Conference PaperPDF Available

3D Dynamic Scene Graphs: Actionable Spatial Perception with Places, Objects, and Humans

Authors:
Robotics: Science and Systems 2020
Corvalis, Oregon, USA, July 12-16, 2020
1
3D Dynamic Scene Graphs: Actionable Spatial
Perception with Places, Objects, and Humans
Antoni Rosinol, Arjun Gupta, Marcus Abate, Jingnan Shi, Luca Carlone
Laboratory for Information & Decision Systems (LIDS)
Massachusetts Institute of Technology
{arosinol,agupta,mabate,jnshi,lcarlone}@mit.edu
Fig. 1: We propose 3D Dynamic Scene Graphs (DSGs) as a unified representation for actionable spatial perception. (a) A
DSG is a layered and hierarchical representation that abstracts a dense 3D model (e.g., a metric-semantic mesh) into higher-
level spatial concepts (e.g., objects, agents, places, rooms) and models their spatio-temporal relations (e.g., “agent A is in
room B at time t”, traversability between places or rooms). We present a Spatial PerceptIon eNgine (SPIN) that reconstructs a
DSG from visual-inertial data, and (a) segments places, structures (e.g., walls), and rooms, (b) is robust to extremely crowded
environments, (c) tracks dense mesh models of human agents in real time, (d) estimates centroids and bounding boxes of
objects of unknown shape, (e) estimates the 3D pose of objects for which a CAD model is given.
Abstract—We present a unified representation for actionable
spatial perception: 3D Dynamic Scene Graphs.Scene graphs
are directed graphs where nodes represent entities in the scene
(e.g., objects, walls, rooms), and edges represent relations (e.g.,
inclusion, adjacency) among nodes. Dynamic scene graphs (DSGs)
extend this notion to represent dynamic scenes with moving
agents (e.g., humans, robots), and to include actionable infor-
mation that supports planning and decision-making (e.g., spatio-
temporal relations, topology at different levels of abstraction).
Our second contribution is to provide the first fully automatic
Spatial PerceptIon eNgine (SPIN) to build a DSG from visual-
inertial data. We integrate state-of-the-art techniques for object
and human detection and pose estimation, and we describe how to
robustly infer object, robot, and human nodes in crowded scenes.
To the best of our knowledge, this is the first paper that reconciles
visual-inertial SLAM and dense human mesh tracking. Moreover,
we provide algorithms to obtain hierarchical representations of
indoor environments (e.g., places, structures, rooms) and their
relations. Our third contribution is to demonstrate the pro-
posed spatial perception engine in a photo-realistic Unity-based
simulator, where we assess its robustness and expressiveness.
Finally, we discuss the implications of our proposal on modern
robotics applications. 3D Dynamic Scene Graphs can have a
profound impact on planning and decision-making, human-robot
interaction, long-term autonomy, and scene prediction. A video
abstract is available at https://youtu.be/SWbofjhyPzI.
I. INTRODUCTION
Spatial perception and 3D environment understanding are
key enablers for high-level task execution in the real world.
In order to execute high-level instructions, such as “search for
survivors on the second floor of the tall building”, a robot
needs to ground semantic concepts (survivor, floor, building)
into a spatial representation (i.e., a metric map), leading to
metric-semantic spatial representations that go beyond the map
models typically built by SLAM and visual-inertial odometry
(VIO) pipelines [15]. In addition, bridging low-level obstacle
avoidance and motion planning with high-level task planning
requires constructing a world model that captures reality
at different levels of abstraction. For instance, while task
planning might be effective in describing a sequence of actions
to complete a task (e.g., reach the entrance of the building,
take the stairs, enter each room), motion planning typically
relies on a fine-grained map representation (e.g., a mesh or
a volumetric model). Ideally, spatial perception should be
able to build a hierarchy of consistent abstractions to feed
both motion and task planning. The problem becomes even
more challenging when autonomous systems are deployed in
crowded environments. From self-driving cars to collaborative
robots on factory floors, identifying obstacles is not sufficient
for safe and effective navigation/action, and it becomes crucial
to reason on the dynamic entities in the scene (in particular,
humans) and predict their behavior or intentions [24].
The existing literature falls short of simultaneously address-
ing these issues (metric-semantic understanding, actionable
hierarchical abstractions, modeling of dynamic entities). Early
work on map representation in robotics (e.g., [16,28,50,51,
103,113]) investigates hierarchical representations but mostly
in 2D and assuming static environments; moreover, these
works were proposed before the “deep learning revolution”,
hence they could not afford advanced semantic understand-
ing. On the other hand, the quickly growing literature on
metric-semantic mapping (e.g., [8,12,30,68,88,96,100])
mostly focuses on “flat” representations (object constellations,
metric-semantic meshes or volumetric models) that are not
hierarchical in nature. Very recent work [5,41] attempts to
bridge this gap by designing richer representations, called
3D Scene Graphs. A scene graph is a data structure com-
monly used in computer graphics and gaming applications that
consists of a graph model where nodes represent entities in
the scene and edges represent spatial or logical relationships
among nodes. While the works [5,41] pioneered the use
of 3D scene graphs in robotics and vision (prior work in
vision focused on 2D scene graphs defined in the image
space [17,33,35,116]), they have important drawbacks.
Kim et al. [41] only capture objects and miss multiple levels
of abstraction. Armeni et al. [5] provide a hierarchical model
that is useful for visualization and knowledge organization, but
does not capture actionable information, such as traversability,
which is key to robot navigation. Finally, neither [41] nor [5]
account for or model dynamic entities in the environment.
Contributions. We present a unified representation for
actionable spatial perception: 3D Dynamic Scene Graphs
(DSGs, Fig. 1). A DSG, introduced in Section III, is a layered
directed graph where nodes represent spatial concepts (e.g.,
objects, rooms, agents) and edges represent pairwise spatio-
temporal relations. The graph is layered, in that nodes are
grouped into layers that correspond to different levels of
abstraction of the scene (i.e., aDSG is a hierarchical repre-
sentation). Our choice of nodes and edges in the DSG also
captures places and their connectivity, hence providing a strict
generalization of the notion of topological maps [85,86] and
making DSGs an actionable representation for navigation and
planning. Finally, edges in the DSG capture spatio-temporal
relations and explicitly model dynamic entities in the scene,
and in particular humans, for which we estimate both 3D poses
over time (using a pose graph model) and a mesh model.
Our second contribution, presented in Section IV, is to
provide the first fully automatic Spatial PerceptIon eNgine
(SPIN) to build a DSG. While the state of the art [5] assumes an
annotated mesh model of the environment is given and relies
on a semi-automatic procedure to extract the scene graph,
we present a pipeline that starts from visual-inertial data and
builds the DSG without human supervision. Towards this goal
(i) we integrate state-of-the-art techniques for object [111] and
human [46] detection and pose estimation, (ii) we describe
how to robustly infer object, robot, and human nodes in
cluttered and crowded scenes, and (iii) we provide algorithms
to partition an indoor environment into places, structures, and
rooms. This is the first paper that integrates visual-inertial
SLAM and human mesh tracking (we use SMPL meshes [64]).
The notion of SPIN generalizes SLAM, which becomes a
module in our pipeline, and augments it to capture relations,
dynamics, and high-level abstractions.
Our third contribution, in Section V, is to demonstrate the
proposed spatial perception engine in a Unity-based photo-
realistic simulator, where we assess its robustness and expres-
siveness. We show that our SPIN (i) includes desirable features
that improve the robustness of mesh reconstruction and human
tracking (drawing connections with the literature on pose
graph optimization [15]), (ii) can deal with both objects
of known and unknown shape, and (iii) uses a simple-yet-
effective heuristic to segment places and rooms in an indoor
environment. More extensive and interactive visualizations are
given in the video attachment (available at [90]).
Our final contribution, in Section VI, is to discuss several
queries aDSG can support, and its use as an actionable spatial
perception model. In particular, we discuss how DSGs can
impact planning and decision-making (by providing a repre-
sentation for hierarchical planning and fast collision check-
ing), human-robot interaction (by providing an interpretable
abstraction of the scene), long-term autonomy (by enabling
data compression), and scene prediction.
II. RE LATE D WOR K
Scene Graphs. Scene graphs are popular computer graphics
models to describe, manipulate, and render complex scenes
and are commonly used in game engines [106]. While in gam-
ing applications, these structures are used to describe 3D en-
vironments, scene graphs have been mostly used in computer
vision to abstract the content of 2D images. Krishna et al. [48]
use a scene graph to model attributes and relations among
objects in 2D images, relying on manually defined natural
language captions. Xu et al. [109] and Li et al. [59] develop
algorithms for 2D scene graph generation. 2D scene graphs
have been used for image retrieval [36], captioning [3,37,47],
high-level understanding [17,33,35,116], visual question-
answering [27,120], and action detection [60,65,114].
Armeni et al. [5] propose a 3D scene graph model to
describe 3D static scenes, and describe a semi-automatic algo-
rithm to build the scene graph. In parallel to [5], Kim et al. [41]
propose a 3D scene graph model for robotics, which however
only includes objects as nodes and misses multiple levels of
abstraction afforded by [5] and by our proposal.
Representations and Abstractions in Robotics. The ques-
tion of world modeling and map representations has been
central in the robotics community since its inception [15,101].
The need to use hierarchical maps that capture rich spatial
and semantic information was already recognized in seminal
papers by Kuipers, Chatila, and Laumond [16,50,51]. Vasude-
van et al. [103] propose a hierarchical representation of object
constellations. Galindo et al. [28] use two parallel hierarchical
representations (a spatial and a semantic representation) that
are then anchored to each other and estimated using 2D
lidar data. Ruiz-Sarmiento et al. [92] extend the framework
in [28] to account for uncertain groundings between spa-
tial and semantic elements. Zender et al. [113] propose a
single hierarchical representation that includes a 2D map, a
navigation graph and a topological map [85,86], which are
then further abstracted into a conceptual map. Note that the
spatial hierarchies in [28] and [113] already resemble a scene
graph, with less articulated set of nodes and layers. A more
fundamental difference is the fact that early work (i) did not
reason over 3D models (but focused on 2D occupancy maps),
(ii) did not tackle dynamical scenes, and (iii) did not include
dense (e.g., pixel-wise) semantic information, which has been
enabled in recent years by deep learning methods.
Metric-Semantic Scene Reconstruction. This line of work
is concerned with estimating metric-semantic (but typically
non-hierarchical) representations from sensor data. While
early work [7,14] focused on offline processing, recent
years have seen a surge of interest towards real-time metric-
semantic mapping, triggered by pioneering works such as
SLAM++ [96]. Object-based approaches compute an object
map and include SLAM++ [96], XIVO [21], OrcVIO [98],
QuadricSLAM [74], and [12]. For most robotics applications,
an object-based map does not provide enough resolution for
navigation and obstacle avoidance. Dense approaches build
denser semantically annotated models in the form of point
clouds [8,22,61,100], meshes [30,88,91], surfels [104,107],
or volumetric models [30,68,72]. Other approaches use both
objects and dense models, see Li et al. [55] and Fusion++ [69].
These approaches focus on static environments. Approaches
that deal with moving objects, such as DynamicFusion [73],
Mask-fusion [94], Co-fusion [93], and MID-Fusion [108] are
currently limited to small table-top scenes and focus on objects
or dense maps, rather than scene graphs.
Metric-to-Topological Scene Parsing. This line of work fo-
cuses on partitioning a metric map into semantically meaning-
ful places (e.g., rooms, hallways). Nüchter and Hertzberg [75]
encode relations among planar surfaces (e.g., walls, floor,
ceiling) and detect objects in the scene. Blanco et al. [10]
propose a hybrid metric-topological map. Friedman et al. [26]
propose Voronoi Random Fields to obtain an abstract model of
a 2D grid map. Rogers and Christensen [87] and Lin et al. [62]
leverage objects to perform a joint object-and-place classifica-
tion. Pangercic et al. [80] reason on the objects’ functionality.
Pronobis and Jensfelt [83] use a Markov Random Field to
segment a 2D grid map. Zheng et al. [118] infer the topology
of a grid map using a Graph-Structured Sum-Product Net-
work, while Zheng and Pronobis [117] use a neural network.
Armeni et al. [4] focus on a 3D mesh, and propose a method
to parse a building into rooms. Floor plan estimation has been
also investigated using single images [32], omnidirectional
images [66], 2D lidar [56,102], 3D lidar [71,76], RGB-
D [63], or from crowd-sourced mobile-phone trajectories [2].
The works [4,71,76] are closest to our proposal, but contrarily
to [4] we do not rely on a Manhattan World assumption, and
contrarily to [71,76] we operate on a mesh model.
SLAM and VIO in Dynamic Environments. This paper is
also concerned with modeling and gaining robustness against
dynamic elements in the scene. SLAM and moving object
tracking has been extensively investigated in robotics [6,105],
while more recent work focuses on joint visual-inertial odom-
etry and target pose estimation [23,29,84]. Most of the
existing literature in robotics models the dynamic targets as
a single 3D point [18], or with a 3D pose and rely on
lidar [6], RGB-D cameras [1], monocular cameras [58], and
visual-inertial sensing [84]. Related work also attempts to gain
robustness against dynamic scenes by using IMU motion infor-
mation [34], or masking portions of the scene corresponding to
dynamic elements [9,13,19]. To the best of our knowledge,
the present paper is the first work that attempts to perform
visual-inertial SLAM, segment dense object models, estimate
the 3D poses of known objects, and reconstruct and track dense
human SMPL meshes.
Human Pose Estimation. Human pose and shape estima-
tion from a single image is a growing research area. While we
refer the reader to [45,46] for a broader review, it is worth
mentioning that related work includes optimization-based ap-
proaches, which fit a 3D mesh to 2D image keypoints [11,45,
54,110,112], and learning-based methods, which infer the
mesh directly from pixel information [39,45,46,79,81,99].
Human models are typically parametrized using the Skinned
Multi-Person Linear Model (SMPL) [64], which provides a
compact pose and shape description and can be rendered as a
mesh with 6890 vertices and 23 joints.
III. 3D DYNAM IC SC EN E GRAPHS
A 3D Dynamic Scene Graph (DSG, Fig. 1) is an action-
able spatial representation that captures the 3D geometry
and semantics of a scene at different levels of abstraction,
and models objects, places, structures, and agents and their
relations. More formally, a DSG is a layered directed graph
where nodes represent spatial concepts (e.g., objects, rooms,
agents) and edges represent pairwise spatio-temporal relations
(e.g., “agent A is in room B at time t”). Contrarily to
Fig. 2: Places and their connectivity shown as a graph. (a)
Skeleton (places and topology) produced by [78] (side view);
(b) Room parsing produced by our approach (top-down view);
(c) Zoomed-in view; red edges connect different rooms.
knowledge bases [49], spatial concepts are semantic concepts
that are spatially grounded (in other words, each node in our
DSG includes spatial coordinates and shape or bounding-box
information as attributes). A DSG is a layered graph, i.e., nodes
are grouped into layers that correspond to different levels of
abstraction. Every node has a unique ID.
The DSG of a single-story indoor environment includes 5
layers (from low to high abstraction level): (i) Metric-Semantic
Mesh, (ii) Objects and Agents, (iii) Places and Structures,
(iv) Rooms, and (v) Building. We discuss each layer and the
corresponding nodes and edges below.
A. Layer 1: Metric-Semantic Mesh
The lower layer of a DSG is a semantically annotated 3D
mesh (bottom of Fig. 1(a)). The nodes in this layer are 3D
points (vertices of the mesh) and each node has the following
attributes: (i) 3D position, (ii) normal, (iii) RGB color, and (iv)
a panoptic semantic label.1Edges connecting triplets of points
(i.e., a clique with 3 nodes) describe faces in the mesh and
define the topology of the environment. Our metric-semantic
mesh includes everything in the environment that is static,
while for storage convenience we store meshes of dynamic
objects in a separate structure (see “Agents” below).
B. Layer 2: Objects and Agents
This layer contains two types of nodes: objects and agents
(Fig. 1(c-e)), whose main distinction is the fact that agents are
time-varying entities, while objects are static.
Objects represent static elements in the environment that
are not considered structural (i.e., walls, floor, ceiling, pillars
1Panoptic segmentation [42,57] segments both object (e.g., chairs, tables,
drawers) instances and structures (e.g., walls, ground, ceiling).
are considered structure and are not modeled in this layer).
Each object is a node and node attributes include (i) a 3D
object pose, (ii) a bounding box, and (ii) its semantic class
(e.g., chair, desk). While not investigated in this paper, we refer
the reader to [5] for a more comprehensive list of attributes,
including materials and affordances. Edges between objects
describe relations, such as co-visibility, relative size, distance,
or contact (“the cup is on the desk”). Each object node is
connected to the corresponding set of points belonging to the
object in the Metric-Semantic Mesh. Moreover, nearby objects
are connected to the same place node (see Section III-C).
Agents represent dynamic entities in the environment,
including humans. While in general there might be many
types of dynamic entities (e.g., vehicles, bicycles in outdoor
environments), without loss of generality here we focus on two
classes: humans and robots.2Both human and robot nodes
have three attributes: (i) a 3D pose graph describing their
trajectory over time, (ii) a mesh model describing their (non-
rigid) shape, and (iii) a semantic class (i.e., human, robot).
A pose graph [15] is a collection of time-stamped 3D poses
where edges model pairwise relative measurements. The robot
collecting the data is also modeled as an agent in this layer.
C. Layer 3: Places and Structures
This layer contains two types of nodes: places and struc-
tures. Intuitively, places are a model for the free space, while
structures capture separators between different spaces.
Places (Fig. 2) correspond to positions in the free-space and
edges between places represent traversability (in particular:
presence of a straight-line path between places). Places and
their connectivity form a topological map [85,86] that can
be used for path planning. Place attributes only include a
3D position, but can also include a semantic class (e.g., back
or front of the room) and an obstacle-free bounding box
around the place position. Each object and agent in Layer 2 is
connected with the nearest place (for agents, the connection is
for each time-stamped pose, since agents move from place to
place). Places belonging to the same room are also connected
to the same room node in Layer 4. Fig. 2(b-c) shows a
visualization with places color-coded by rooms.
Structures (Fig. 3) include nodes describing structural
elements in the environment, e.g., walls, floor, ceiling, pillars.
The notion of structure captures elements often called “stuff”
in related work [57], while we believe the name “structure”
is more evocative and useful to contrast them to objects.
Structure nodes’ attributes are: (i) 3D pose, (ii) bounding box,
and (iii) semantic class (e.g., walls, floor). Structures may have
edges to the rooms they enclose. Structures may also have
edges to an object in Layer 3, e.g., a “frame” (object) “is
hung” (relation) on a “wall” (structure), or a “ceiling light is
mounted on the ceiling”.
2These classes can be considered instantiations of more general concepts:
“rigid” agents (such as robots, for which we only need to keep track a 3D
pose), and “deformable” agents (such as humans, for which we also need to
keep track of a time-varying shape).
Fig. 3: Structures: exploded view of walls and floor.
D. Layer 4: Rooms
This layer includes nodes describing rooms, corridors, and
halls. Room nodes (Fig. 2) have the following attributes: (i) 3D
pose, (ii) bounding box, and (iii) semantic class (e.g., kitchen,
dining room, corridor). Two rooms are connected by an edge
if they are adjacent (i.e., there is a door connecting them).
A room node has edges to the places (Layer 3) it contains
(since each place is connected to nearby objects, the DSG also
captures which object/agent is contained in each room). All
rooms are connected to the building they belong to (Layer 5).
E. Layer 5: Building
Since we are considering a representation over a single
building, there is a single building node with the following
attributes: (i) 3D pose, (ii) bounding box, and (iii) semantic
class (e.g., office building, residential house). The building
node has edges towards all rooms in the building.
F. Composition and Queries
Why should we choose this set of nodes or edges rather
than a different one? Clearly, the choice of nodes in the DSG
is not unique and is task-dependent. Here we first motivate
our choice of nodes in terms of planning queries the DSG
is designed for (see Remark 1and the broader discussion
in Section VI), and we then show that the representation is
compositional, in the sense that it can be easily expanded to
encompass more layers, nodes, and edges (Remark 2).
Remark 1 (Planning Queries): The proposed DSG is de-
signed with task and motion planning queries in mind. The
semantic node attributes (e.g., semantic class) support planning
from high-level specification (“pick up the red cup from the
table in the dining room”). The geometric node attributes (e.g.,
meshes, positions, bounding boxes) and the edges are used for
motion planning. For instance, the places can be used as a
topological graph for path planning, and the bounding boxes
can be used for fast collision checking.
Remark 2 (Composition of DSGs): A second re-ensuring
property of a DSG is its compositionality: one can easily
concatenate more layers at the top and the bottom of the DSG
in Fig. 1(a), and even add intermediate layers. For instance, in
a multi-story building, we can include a “Level” layer between
the “Building” and “Rooms” layers in Fig. 1(a). Moreover, we
can add further abstractions or layers at the top, for instance
going from buildings to neighborhoods, and then to cities.
IV. SPATIA L PERCEPTION ENGINE:
BUILDING A 3D DSGs F ROM SE NS OR DATA
This section describes a Spatial PerceptIon eNgine (SPIN)
that populates the DSG nodes and edges using sensor data. The
input to our SPIN is streaming data from a stereo camera and an
Inertial Measurement Unit (IMU). The output is a 3D DSG. In
our current implementation, the metric-semantic mesh and the
agent nodes are incrementally built from sensor data in real-
time, while the remaining nodes (objects, places, structure,
rooms) are automatically built at the end of the run.
Section IV-A describes how to obtain the metric-semantic
mesh and agent nodes from sensor data. Section IV-B de-
scribes how to segment and localize objects. Section IV-C
describes how to parse places, structures, and rooms.
A. From Visual-Inertial data to Mesh and Agents
Metric-Semantic Mesh. We use Kimera [88] to reconstruct
a semantically annotated 3D mesh from visual-inertial data in
real-time. Kimera is open source and includes four main mod-
ules: (i) Kimera-VIO: a visual-inertial odometry module im-
plementing IMU preintegration and fixed-lag smoothing [25],
(ii) Kimera-RPGO: a robust pose graph optimizer [67], (iii)
Kimera-Mesher: a per-frame and multi-frame mesher [89], and
(iv) Kimera-Semantics: a volumetric approach to produce a se-
mantically annotated mesh and an Euclidean Signed Distance
Function (ESDF) based on Voxblox [77]. Kimera-Semantics
uses a panoptic 2D semantic segmentation of the left camera
images to label the 3D mesh using Bayesian updates. We take
the metric-semantic mesh produced by Kimera-Semantics as
Layer 1 in the DSG in Fig. 1(a).
Robot Node. In our setup the only robotic agent is the one
collecting the data, hence Kimera-RPGO directly produces a
time-stamped pose graph describing the poses of the robot
at discrete time stamps. Since our robot moves in crowded
environments, we replace the Lukas-Kanade tracker in the VIO
front-end of [88] with an IMU-aware optical flow method,
where feature motion between frames is predicted using IMU
motion information, similar to [34]. Moreover, we use a 2-
point RANSAC [43] for geometric verification, which directly
uses the IMU rotation to prune outlier correspondences in the
feature tracks. To complete the robot node, we assume a CAD
model of the robot to be given (only used for visualization).
Human Nodes. Contrary to related work that models dy-
namic targets as a point or a 3D pose [1,6,18,58,84], we
track a dense time-varying mesh model describing the shape
of the human over time. Therefore, to create a human node
our SPIN needs to detect and estimate the shape of a human
in the camera images, and then track the human over time.
For shape estimation, we use the Graph-CNN approach of
Kolotouros et al. [46], which directly regresses the 3D location
of the vertices of an SMPL [64] mesh model from a single
image. An example is given in Fig. 4(a-b). More in detail,
given a panoptic 2D segmentation, we crop the left camera
image to a bounding box around each detected human, and
we use the approach [46] to get a 3D SMPL. We then extract
the full pose in the original perspective camera frame ([46]
uses a weak perspective camera model) using PnP [119].
To track a human, our SPIN builds a pose graph where each
node is assigned the pose of the torso of the human at a
discrete time. Consecutive poses are connected by a factor [20]
modeling a zero velocity prior. Then, each detection at time tis
modeled as a prior factor on the pose at time t. For each node
of the pose graph, our SPIN also stores the 3D mesh estimated
by [46]. For this approach to work reliably, outlier rejection
and data association become particularly important. The ap-
proach of [46] often produces largely incorrect poses when
the human is partially occluded. Moreover, in the presence of
multiple humans, one has to associate each detection dtto one
of the human pose graphs h(i)
1:t1(including poses from time
1 to t1for each human i= 1,2, . . .). To gain robustness,
our SPIN (i) rejects detections when the bounding box of the
human approaches the boundary of the image or is too small
(30 pixels in our tests), and (ii) adds a measurement to the
pose graph only when the human mesh detected at time tis
“consistent” with the mesh of one of the humans at time t1.
To check consistency, we extract the skeleton at time t1
(from the pose graph) and t(from the current detection) and
check that the motion of each joint (Fig. 4(c)) is physically
plausible in that time interval (i.e., we leverage the fact that
the joint and torso motion cannot be arbitrarily fast). We use
a conservative bound of 3m on the maximum allowable joint
displacement in a time interval of 1 second. If no pose graph
meets the consistency criterion, we initialize a new pose graph
with a single node corresponding to the current detection.
Besides using them for tracking, we feed back the human
detections to Kimera-Semantics, such that dynamic elements
are not reconstructed in the 3D mesh. We achieve this by only
using the free-space information when ray casting the depth
for pixels labeled as humans, an approach we dubbed dynamic
masking (see results in Fig. 5).
(a) Image (b) Detection (c) Tracking
Fig. 4: Human nodes: (a) Input camera image from Unity, (b)
SMPL mesh detection and pose/shape estimation using [46],
(c) Temporal tracking and consistency checking on the maxi-
mum joint displacement between detections.
B. From Mesh to Objects
Our spatial perception engine extracts static objects from the
metric-semantic mesh produced by Kimera. We give the user
the flexibility to provide a catalog of CAD models for some
of the object classes. If a shape is available, our SPIN will try
to fit it to the mesh (paragraph “Objects with Known Shape”
below), otherwise will only attempt to estimate a centroid and
bounding box (paragraph “Objects with Unknown Shape”).
Objects with Unknown Shape. The metric semantic mesh
from Kimera already contains semantic labels. Therefore,
our SPIN first exacts the portion of the mesh belonging to
a given object class (e.g., chairs in Fig. 1(d)); this mesh
potentially contains multiple object instances belonging to
the same class. Then, it performs Euclidean clustering using
PCL [95] (with a distance threshold of twice the voxel size
used in Kimera-Semantics, which is 0.1m) to segment the
object mesh into instances. From the segmented clusters,
our SPIN obtains a centroid of the object (from the vertices of
the corresponding mesh), and assigns a canonical orientation
with axes aligned with the world frame. Finally, it computes a
bounding box with axes aligned with the canonical orientation.
Objects with Known Shape. For objects with known shape,
our SPIN isolates the mesh corresponding to an object instance,
similarly to the unknown-shape case. However, if a CAD
model for that class of objects is given, our SPIN attempts
fitting the known shape to the object mesh. This is done in
three steps. First, we extract 3D keypoints from the CAD
model of the object, and the corresponding object mesh from
Kimera. The 3D keypoints are extracted by transforming each
mesh to a point cloud (by picking the vertices of the mesh)
and then extracting 3D Harris corners [95] with 0.15m radius
and 104non-maximum suppression threshold. Second, we
match every keypoint on the CAD model with any keypoint on
the Kimera model. Clearly, this step produces many incorrect
putative matches (outliers). Third, we apply a robust open-
source registration technique, TEASER++ [111], to find the best
alignment between the point clouds in the presence of extreme
outliers. The output of these three steps is a 3D pose of the
object (from which it is also easy to extract an axis-aligned
bounding box), see result in Fig. 1(e).
C. From Mesh to Places, Structures, and Rooms
This section describes how our SPIN leverages existing
techniques and implements simple-yet-effective methods to
parse places, structures, and rooms from Kimera’s 3D mesh.
Places. Kimera uses Voxblox [77] to extract a global mesh
and an ESDF. We also obtain a topological graph from the
ESDF using [78], where nodes sparsely sample the free space,
while edges represent straight-line traversability between two
nodes. We directly use this graph to extract the places and their
topology (Fig. 2(a)). After creating the places, we associate
each object and agent pose to the nearest place to model a
proximity relation.
Structures. Kimera’s semantic mesh already includes dif-
ferent labels for walls, ground floor, and ceiling, so isolating
these three structural elements is straightforward (Fig. 3). For
each type of structure, we then compute a centroid, assign
a canonical orientation (aligned with the world frame), and
compute an axis-aligned bounding box.
Rooms. While floor plan computation is challenging in
general, (i) the availability of a 3D ESDF and (ii) the
knowledge of the gravity direction given by Kimera enable
a simple-yet-effective approach to partition the environment
into different rooms. The key insight is that an horizontal 2D
section of the 3D ESDF, cut below the level of the detected
ceiling, is relatively unaffected by clutter in the room. This
2D section gives a clear signature of the room layout: the
voxels in the section have a value of 0.3m almost everywhere
(corresponding to the distance to the ceiling), except close to
the walls, where the distance decreases to 0m. We refer to this
2D ESDF (cut at 0.3m below the ceiling) as an ESDF section.
To compensate for noise, we further truncate the ESDF
section to distances above 0.2m, such that small openings
between rooms (possibly resulting from error accumulation)
are removed. The result of this partitioning operation is a
set of disconnected 2D ESDFs corresponding to each room,
that we refer to as 2D ESDF rooms. Then, we label all the
“Places” (nodes in Layer 3) that fall inside a 2D ESDF room
depending on their 2D (horizontal) position. At this point,
some places might not be labeled (those close to walls or
inside door openings). To label these, we use majority voting
over the neighborhood of each node in the topological graph
of “Places” in Layer 3; we repeat majority voting until all
places have a label. Finally, we add an edge between each
place (Layer 3) and its corresponding room (Layer 4), see
Fig. 2(b-c), and add an edge between two rooms (Layer 4)
if there is an edge connecting two of its places (red edges in
Fig. 2(b-c)). We also refer the reader to the video attachment.
V. EX PE RI ME NT S IN PH OTO -REALISTIC SIM UL ATOR
This section shows that the proposed SPIN (i) produces
accurate metric-semantic meshes and robot nodes in crowded
environments (Section V-A), (ii) correctly instantiates object
and agent nodes (Section V-B), and (iii) reliably parses large
indoor environments into rooms (Section V-C).
Testing Setup. We use a photo-realistic Unity-based sim-
ulator to test our spatial perception engine in a 65m×65m
simulated office environment. The simulator also provides the
2D panoptic semantic segmentation for Kimera. Humans are
simulated using the realistic 3D models provided by the SMPL
project [64]. The simulator provides ground-truth poses of
humans and objects, which are only used for benchmarking.
Using this setup, we create 3 large visual-inertial datasets, that
we release as part of the uHumans dataset [90]. The datasets,
labeled as uH_01,uH_02,uH_03, include 12, 24, and 60 humans,
respectively. We use the human pose and shape estimator [46]
out of the box, without any domain adaptation or retraining.
A. Robustness of Mesh Reconstruction in Crowded Scenes
Here we show that IMU-aware feature tracking and the use
of a 2-point RANSAC in Kimera enhance VIO robustness.
Moreover, we show that this enhanced robustness, combined
with dynamic masking (Section IV-A), results in robust and
accurate metric-semantic meshes in crowded environments.
Enhanced VIO. Table Ireports the absolute trajectory
errors of Kimera with and without the use of 2-point RANSAC
and when using 2-point RANSAC and IMU-aware feature
tracking (label: DVIO). Best results (lowest errors) are shown
in bold. The left part of the table (MH_01–V2_03) corresponds
to tests on the (static) EuRoC dataset. The results confirm
that in absence of dynamic agents the proposed approach
performs on-par with the state of the art, while the use of
2-point RANSAC already boosts performance. The last three
columns (uH_01uH_03), however, show that in the presence
of dynamic entities, the proposed approach dominates the
baseline (Kimera-VIO).
Dynamic Masking. Fig. 5visualizes the effect of dynamic
masking on Kimera’s metric-semantic mesh reconstruction.
Fig. 5(a) shows that without dynamic masking a human
walking in front of the camera leaves a “contrail” (in cyan)
and creates artifacts in the mesh. Fig. 5(b) shows that dynamic
TABLE I: VIO errors in centimeters on the EuRoC (MH and
V) and uHumans (uH) datasets.
Seq.
MH_01
MH_02
MH_03
MH_04
MH_05
V1_01
V1_02
V1_03
V2_01
V2_02
V2_03
uH_01
uH_02
uH_03
5-point 9.3 10 11 42 21 6.7 12 17 5 8.1 30 92 145 160
2-point 9.0 10 10 31 16 4.7 7.5 14 5.8 9 20 78 79 111
DVIO 8.1 9.8 14 23 20 4.3 7.8 17 6.2 11 30 59 78 88
(a) (b)
Fig. 5: 3D mesh reconstruction (a) without and (b) with
dynamic masking.
masking avoids this issue and leads to clean mesh reconstruc-
tions. Table II reports the RMSE mesh error (see accuracy
metric in [89]) with and without dynamic masking (label:
“with DM” and “w/o DM”). To assess the mesh accuracy
independently from the VIO accuracy, we also report the
mesh error when using ground-truth poses (label: “GT Poses”
in the table), besides the results with the VIO poses (label:
“DVIO Poses”). The “GT Poses” columns in the table show
that even with a perfect localization, the artifacts created by
dynamic entities (and visualized in Fig. 5(a)) significantly
hinder the mesh accuracy, while dynamic masking ensures
highly accurate reconstructions. The advantage of dynamic
masking is preserved when VIO poses are used.
TABLE II: Mesh error in meters with and without dynamic
masking (DM).
Seq.
GT Pose
w/o DM
GT Poses
with DM
DVIO Poses
w/o DM
DVIO Poses
with DM
uH_01 0.089 0.060 0.227 0.227
uH_02 0.133 0.061 0.347 0.301
uH_03 0.192 0.061 0.351 0.335
B. Parsing Humans and Objects
Here we evaluate the accuracy of human tracking and object
localization on the uHumans datasets.
Human Nodes. Table III shows the average localization
error (mismatch between the torso estimated position and
the ground truth) for each human on the uHumans datasets.
The first column reports the error of the detections produced
by [46] (label: “Single-img.”). The second column reports the
error for the case in which we filter out detections when the
human is only partially visible in the camera image, or when
the bounding box of the human is too small (30 pixels, label:
“Single-img. filtered”). The third column reports errors with
the proposed pose graph model discussed in Section IV-A (la-
bel: “Tracking”). The approach [46] tends to produce incorrect
estimates when the human is occluded. Filtering out detections
improves the localization performance, but occlusions due to
objects in the scene still result in significant errors. Instead,
the proposed approach ensures accurate human tracking.
TABLE III: Human and object localization errors in meters.
Humans Objects
Seq.
Single-img.
[46]
Single-img.
filtered
Tracking
(proposed)
Unknown
Objects
Known
Objects
uH_01 1.07 0.88 0.65 1.31 0.20
uH_02 1.09 0.78 0.61 1.70 0.35
uH_03 1.20 0.97 0.63 1.51 0.38
Object Nodes. The last two columns of Table III report the
average localization errors for objects of unknown and known
shape detected in the scene. In both cases, we compute the
localization error as the distance between the estimated and the
ground truth centroid of the object (for the objects with known
shape, we use the centroid of the fitted CAD model). We use
CAD models for objects classified as “couch”. In both cases,
we can correctly localize the objects, while the availability of
a CAD model further boosts accuracy.
C. Parsing Places and Rooms
The quality of the extracted places and rooms can be seen
in Fig. 2. We also compute the average precision and recall for
the classification of places into rooms. The ground truth labels
are obtained by manually segmenting the places. For uH_01 we
obtain an average precision of 99.89% and an average recall
of 99.84%. Incorrect classifications typically occur near doors,
where room misclassification is inconsequential.
VI. DISCUSSION: QUERIES AND OPPORTUNITIES
We highlight the actionable nature of a 3D Dynamic Scene
Graph by providing examples of queries it enables.
Obstacle Avoidance and Planning. Agents, objects, and
rooms in our DSG have a bounding box attribute. Moreover,
the hierarchical nature of the DSG ensures that bounding boxes
at higher layers contain bounding boxes at lower layers (e.g.,
the bounding box of a room contains the objects in that
room). This forms a Bounding Volume Hierarchy (BVH) [53],
which is extensively used for collision checking in computer
graphics. BVHs provide readily available opportunities to
speed up obstacle avoidance and motion planning queries
where collision checking is often used as a primitive [40].
DSGs also provide a powerful tool for high-level planning
queries. For instance, the (connected) subgraph of places and
objects in a DSG can be used to issue the robot a high-level
command (e.g., object search [38]), and the robot can directly
infer the closest place in the DSG it has to reach to complete
the task, and can plan a feasible path to that place.
The multiple levels of abstraction afforded by a DSG have
the potential to enable hierarchical and multi-resolution plan-
ning approaches [52,97], where a robot can plan at different
levels of abstraction to save computational resources.
Human-Robot Interaction. As already explored in [5,41],
a scene graph can support user-oriented tasks, such as inter-
active visualization and Question Answering. Our Dynamic
Scene Graph extends the reach of [5,41] by (i) allowing visu-
alization of human trajectories and dense poses (see visualiza-
tion in the video attachment), and (ii) enabling more complex
and time-aware queries such as “where was this person at
time t?”, or “which object did this person pick in Room A?”.
Furthermore, DSGs provide a framework to model plausible
interactions between agents and scenes [31,70,82,115]. We
believe DSGs also complement the work on natural language
grounding [44], where one of the main concerns is to reason
over the variability of human instructions.
Long-term Autonomy. DSGs provide a natural way to “for-
get” or retain information in long-term autonomy. By construc-
tion, higher layers in the DSG hierarchy are more compact and
abstract representations of the environment, hence the robot
can “forget” portions of the environment that are not frequently
observed by simply pruning the corresponding branch of the
DSG. For instance, to forget a room in Fig. 1, we only need
to prune the corresponding node and the connected nodes
at lower layers (places, objects, etc.). More importantly, the
robot can selectively decide which information to retain: for
instance, it can keep all the objects (which are typically fairly
cheap to store), but can selectively forget the mesh model,
which can be more cumbersome to store in large environments.
Finally, DSGs inherit memory advantages afforded by standard
scene graphs: if the robot detects Ninstances of a known
object (e.g., a chair), it can simply store a single CAD model
and cross-reference it in Nnodes of the scene graph; this
simple observation enables further data compression.
Prediction. The combination of a dense metric-semantic
mesh model and a rich description of the agents allows
performing short-term predictions of the scene dynamics and
answering queries about possible future outcomes. For in-
stance, one can feed the mesh model to a physics simulator
and roll out potential high-level actions of the human agents;
VII. CONCLUSION
We introduced 3D Dynamic Scene Graphs as a unified
representation for actionable spatial perception, and presented
the first Spatial PerceptIon eNgine (SPIN) that builds a DSG
from sensor data in a fully automatic fashion. We showcased
our SPIN in a photo-realistic simulator, and discussed its
application to several queries, including planning, human-
robot interaction, data compression, and scene prediction. This
paper opens several research avenues. First of all, many of the
queries in Section VI involve nontrivial research questions and
deserve further investigation. Second, more research is needed
to expand the reach of DSGs, for instance by developing
algorithms that can infer other node attributes from data
(e.g., material type and affordances for objects) or creating
new node types for different environments (e.g., outdoors).
Third, this paper only scratches the surface in the design
of spatial perception engines, thus leaving many questions
unanswered: is it advantageous to design SPINs for other sensor
combinations? Can we estimate a scene graph incrementally
and in real-time? Can we design distributed SPINs to estimate
aDSG from data collected by multiple robots?
ACKNOWLEDGMENTS
This work was partially funded by ARL DCIST CRA
W911NF-17-2-0181, ONR RAIDER N00014-18-1-2828,
MIT Lincoln Laboratory, and “la Caixa” Foundation (ID
100010434), LCF/BQ/AA18/11680088 (A. Rosinol).
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... Both PanopticFusion and ATLAS works only for Paper Sem Obj Pan Dyn Opt Syn MeshRCNN [17] Total3D [43] Atlas [40] SLAM++ [54] . PanopticFusion [41] Kimera [52] DynSceneGraphs [53] SemanticNerF [70] NSG [44] ObjectNeRF [65] PNF (Ours) Table 1. Comparison to properties of related work. ...
... Kimera [52] takes a stereo sequence and does online reconstruction, meshing, and semantic labeling of the mesh using ground-truth labels, as a proxy for any 2D segmentation method. Dynamic Scene Graphs [53] expands on that by inferring object instances, even dynamic ones in case of people. Both methods, though representing impressive systems, were only demonstrated in simulation and rely on ground truth semantic labels. ...
Preprint
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We present Panoptic Neural Fields (PNF), an object-aware neural scene representation that decomposes a scene into a set of objects (things) and background (stuff). Each object is represented by an oriented 3D bounding box and a multi-layer perceptron (MLP) that takes position, direction, and time and outputs density and radiance. The background stuff is represented by a similar MLP that additionally outputs semantic labels. Each object MLPs are instance-specific and thus can be smaller and faster than previous object-aware approaches, while still leveraging category-specific priors incorporated via meta-learned initialization. Our model builds a panoptic radiance field representation of any scene from just color images. We use off-the-shelf algorithms to predict camera poses, object tracks, and 2D image semantic segmentations. Then we jointly optimize the MLP weights and bounding box parameters using analysis-by-synthesis with self-supervision from color images and pseudo-supervision from predicted semantic segmentations. During experiments with real-world dynamic scenes, we find that our model can be used effectively for several tasks like novel view synthesis, 2D panoptic segmentation, 3D scene editing, and multiview depth prediction.
... Recent works take this a step further by introducing objects into the map representation. Some methods use 3D CAD models to identify a set of pre-defined objects in the scene [27], while others construct unseen objects on-the-fly [28,29]. However, these methods still adopt the static world assumption, which is unrealistic for a robot deployed in the real world for extended operations. ...
... 3) The poses of the robot can be obtained from an existing localization system such as in [29]. 4) Scene changes are due to the addition, removal, or planar motion of objects between robot traversals. ...
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... Besides ceilings, floors and wall surfaces, some approaches also aim at reconstructing other building elements such as furniture [588,647,492], elements affixed to room surfaces such as fire alarms or power plugs [6,556] or door openings [143,38,176,653]. The detection of door openings as transition spaces between rooms is also of importance in the context of applications aiming to reconstruct the room topology of indoor environments [575,492,653]. ...
... Besides ceilings, floors and wall surfaces, some approaches also aim at reconstructing other building elements such as furniture [588,647,492], elements affixed to room surfaces such as fire alarms or power plugs [6,556] or door openings [143,38,176,653]. The detection of door openings as transition spaces between rooms is also of importance in the context of applications aiming to reconstruct the room topology of indoor environments [575,492,653]. Besides the topology of rooms, some approaches also focus on extracting the topology of wall structures [49,673]. ...
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... They provide an example of a single-layer indoor environment which includes 5 layers (from low to high abstraction level): Metric-Semantic Mesh, Objects and Agents, Places and Structures, Rooms, and Building. Whether it is a four [44] -or five-story [172] structure, we can get a hint that 3D scene contains rich semantic information that goes far beyond the 2D scene graph representation. ...
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... Recent approaches such as [7], [8] model the scene as a graph, in order to efficiently represent the environment and its semantic elements in a hierarchical representation with structural and topological constraints between the elements. Scene graphs might enable the robots to understand and navigate the environment similarly to humans, using highlevel abstractions (such as chairs, tables, walls) and the inter-connections between them (such as a set of walls forming a room or a corridor). ...
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Robot planning in partially observable domains is difficult, because a robot needs to estimate the current state and plan actions at the same time. When the domain includes many objects, reasoning about the objects and their relationships makes robot planning even more difficult. In this paper, we develop an algorithm called scene analysis for robot planning (SARP) that enables robots to reason with visual contextual information toward achieving long-term goals under uncertainty. SARP constructs scene graphs, a factored representation of objects and their relations, using images captured from different positions, and reasons with them to enable context-aware robot planning under partial observability. Experiments have been conducted using multiple 3D environments in simulation, and a dataset collected by a real robot. In comparison to standard robot planning and scene analysis methods, in a target search domain, SARP improves both efficiency and accuracy in task completion. Supplementary material can be found at https://tinyurl.com/sarp22
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This paper presents an algorithm for indoor layout estimation and reconstruction through the fusion of a sequence of captured images and LiDAR data sets. In the proposed system, a movable platform collects both intensity images and 2D LiDAR information. Pose estimation and semantic segmentation is computed jointly by aligning the LiDAR points to line segments from the images. For indoor scenes with walls orthogonal to floor, the alignment problem is decoupled into top-down view projection and a 2D similarity transformation estimation and solved by the recursive random sample consensus (R-RANSAC) algorithm. Hypotheses can be generated, evaluated and optimized by integrating new scans as the platform moves throughout the environment. The proposed method avoids the need of extensive prior training or a cuboid layout assumption, which is more effective and practical compared to most previous indoor layout estimation methods. Multi-sensor fusion allows the capability of providing accurate depth estimation and high resolution visual information.
Article
2020 IEEE. We provide an open-source C++ library for real-time metric-semantic visual-inertial Simultaneous Localization And Mapping (SLAM). The library goes beyond existing visual and visual-inertial SLAM libraries (e.g., ORB-SLAM, VINS-Mono, OKVIS, ROVIO) by enabling mesh reconstruction and semantic labeling in 3D. Kimera is designed with modularity in mind and has four key components: a visual-inertial odometry (VIO) module for fast and accurate state estimation, a robust pose graph optimizer for global trajectory estimation, a lightweight 3D mesher module for fast mesh reconstruction, and a dense 3D metric-semantic reconstruction module. The modules can be run in isolation or in combination, hence Kimera can easily fall back to a state-of-the-art VIO or a full SLAM system. Kimera runs in real-time on a CPU and produces a 3D metric-semantic mesh from semantically labeled images, which can be obtained by modern deep learning methods. We hope that the flexibility, computational efficiency, robustness, and accuracy afforded by Kimera will build a solid basis for future metric-semantic SLAM and perception research, and will allow researchers across multiple areas (e.g., VIO, SLAM, 3D reconstruction, segmentation) to benchmark and prototype their own efforts without having to start from scratch.