We perform vertex de-
duplication and reduce position and normal data to 16 bit
floats on the GPU before transfer to host memory for se-
rialization. During conversion to our wire format, we use
LZ4 compression on index data to further reduce frame size.
For our current capture resolution this results in a per-frame
transmission requirement of approximately 2MB per frame
for mesh geometry (60K vertices, 40K triangles). Note that
for proper compositing of remote content in AR scenarios,
the reconstruction of local geometry must still be available to
support occlusion testing.
Color Imagery
For color rendering, the projective texturing described above
is used for both local and remote rendering, which requires
that we transfer relevant color imagery from all eight color
cameras. As we only need to transmit those sections of the
image that contribute to the reconstructed foreground ob-
jects, we leverage the foreground segmentation calculated
in earlier stages to set non-foreground regions to a constant
background color value prior to transmission. With this in
place, the LZ4 compression of the color imagery (8 4MP
Bayer images) reduces the average per-frame storage require-
ments from 32MB to 3MB per frame. Calibration parameters
for projective mapping from camera to model space may be
transferred per frame, or only as needed.
Audio Transmission
For users wearing an AR or VR headset, the associated micro-
phone audio and user pose streams are captured and transmit-
ted to all remote participants to generate spatial audio sources
Figure 9. Render Offloading: (a) Rendering with predicted pose on PC,
(b) Rendered image encoding, (c) Rendering with actual pose on HMD,
(d) Predicted view on PC, and its over-rendering with enlarged FOV,
(e) Pixels in decoded video stream, used during reprojection, (f) Repro-
jected actual view.
correlated to their rendered location, as described in the Au-
dio subsection above. Each captured audio stream is monau-
ral, sampled at 11 KHz, and represented as 16-bit PCM, re-
sulting in a transmitted stream of 11000*16 = 176 Kbps, plus
9.6 Kbps for the pose information. At the receiving side, the
audio is buffered for playback. Network jitter can cause the
buffer to shrink when packets are not being delivered on time,
and then to grow again when they are delivered in a burst. If
the buffer underflows (becomes empty), zero samples are in-
serted. There is no buffer overflow, but the audio playback
rate is set to 11025 samples per second, to provide a slight
downward force on the playback buffer size. This keeps the
playback buffer in check even if the receiving clock is some-
what slower than the sending clock. The audio+pose data
is transmitted independently and bi-directionally between all
pairs of remote participants. The audio communication sys-
tem is peer-to-peer, directly between headsets, and is com-
pletely independent of the visual communication system. We
do not provide AV sync, and find that the audio and visual
reproductions are sufficiently synchronized; any difference in
delay between audio and visual systems is not noticeable.
Bandwidth and Network Topology
For low-latency scenarios, these approaches reduce the aver-
age per-frame transmission size to a 1-2 Gbps transfer rate for
an average capture stream at 30fps, while adding only a small
overhead ( <10ms) for compression. Compressed frames of
mesh and color data are transmitted between capture stations
via TCP to each active rendering client. For ‘Living Mem-
ories’ scenarios, described later in the applications section,
these packets may be intercepted, stored, and replayed by an
intermediary recording server. Large one-to-many broadcast
scenarios, such as music or sports performance, requires fur-
ther compression and multicast infrastructures.
Render Offloading and Latency Compensation
For untethered VR or AR HMDs, like HoloLens, the cost of
rendering a detailed, high quality 3D model from the user’s
perspective natively on the device can be considerable. It also
increases motion-to-photon latency, which worsens the user
experience. We mitigate this by offloading the rendering costs
to a dedicated desktop PC on the receiving side, i.e., perform
remote rendering, similar to [41, 34]. This allows us to main-
tain consistent framerate, to reduce perceived latency, and to
conserve battery life on the device, while also enabling high-
end rendering capabilities, which are not always available on
mobile GPUs.
Offloading is done as follows: the rendering PC is connected
to an untethered HMD via WiFi, and constantly receives
user’s 6DoF (six degree of freedom) pose from the HMD.
It predicts a headset pose at render time and performs scene
rendering with that pose for each eye (Fig. 9a), encodes them
to a video stream (Fig. 9b), and transmits the stream and
poses to the HMD. There, the video stream is decoded and
displayed for each eye as a textured quad, positioned based
on predicted rendered pose (Fig. 9c), and then reprojected to
the latest user pose (Fig. 9f) [60].
To compensate for pose mispredictions and PC-to-HMD la-
tency, we perform speculative rendering on the desktop side,
based on the likelihood of the user pose. The orientation mis-
prediction can be compensated by rendering into a larger FoV
(field of view) (Fig. 9d), centered around the predicted user
direction. The HMD renders the textured quad with actual
display FoV, thus allowing some small misprediction in rota-
tion (Fig. 9e). To handle positional misprediction, we could
perform view interpolation techniques as in [34]. However, it
would require streaming the scene depth buffer and its repro-
jection, which would increase the bandwidth and HMD-side
rendering costs. Since the number of objects are small and
they are localized in the capture cube, we approximate the
scene depth complexity with geometry of the same (textured)
quad and dynamically adjust its distance to the user, based on
the point of interest.
IMPLEMENTATION
We use multiple GPUs across multiple machines to achieve
real-time performance. At each capture site, 4 PCs compute
depth and foreground segmentation (each PC handles two
capture pods). Each PC is an Intel Core i7 3.4 Ghz CPU,
16 GB of RAM and it uses two NVIDIA Titan X GPUs. The
resulting depth maps and segmented color images are then
transmitted to a dedicated dual-GPU fusion machine over
point-to-point 10Gbps connections. The dual-GPU unit is an
Intel Core i7 3.4GHz CPU, 16GB of RAM, with two NVIDIA
Titan X GPUs.
Depth Estimation: GPU Implementation
The depth estimation algorithm we use in this work comprises
of three main stages: initialization, propagation, and filtering.
Most of these stages are highly parallelizable and its compu-
tation is pixel independent (initialization). Therefore, a huge
benefit in terms of compute can be obtained by implementing
this directly on the GPU. Porting PatchMatch stereo [8] to the
GPU required a modification of the propagation stage in com-
parison with the original CPU implementation. The original
propagation stage is inherently iterative and is performed in
row order starting at the top-left pixel to the end of the im-
age and in the reverse order, iterating twice over the image.
Our GPU implementation modifies this step in order to take
advantage of the massive parallelism of current GPUs. The
image is subdivided into neighborhoods of regular size and
the propagation happens locally on each of these neighbor-
hoods. With this optimization, whole rows and columns can
be processed independently on separate threads in the GPU.
In this case, the algorithm iterates through four propagation
directions in the local neighborhood: from left to right, top to
bottom, right to left, and bottom to top.
Finally, the filtering stage handles the removal of isolated re-
gions that do not represent correct disparity values. For this
stage, we implemented a Connected Components labeling al-
gorithm on the GPU, so regions below a certain size are re-
moved. This step is followed by an efficient GPU-based me-
dian algorithm to remove noisy disparity values while pre-
serving edges.
Temporal Reconstruction: Dual-GPU Implementation
The implementation of [14] is the most computationally ex-
pensive part of the system. Although with [14] we can recon-
struct multiple people in real time, room-sized dynamic scene
reconstructions require additional compute power. Therefore,
we had to revisit the algorithm proposed in [14] and imple-
ment a novel dual-GPU scheme. We pipeline [14] into two
parts, where each part lives on a dedicated GPU. The first
GPU estimates a low resolution frame-to-frame motion field;
the second GPU refines the motion field and performs the vol-
umetric data fusion. The coarse frame-to-frame motion field
is computed from the raw data (depth and RGB). The second
GPU uses the frame-to-frame motion to initialize the model-
to-frame motion estimation, which is then used to fuse the
data volumetrically. Frame-to-frame motion estimation does
not require the feedback from the second GPU, so two GPUs
can run in parallel. For this part we use a coarser deforma-
tion graph (i.e. we sample an ED node every 8 cm) and run
8 Levenberg-Marquardt (LM) solver iterations, with coarser
voxel resolution and only 2 iterations are used for model-to-
frame motion estimation.
Throughput-oriented Architecture
We avoid stalls between CPU and GPU in each part of our
pipeline by using pinned (non-pageable) memory for CPU-
GPU DMA data transfers, organized as ring buffers; overlap-
ping data upload, download, and compute; and using sync-
free kernel launches, maintaining relevant subproblem sizes
on the GPU, having only its max-bounds estimations on
the CPU. We found that overhead of using max-bounds is
lower than with CUDA Dynamic Parallelism for nested ker-
nel launches. Ring buffers introduce only processing latency
into the system and maintain the throughput we want to pre-
serve.
Computational Time
Image acquisition happens in a separate thread overlapping
the computing of the previous frame with the acquisition of
Figure 10. Applications. A) One-to-one communication. B) Business meeting. C) Live concert broadcasting. D) Living memory. E) Out-of-body
dancing instructions. F) Family gathering. G) Miniature view. H) Social VR/AR.
the next one and introducing 1frame latency. The average
time for image acquisition is 4ms.
Each machine on the capture side generates two depth maps
and two segmentation masks in parallel. The total time is
21ms and 4ms for the stereo matching and segmentation, re-
spectively. In total each machine uses no more than 25ms
to generate the input for the multiview non-rigid 3D recon-
struction pipeline. Segmented RGBD frames are generated
in parallel to the nonrigid pipeline, but do also introduce 1
frame of latency.
A master PC, aggregates and synchronizes all the depth maps
and segmentation masks. Once the RGBD inputs are avail-
able, the average processing time (second GPU) to fuse all the
depth maps is 29ms (i.e., 34fps) with 5ms for preprocessing
(18% of the whole fusion pipeline), 14ms (47%) for the non-
rigid registration (2 LM iterations, with 10 PCG iterations),
and 10ms (35%) for the fusion stage. The visualization for
a single eye takes 6ms on desktop GPU, and thus enables to
display graphics at native refresh rate of 60Hz for HoloLens
and 75Hz for Oculus Rift DK2.
APPLICATIONS
As shown in the supplementary video and Fig. 10, we en-
vision many applications for Holoportation. These fall into
two broad categories. First, are one-to-one applications: these
are scenarios where two remote capture rigs establish a direct
connection so that the virtual and physical spaces of each rig
are in one-to-one correspondence. Once segmentation is per-
formed any new object in one space will appear in the other
and vice versa. As shown in the video, this allows remote
participants to be correctly occluded by the objects in the lo-
cal space. Because our system is agnostic to what is actually
captured, objects, props, and even furniture can be captured.
One to one calls are analogous to a telephone or video chat be-
tween two parties. However, with the ability to move around
the space, and benefit from many physical cues. The second
category consists of one-to-many applications: this is where
a single capture rig is broadcasting a live stream of data to
many receivers. In this case the physical space within the cap-
ture rig corresponds to the many virtual spaces of the remote
viewers. This is analogous to the broadcast television model,
where a single image is displayed on countless screens.
One-to-one
One-to-one applications are communication and collabora-
tion scenarios. This could be as simple as a remote conver-
sation. The distinguishing characteristic that Holoportation
brings is a sense of physical presence through cues such as
movement in space, gestures, and body language.
Specific one-to-one applications we foresee include:
Family gatherings, where a remote participant could visit
with loved ones or join a birthday celebration.
Personal instruction, where a remote teacher could provide
immediate feedback on dance moves, or yoga poses.
Doctor patient, where a remote doctor could examine and
interact with a patient to provide a diagnosis.
Design meetings, where remote parties could share and in-
teract with physical or virtual objects.
One-to-many
A unique characteristic of Holoportation for one-to-many
scenarios is live streaming. Other technologies can broad-
cast pre-processed content, as in Collet et al [11]. We believe
that live streaming provides a much more powerful sense of
engagement and shared experience. Imagine a concert view-
able from any seat in the house, or even from on stage. Sim-
ilarly, live sporting events could be watched, or re-watched,
from any point. When humans land on Mars, imagine waiting
on the surface to greet the astronaut as she steps foot on the
planet.
In both one-to-one or one-to many scenarios, Holoportation
content can be recorded for replay from any viewpoint, and
any scale. Room-sized events can be scaled to fit on a cof-
fee table for comfortable viewing. Content can be paused,
rewound, fast forwarded, to view any event of interest, from
any point of view.
Beyond Augmented Reality: A Body in VR
Finally, we point out that Holoportation can also be used in
a single or multi player immersive VR experiences, where
one’s own body is captured by the Holoportation system and
inserted into the VR world in real-time. This provides a vir-
tual body that moves and appears like ones own physical
body, providing a sense of presence.
As shown in the supplementary video, these experiences cap-
ture many new possibilities for ‘social’ AR and VR applica-
tions in the future.
USER STUDY
Having solved many of the technical challenges for real-time
high quality immersive telepresence experiences, we wanted
to test our prototype in a preliminary user study. We aimed
to unveil opportunities and challenges for interaction design,
explore technological requirements, and better understand the
tradeoffs of AR and VR for this type of communication.
Procedure and Tasks
We recruited a total of 10 participants (3 females; age 22 to
56) from a pool of partners and research lab members not
involved with this project. In each study session, two par-
ticipants were placed in different rooms and asked to per-
form two tasks (social interaction and physical object ma-
nipulation) using each of the target technologies, AR and
VR. We counterbalanced the order of technology conditions
as well as the order of the tasks using the Latin square de-
sign. The whole procedure took approximately 30 minutes.
For the duration of the study, two researchers observed and
recorded participants’ behavior, including their interaction
patterns, body language, and strategies for collaboration in
the shared space. After the study, researchers also conducted
semi-structured interviews to obtain additional insight about
the most salient elements of the users’ experience, challenges,
and potential usage scenarios.
Social Interaction Task: Tell-a-lie
To observe how people use Holoportation for verbal commu-
nication, we used a tell-a-lie task [66]. All participants were
asked to state three pieces of information about themselves,
with one of the statements being false. The goal of the part-
ner was to identify the false fact by asking any five questions.
Both deception and deception detection are social behaviors
that encourage people to pay attention to and accurately inter-
pret verbal and non-verbal communication. Thus, it presents
a very conservative testing scenario for our technology.
Physical Interaction Task: Building Blocks
To explore the use of technology for physical interaction in
the shared workspace, we also designed an object manipula-
tion task. Participants were asked to collaborate in AR and
VR to arrange six 3D objects (blocks, cylinders, etc.) in a
given configuration (Fig. 11). Each participant had only three
physical objects in front of him on a stool, and could see the
blocks of the other person virtually. During each task, only
one of the participants had a picture of the target layout, and
had to instruct the partner.
Study Results and Discussion
Prior to analysis, researchers compared observation notes and
partial transcripts of interviews for congruency. Further qual-
itative analysis revealed insights that fall into 5 categories.
Adapting to Mixed-reality Setups
Participants experienced feelings of spatial and social co-
presence with their remote partners, which made their inter-
action more seamless. P4: “It’s way better than phone calls.
Figure 11. User study setting: Two participants performing the building
blocks task in an AR condition (left) and a VR condition (right).
[...] Because you feel like you’re really interacting with the
person. It’s also better than a video chat, because I feel like
we can interact in the same physical space and that we are
modifying the same reality.
The spatial and auditory cues gave an undeniable sense of
co-location; so much so that many users even reported a
strong sense of interpersonal space awareness. For exam-
ple, most participants showed non-verbal indicators/body lan-
guage typical to face-to-face conversations in real life (e.g.
adopting the “closed” posture when lying in the social task;
automatically using gestures to point or leaning towards each
other in the building task). P6: “It made me conscious of [my
own image]. It was kind of realistic in that sense, because
whenever we are talking to someone around the table we are
conscious of not wanting to slouch in front of the other person
[and] about how we look to the other person.
Participants also quickly adapted and developed strategies for
collaborating in the mixed-reality setup. For example, several
participants independently came up with the idea to remove
their own blocks to let their partner arrange their shapes first,
to avoid confusing the real and virtual objects. While partic-
ipants often started by verbally instructing and simple point-
ing, they quickly figured out that it is easier and more natural
for them to use gestures or even their own objects or hands
to show the intended position and orientation. P2: “When I
started I was kind of pointing at the shape, then I was just
doing the same with my shape and then just say ’Ok, do that.
Because then she could visually see, I could almost leave my
shapes and wait until she put her shapes exactly in the same
location and then remove mine.
Natural Interaction for Physical and Collaborative tasks
The shared spatial frame of reference, being more natural
than spatial references in a video conversation, was men-
tioned as a main advantage of Holoportation. For example,
P9 found that “[the best thing was] being able to interact
remotely and work together in a shared space. In video, in
Skype, even just showing something is still a problem: you
need to align [the object] with the camera, then ’Can you see
it?’, then ’Can you turn it?’ Here it’s easy.
The perception of being in the same physical space also al-
lows people to interact simply more naturally, even for full
body interaction, as nicely described by P4: “I’m a dancer
[...] and we had times when we tried to have Skype re-
hearsals. It’s really hard, because you’re in separate rooms
completely. There’s no interaction, they might be flipped, they
might not. Instead with this, it’s like I could look and say ’ok,
this is his right’ so I’m going to tell him move it to the right
or towards me.
Influence of Viewer Technology
To probe for the potential uses of Holoportation, we used two
viewer technologies with different qualities and drawbacks.
Characteristics of the technology (e.g. FoV, latency, image
transparency, and others) highly influenced participants’ ex-
periences, and, specifically, their feelings of social and spa-
tial co-presence. To some participants, AR felt more realis-
tic/natural because it integrated a new image with their own
physical space. In the AR condition, they felt that their part-
ner was “here”, coming to their space as described by P7
“In the AR it felt like somebody was transported to the room
where I was, there were times I was forgetting they were in a
different room. The VR condition gave participants the im-
pression of being in the partner’s space, or another space al-
together. P6: “[With VR] I felt like I was in a totally different
place and it wasn’t all connected to my actual surroundings.
Another interesting finding in the VR condition was the con-
fusion caused by seeing a replica of both local and remote
participants and objects. There were many instances where
users failed to determine if the block they were about to touch
was real or not, passing their hands directly through the vir-
tual object. Obviously, rendering effects could help in this
disambiguation, although if these are subtle then they may
still cause confusions.
Related to AR and VR conditions was also the effects of la-
tency. In VR the user’s body suffered from latency due to
our pipeline. Participants could tolerate the latency in terms
of interacting and picking up objects, but a few users suf-
fered from discomfort during the VR condition, which was
not perceived in the AR condition. Despite this we were sur-
prised how well participants performed in VR and how they
commented on enjoying the greater sense of presence due to
seeing their own bodies.
Freedom to Choose a Point of View
One of the big benefits mentioned, was that the technology
allows the viewer to assume the view they wanted, and move
freely without constraints. This makes it very suitable for
exploring objects or environments, or to explain complicated
tasks that benefit from spatial instructions. P10 commented:
“I do a lot of design reviews or things like that where you
actually want to show somebody something and then explain
in detail and it’s a lot easier if you could both see the same
things. So if you have a physical object and you want to show
somebody the physical object and point to it, it just makes
it a lot easier. However, this freedom of choice might be
less suitable in case of a “directors narrative” in which the
presenter wants to control the content and the view on it.
Requirements for Visual Quality
While a quantitative analysis of small sample sizes needs
careful interpretation, we found that most participants (70%)
tended to agree or strongly agree that they perceived the con-
versation partner to look like a real person. However, some
feedback about visual quality was less positive; e.g., occa-
sional flicker that occurred when people were at the edge of
the reconstruction volume was perceived to be off-putting.
Eye Contact through the Visor
One key factor that was raised during the study was the pres-
ence of the headset in both AR and VR conditions, which
clearly impacted the ability to make direct eye contact. Whilst
we perceived many aspects of physical co-located interaction,
users did mention this as a clear limitation.
Figure 12. Visor Removal. The left image in the top row shows the
placement of the inward looking cameras on the HMD while those on the
center and right show the projections. The bottom row shows the facial
landmarks localized by our pipeline to enable the projective texturing.
To overcome this problem, we have begun to investigate
a proof-of-concept prototype for augmenting existing see-
through displays with headset removal. We utilize tiny wire-
less video cameras, one for each eye region that is mounted
on the outer rim of the display (see Fig. 12). We then project
the texture associated with the occluded eye-region to a 3D
reconstruction of the user’s face.
To perform the projective texturing, we need to not only esti-
mate the intrinsics calibration of the camera, but also the ex-
trinsic parameters, which in this instance corresponds to the
3D position of the camera with respect to the face of the user.
Given the 3D position of facial keypoints (e.g. eye corners,
wings of the nose) in an image, it is possible to estimate the
extrinsics of the camera by leveraging the 2D-3D position of
these landmarks using optimization methods like [35]. We
use a discriminately trained cascaded Random Forest to gen-
erate series of predictions of the most likely position of facial
keypoints in a manner similar to [50]. To provide predictions
that are spatially smooth over time, we perform a mean-shift
filtering of the predictions made by Random Forest.
The method we just described requires the 3D location of
keypoints to estimate the camera extrinsics. We extract this
information from a high fidelity reconstruction of the user’s
face obtained by aggregating frames using KinectFusion [23].
This process requires human intervention, but only takes a
few seconds to complete. Note that once we have the 3D
model, we render views (from known camera poses) and pass
these to our Random Forest to estimate the 2D location of the
facial keypoints. Knowing the camera poses used to render,
we can ray-cast and estimate where the keypoints lie on the
user’s 3D face model. We also perform color correction to
make sure that the texture coming from both cameras look
compatible. At this stage, we have all the components to per-
form real-time projective texture mapping of eye camera im-
ages onto 3D face geometry and to perform geometric mesh
blending of the eye region and the live mesh. Our preliminary
results are shown in Fig. 12 and more results are presented in
the attached video.
LIMITATIONS
While we presented the first high quality 360oimmersive 3D
telepresence system, our work does have many limitations.
The amount of high-end hardware required to run the sys-
tem is very high, with pairs of depth cameras requiring a
GPU-powered PC, and a seperate GPU-based PC for tem-
poral reconstruction, meshing and transmission. Currently, a
10 Gigabit Ethernet connection is used to communicate be-
tween rooms, allowing low latency communication between
the users, which limits many Internet scenarios. More effi-
cient compressions schemes need to be developed to address
this requirement and compress geometry and color data for
lower bandwidth connections. Moreover, during the textur-
ing process we still observed color artifacts caused by ex-
treme occlusions in the scene. More effective algorithms that
take further advantage of the non-rigid tracking could fur-
ther reduce the artifacts and the number of color cameras that
are required. Regarding the 3D non-rigid reconstruction, we
note that for many fine-grained interaction tasks, the 3D re-
construction of smaller geometry such as fingers produced
artifacts, such as missing or merged surfaces. Finally, we
note that developing algorithms for making direct eye con-
tact through headset removal is challenging, and as yet we
are not fully over the uncanny valley.
CONCLUSION
We have presented Holoportation: an end-to-end system for
high-quality real-time capture, transmission and rendering of
people, spaces, and objects in full 3D. We combine a new
capture technology with mixed reality displays to allow users
to see, hear and interact in 3D with remote colleagues. From
an audio-visual perspective, this creates an experience akin to
physical presence. We have demonstrated many different in-
teractive scenarios, from one-to-one communication and one-
to-many broadcast scenarios, both for live/real-time interac-
tion, to the ability to record and playback ‘living memories’.
We hope that practitioners and researchers will begin to ex-
pand the technology and application space further, leveraging
new possibilities enabled by this type of live 3D capture.
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