Visual sensing and 3D mapping

Report
3D Sensing and
Mapping
Intelligent Robotics
Spring 2014
Kris Hauser
Agenda
• A high-level overview of visual sensors and perception
algorithms
• Core concepts
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Camera / projective geometry
Point clouds
Occupancy grids
Iterative closest points algorithm
• Proprioceptive: sense one’s own body
• Motor encoders (absolute or relative)
• Contact switches (joint limits)
• Inertial: sense accelerations of a link
• Accelerometers
• Gyroscopes
• Inertial Measurement Units (IMUs)
• Visual: sense 3D scene with reflected light
• RGB: cameras (monocular, stereo)
• Depth: lasers, radar, time-of-flight, stereo+projection
• Infrared, etc
• Tactile: sense forces
• Contact switches
• Force sensors
• Pressure sensors
• Other
• Motor current feedback: sense effort
• GPS
• Sonar
Sensing vs Perception
• Sensing: acquisition of signals from hardware
• Perception: processing of “raw” signals into “meaningful”
representations
Digits
• Example:
• Reading pixels from a camera is sensing.
• Declaring “it’s a rounded shape with skin color”, “it’s a face”, or
“it’s Joe’s smiling face” are different levels of perception.
• Especially at lower levels of perception like signal processing,
the line is blurry, and the often the processed results can be
essentially considered “sensed”.
3D Perception Topics
• Sensors: visible-light cameras, depth sensors, laser sensors
• (Some) perception tasks:
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Stereo reconstruction
Object recognition
3D mapping
Object pose recognition
• Key issues:
• How to represent and optimize camera transforms?
• How to fit models in the presence of noise?
• How to represent large 3D models?
Visual Sensors
• Visible-light cameras: cheap, low power,
high resolution, high frame rates
• Data: 2D field of RGB pixels
• Stereo cameras
Bumblebee stereo camera
• Depth field sensors
• Two major types: infrared pattern
projection (Kinect, ASUS, PrimeSense)
and time-of-flight (SwissRanger)
• Data: 2D field of depth values
ASUS Xtion depth sensor
• Sweeping laser sensors
• Data: 1D field of depth values
• Hokuyo, SICK, Velodyne
• Can be mounted on a tilt/spin mount to
get 3D field of view
Hokuyo laser sensor
Sensors vary in
strengths / weaknesses
• Velodyne (DARPA grand challenge)
• 1.3 million readings/s
• $75k price tag
Image formation
• Light bounces off an object, passes through a lens and lands
on a CCD pixel on the image plane
• Depth of focus
• Illumination and aperture
• Color: accomplished through use
of filters, e.g. Bayer filter
• Each channel’s in-between pixels
are interpolated
Idealized projective geometry
• Let:
• Zim: distance from image plane to focal point along the depth axis
• (X,Y,Z): point in 3D space relative to focal point, Z > 0
• Then, image space point is:
• Xc = Zim X / Z
• Yc = Zim Y / Z
x
Focal point
…Which get scaled and offset to get
pixel coordinates
Image plane
(X,Z)
(Xc,Zim)
z
Issues with real sensors
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Motion blur
Distortion caused by lenses
Non-square pixels
Exposure
Noise
Motion blur
• Film grain
• Salt-and-pepper noise
• Shot noise
Distortion
Exposure
Calibration
• Determine camera’s intrinsic
parameters
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Focal length
Field of view
Pixel dimensions
Radial distortion
• That determine the mapping
from image pixels to an
idealized pinhole camera
• Rectification
Stereo vision processing
• Dense reconstruction
• Given two rectified images, find binocular disparity at each pixel
• Take a small image patch around each pixel in the left image,
search for the best horizontal shifted copy in the right image
• What size patch? What search size? What matching criterion?
• Works best for highly textured scenes
Point Clouds
• Unordered list of 3D points P={p1,…,pn}
• Each point optionally annotated by:
• Color (RGB)
• Sensor reading ID# (why?)
• Estimated surface normal (nx,ny,nz)
• No information about objects,
occlusions, topology
• Point Cloud Library (PCL)
• http://pointclouds.org
3D Mapping
• Each frame of a depth sensor gives a narrow snapshot of the
world geometry from a given position
• 3D mapping is the process of stitching multiple views into a
global model
Three scenarios:
• Consider two raw point clouds P1 and P2 from cameras with
transformations T1 and T2.
• Goal: build point cloud P in frame T1 (assume identity)
• Case 1: relative transformations known
• Simple union P = P1  (T2  P2)
• Case 2: small transformation
• Pose registration problem
• Vast majority of points correspond between scenes
• Case 3: large transformation
• Significant fraction of points do not correspond, lighting
differences, more occlusions
• Pose registration must be more robust to outliers
Case 2: Small transformations
• Visual odometry: estimate relative motion of subsequent
frames using optical flow
• Define feature points in P1 (e.g., corner detector)
• Estimate a transformation of an image patch around feature that
best matches P2 (defines optical flow field)
• Transformation: translation, rotation, scale
• Fit T2 to match these feature transforms
Case 3: Large transformations
• Iterative closest point algorithm
• Input: initial guess for T2
• Repeat until convergence:
• Find nearest neighbor pairings between P1 and T2P2
• Select those pairs that fall below some distance threshold (outlier
rejection)
• Assign an error metric and optimize T2 to minimize this metric
Case 3: Large transformations
• Iterative closest point algorithm
• Input: initial guess for T2
• Repeat until convergence:
What metric?
• Find nearest neighbor pairings between P1 and T2P2
• Select those pairs that fail to satisfy some distance criteria
(outlier rejection)
What criteria?
• Assign an error metric and optimize T2 to minimize this metric
How to minimize?
What metrics for matching?
• Position
• Surface normal
• Color
• Nearest neighbor methods
• Fast data structures, e.g. K-D trees
• For large scans usually want a constant sized subsample
• Projection-based methods
• Render scene from perspective of T1 to determine a match
• Very fast (used in Kinect Fusion algorithm)
• Only uses position information
What criteria for outlier
rejection?
• Distance too large (e.g., top X%)
• Inconsistencies with neighboring pairs
• On boundary of scan
How to optimize?
• Want to find rotation R, translation t of T2 to minimize some
error function
• Sum of squared point-to-point differences
•  ,  = =1  − ( + )
• Closed-form solution (SVD)
• Very fast per step
2
• Sum of squared point-to-plane differences
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=1
 
 (
2
 ,  =
−
+ )
Must use numerical methods
Deal with rotation variable
Tends to lead ICP to converge with fewer iterations
Other applications of ICP
• Fitting known, complete 3D triangulated models to partial
point clouds for object recognition / pose estimation
Stitching multiple scans
• In its most basic form, multi-view 3D mapping is simply a
repetition of the two-camera case
• But two major issues:
• Drift and “closing the loop”
• Point clouds become massive after many scans
This time
Point cloud growth problem
• With N points, at F frames/sec, and T seconds of run time,
NFT points are gathered
• Kinect: N=307200, F=30, T=60 => 552,960,000 points
• With RGB in 4 bytes, XYZ in 12 bytes => 8 GB / min
• Solutions:
• Forget earlier scans (short term memory)
• Build persistent, “collapsed” representation of environment
geometry
• Polygon meshes
• Occupancy grids
• Key issue: how to estimate with low # of points and update later?
Occupancy grids
• Store a grid G with a fixed minimum resolution
• Mark which cells (voxels) are occupied by a point
• Representation size
is independent of T
Two options for updating
on a new scan
1. Compute ICP to align
current scan to prior
scan, then add points
to G
2. Modify ICP to work
directly with the
representation G
Probabilistic Occupancy Grids
with Ray Casting
• Scans are noisy, so simply adding points is likely to
overestimate occupied cells
• Ray casting approaches:
• Each cell has a probability of being free/occupied/unseen
• Each scan defines a line segment that passes through free space
and ends in an occupied cell (or near one)
• Walk along the segment, increasing P(free(c)) of each
encountered cell c, and finally increase P(occupied(c)) for the
terminal cell c
Probabilistic Occupancy Grids
with Ray Casting
• Scans are noisy, so simply adding points is likely to
overestimate occupied cells
• Ray casting approaches:
• Each cell has a probability of being free/occupied/unseen
• Each scan defines a line segment that passes through free space
and ends in an occupied cell (or near one)
• Walk along the segment, increasing P(free(c)) of each
encountered cell c, and finally increase P(occupied(c)) for the
terminal cell c
Compact geometry
representations within a cell
• On-line averaging
• On-line least squares estimation of fitting plane
Handling large 3D grids
• Problem: tabular 3D grid storage
increases with O(N3)
• 10243 is 1Gb
• Solutions
• Store hash table only of occupied
cells
• Octree data structure
• OctoMap Library
(http://octomap.sourceforge.net)
Dynamic Environments
• Real environments have people, animals, objects that move
around
• Two options:
• Map static parts by assuming dynamic objects will average out as
noise over time (probabilistic occupancy grids)
• Segment (and possibly model) dynamic objects
Related topics
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Sensor fusion
Object segmentation and recognition
Simultaneous Localization and Mapping (SLAM)
Next-best-view planning
Next week
• Computer Vision guest lecture
• Kalman filtering
• Principles Ch. 8
• Zeeshan

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