Image Segmentation with a Bounding Box...
Transcript of Image Segmentation with a Bounding Box...
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Image Segmentation with a Bounding Box Prior
Victor Lempitsky, Pushmeet Kohli, Carsten Rother, Toby SharpMicrosoft Research Cambridge
Dylan Rhodes and Jasper Lin
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Presentation Overview
● Segmentation problem description● Background and Previous Work● Problems and Proposed Solutions
○ Formalizing tightness○ Defining tractable optimization problem for
segmentation○ Discretizing continuous approximation of solution
● Experiments and Results
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Presentation Overview
● Segmentation problem description● Background and Previous Work● Problems and Proposed Solutions
○ Formalizing tightness○ Defining tractable optimization problem for
segmentation○ Discretizing continuous approximation of solution
● Experiments and Results
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Segmentation Problem
How does one separate the foreground from the background with minimal user input?
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Bounding Box
● Allows the algorithm to focus on subimage● Desired segmentation is close to sides of bounding
box
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Bounding Box
● Allows the algorithm to focus on subimage● Desired segmentation is close to sides of bounding
box
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Presentation Overview
● Segmentation problem description● Background and Previous Work● Problems and Proposed Solutions
○ Formalizing tightness○ Defining tractable optimization problem for
segmentation○ Discretizing continuous approximation of solution
● Experiments and Results
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Basic Formulation
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Basic Formulation
B is the set of pixels within the bounding box
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Basic Formulation
E is the set of adjacent pixels within the bounding box
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Basic Formulation
p and q are pixel indices
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Basic Formulation
x_p can take a label of 1 for foreground or 0 for background
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Basic Formulation
Unary potentials encode preference for foreground or background
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Basic Formulation
Pairwise potentials enforce smoothness of the solution
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Related Work
● Nowozin and Lampert derived framework for segmentation under connectivity constraint
● Relax NP-hard integer problem and solve resulting LP
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Nowozin and Lampert
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Nowozin and Lampert
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Nowozin and Lampert
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Presentation Overview
● Segmentation problem description● Background and Previous Work● Problems and Proposed Solutions
○ Formalizing tightness○ Defining tractable optimization problem for
segmentation○ Discretizing continuous approximation of solution
● Experiments and Results
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Presentation Overview
● Segmentation problem description● Background and Previous Work● Problems and Proposed Solutions
○ Formalizing tightness○ Defining tractable optimization problem for
segmentation○ Discretizing continuous approximation of solution
● Experiments and Results
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Why tightness?
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Tightness Definition
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Corollary
A shape x is strongly tight if and only if its intersection with the middle box has a connected component touching all four sides of the middle box
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Energy Minimization Problem
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Presentation Overview
● Segmentation problem description● Background and Previous Work● Problems and Proposed Solutions
○ Formalizing tightness○ Defining tractable optimization problem for
segmentation○ Discretizing continuous approximation of solution
● Experiments and Results
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Energy Minimization Problem
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Energy Minimization Problem
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Convex Continuous Relaxation
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Convex Continuous Relaxation
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Continuous Optimization
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Continuous Optimization
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Additional Approximation
Intuition: Solve LP with a subset Γ' of the constraints in 3c activated
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Calculating Γ'
1. Begin with Γ' = ∅2. Solve the LP3. Pick a group of crossing paths from Γ \ Γ' which are
violated by more than a small tolerance 4. Add these paths to Γ'5. Repeat steps 2 through 4 until all paths in Γ are
satisfied within the tolerance
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Final Form
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Presentation Overview
● Segmentation problem description● Background and Previous Work● Problems and Proposed Solutions
○ Formalizing tightness○ Defining tractable optimization problem for
segmentation○ Discretizing continuous approximation of solution
● Experiments and Results
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Pinpointing Algorithm
Normally, output of LP is rounded to integer solution
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Pinpointing Algorithm
● Pinpoint set Π contains pixels hard-assigned to foreground
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Pinpoint Algorithm
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Pinpoint Algorithm
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Challenges
● Existing methods perform energy-driven shrinking over bounding box○ No guarantees optimization won’t shrink excessively
○ Stuck at poor local minima
○ Discretization of approximate solution is noisy
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Paper’s Contributions
● Common methods initialize foreground region and perform energy-driven shrinking○ No guarantees optimization won’t shrink excessively
Solution: new tightness prior○ Stuck at poor local minima
○ Discretization of approximate solution is noisy
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Paper’s Contributions
● Common methods initialize foreground region and perform energy-driven shrinking○ No guarantees optimization won’t shrink excessively
Solution: new tightness prior○ Stuck at poor local minima
Solution: new approximation strategies○ Discretization of approximate solution is noisy
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Paper’s Contributions
● Common methods initialize foreground region and perform energy-driven shrinking○ No guarantees optimization won’t shrink excessively
Solution: new tightness prior○ Stuck at poor local minima
Solution: new approximation strategy○ Discretization of approximate solution is noisy
Solution: new pinpointing algorithm
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Presentation Overview
● Segmentation problem description● Background and Previous Work● Problems and Proposed Solutions
○ Formalizing tightness○ Defining tractable optimization problem for
segmentation○ Discretizing continuous approximation of solution
● Experiments and Results
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Experiments
● Evaluated over 50 image GrabCut dataset○ Each image comes with bounding box
● Comparison with competing methods and initialization strategies
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GrabCut Dataset
● 50 natural images with bounding box annotations○ Includes background, outside strip, and foreground
bounding boxes
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GrabCut Dataset
● 50 natural images with bounding box annotations○ Includes background, outside strip, and foreground
bounding boxes
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Unary and Pairwise Terms
● Pairwise terms over 8-connected edge set
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Relative Performance
Error rate - mislabeled pixels inside bounding boxOptimum Rank - average rank of energy of final integer program solutions
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Relative Performance
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Iterative Process
● Compare the following algorithms on the segmentation task:○ GrabCut with standard graph cut minimization for
all segmentation steps○ GrabCut which enforces the tightness prior for all
segmentation steps● 5 iterations each
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Initialization Strategies
● Compare two methods for initializing foreground/background GMMs:○ InitThirds = same as Experiment 1 (outside strip +
best matches vs. poor matches) ○ InitFullBox which sets background GMM to
outside strip and foreground to whole interior of bounding box
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Iterative Process
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Effect of Margin Thickness
Error rates as function of margin thickness
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Strong vs. Weak Tightness
● Strong and weak tightness lead to similar error rates in general○ same error rate (3.7%) for best model (GrabCut-
Pinpoint/InitThirds)
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Iterative Process Comparisons
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Conclusions
● New bounding-box based prior for interactive image segmentation
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Conclusions
● New bounding-box based prior for interactive image segmentation
● Demonstrated segmentation tasks under this prior can be formulated as integer programs
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Conclusions
● New bounding-box based prior for interactive image segmentation
● Demonstrated segmentation tasks under this prior can be formulated as integer programs
● Developed new optimization approaches for approximate solution of these NP-hard problems○ Can be applied to other computer vision problems e.g.
other image segmentation or silhouettes in multi-view stereo
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Thank you!
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