1 Motion Segmentation and Dense Reconstruction of Scenes Containing Moving Objects Observed by a Moving Camera Chang Yuan Institute of Robotics and Intelligent Systems Computer Science Department Viterbi School of Engineering University of Southern California 2/49 Problem Definition • Scenario: rigidly moving objects + moving camera • Goal • 2D motion segmentation: motion regions / background area • 3D dense reconstruction: object shape / background structure 3/49 2D Motion Segmentation 4/49 3D Shape + Trajectory Reconstruction
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Motion Segmentation andDense Reconstruction of Scenes
Containing Moving Objects Observed by a Moving Camera
Chang Yuan
Institute of Robotics and Intelligent Systems
Computer Science Department
Viterbi School of Engineering
University of Southern California2/49
Problem Definition
• Scenario: rigidly moving objects + moving camera
• Goal• 2D motion segmentation: motion regions / background area
• 3D dense reconstruction: object shape / background structure
3/49
2D Motion Segmentation
4/49
3D Shape + Trajectory Reconstruction
2
5/49
Challenges & Applications
• Information sources
• Pixel colors + 2D coordinates
• No object model information is available
• Difficulties
• Camera motion
• Multiple moving objects
• 3D static structures (parallax)
• Applications
• Video surveillance
• Image synthesis
• …
6/49
Overview of the Approach
Dynamic voxel
coloring scheme (CVPR’07, Journal(?))
My contributions:
2D => 3D
Sparse => Dense
Sparse => Dense
Parallax rigidity constraint
(ICCV’05, PAMI)
Planar-motion constraint
(CVPR’06, PAMI(?))
7/49
Outline
• Introduction
• 2D Shape Recovery
• Multi-image registration
• Motion segmentation
• Object tracking
• 3D Shape Recovery
• Sparse reconstruction
• Dense volumetric reconstruction
• Summary and Discussion
Math background:
• Linear algebra
• Optimization
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Outline
• Introduction
• 2D Shape Recovery
• Multi-image registration
• Motion segmentation
• Object tracking
• 3D Shape Recovery
• Sparse reconstruction
• Dense volumetric reconstruction
• Summary and Discussion
3
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Motion Segmentation – Overview
• Task: to detect moving objects and track them
• Assumptions• General camera motion
• Distant scene
• Textured background
10/49
Motion Segmentation – Related Work
• Detecting moving objects from static cameras
• Background modeling
• Frame subtraction
• Optical flow based segmentation
• Motion layers (not necessarily a moving object)
• Point clustering
• Divide sparse feature matches into different motion groups
• “Plane+Parallax” approaches
• A constant reference plane + off-plane structure (parallax)
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Feature Extraction & Matching
• Salient parts of the scene
• Extraction• Harris corners
• Multi-scale
• Multi-orientation
• Sub-pixel accuracy
• Matching• Small inter-frame motion
• Gray-scale windows
• Cross correlation
• Large viewpoint change
• Gradient histogram
• Vector angle
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Multiple Image Registration
• Frame motion model
• Assumptions:
• Small inter-frame motion
• Distant planar scene
• 2D affine transform
• Robust estimation
• Random Sample Consensus
(RANSAC)
• Keep the model with the
largest number of inliers
• Non-linear refinement over
the inliers
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Frame t-w Frame t+w
Frame t
t: reference framew: half size of the window
Initial Motion Segmentation (1)
• Two-frame pixel-level segmentation?
• Segmentation within a temporal window
• Accumulate the pixels warped from adjacent frames
• K-Means to find the most representative pixel
• Frame differencing and thresholding: |Ioriginal-Imodel|>ΔI