Fitting multiple structures to geometric data the j linkage approach
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Fitting multiple structures to geometric data: the J-linkage approach. Roberto Toldo and Andrea Fusiello University of Verona University of Udine. The problem. Fitting multiple instances of a model to data corrupted by noise and outliers. Two types of outliers. Gross outlliers

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Fitting multiple structures to geometric data: the J-linkage approach

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Fitting multiple structures to geometric data the j linkage approach

Fitting multiple structures to geometric data: the J-linkage approach

Roberto Toldo and

Andrea Fusiello

University of Verona

University of Udine

Big Data - Udine 5/6/2013


The problem

The problem

  • Fitting multiple instancesof a modelto data corruptedbynoise and outliers


Two types of outliers

Twotypesofoutliers

  • Grossoutlliers

  • Pseudo-outliers


Previous work

Previous work

  • Sequential RANSAC: theoretically wrong

  • MultiRANSAC [Zuliani et al. ICIP05]: problems with intersecting models

  • Residual Histogram Analysis [Zhang&Koseka ECCV06]: peaks finding is unreliable

  • Mode finding in parameter space.

    • Randomized HT: discretization is critical

    • Mean Shift clustering: not rubust enough


Random sampling consensus

Randomsamplingconsensus

  • Draw minimal sample sets (MSS) from data points

  • Fit a model to each MSS

  • Build the consensus set of the model: the set of points such that their distance to the model is below a given threshold (inlier band)

  • Select the model with the highest consensus


Random sampling consensus1

Randomsamplingconsensus

  • The number of MSS to be drawn must be large enough to guarantee that at least an outlier-free MSS is selected with high probability.

  • The assumption is that an outliers-free MSS will achieve the highest consensus, because inliers are structured whereas outliers are random

  • RANSAC looks at the problem from the model’s viewpoint


The preference set

The preference set

  • Let us look at the problem from the perspective of the points.

  • The Preference Set (PS) of a point is the set of models it belongs to.

CS of model j

PS of point i


Conceptual representation

Conceptualrepresentation

  • The PS (or its characteristic function) is a conceptual representationof a point.

  • In Pattern Recognition, the conceptual representation of an object x given C classes is: [ P(x | class 1) · · · P(x| class C ) ].

  • Conjecture: points belonging to the same model have “similar” conceptual representations.

  • In other words, they cluster in the conceptual space.


Jaccard distance

Jaccarddistance

  • Models are extracted by agglomerative clustering in the conceptual space using the Jaccard distance:

A

B

A ∪ B

A ∩ B

∑=8

∑=2

dj=6/8=0.75


J linkage

J-linkage

  • Define the PS of a cluster as the intersection of the PS of all its points.

  • Start with one cluster for each point

  • Pick the two cluster with the smallest J-distance and merge them

  • Repeat 2 while the smallest J-distance < 1

  • Postcondition: all the clusters have distance 1 (their PS do not overlap)


J linkage properties

J-linkage: properties

model that fits all the points of cluster 2

model that fits all the points of clusters 1 & 2

model that fits all the points of cluster 1

  • Foreach cluster, thereis a modelthatfitsall the point, otherwisetheywouldhavedistance = 1

  • A modelcannotfitall the pointsoftwoclusters, otherwisetheywouldhavedistance < 1

cluster 2

points

cluster 1


How many clusters

How many clusters?

  • Outliersemergesassmallclusters

  • If the numberMofmodelsisknown, the largestMclusters are retained

  • If the overallnumberofinlier can beestimated, the largestclusters up to the numbetofinliers are retained

  • Modelselectiontools can help to solve thisissue


What about the inlier threshold

What about the inlier threshold?

  • We presented [ICIAP 09] a technique based on clustering validation that is able to automatically select the “just right” threshold.


Continous relaxation

Continousrelaxation

  • The votingfunctionin J-linkageis a stepfunction (indicatorfunctionof the inlier band)

  • Idea: choose a soft votingfunctionswithvalues in [0,1]

  • ris the residual

  • the time constant τplays the role of the inlier theshold


Continous relaxation1

Continousrelaxation

  • Instead of the characteristic function of the preference set now the preference vector of a point has entries in [0,1] as produced by the soft voting function.

  • The Jaccard distance is generalized by the Tanimoto distance:

  • where p,q are the preference vectors


Continous relaxation2

Continous relaxation

  • The preferencevectorof a cluster isobtainedas the componentwise minimum among the preferencevectorsof the cluster (generalizes the logical AND ofpreferencevectors)

  • The soft J-linkageproceedsasits discrete version

  • Post-conditions:

    • Atleastonemodel in a cluster hasvotes >0fromall the points in the cluster (itfitsall the points)

    • A modelcannothavevotes >0fromall the points in two separate cluster


Experiments

Experiments

  • Each model consists of 50 inliers, corrupted by variable Gaussian noise and variable outliers percentage.

  • Compared to: sequential RANSAC, multiRANSAC, residual histogram analysis (RHA) and Mean-Shift.

  • Same samples.

  • Same inlier threshold

  • Parameters needed by MS (bandwidth) and by RHA have been optimized manually.

  • Number of models is given.


Real data

Real data

  • The motivation for this work is fitting 3D primitives (planes, cylinders) to cloud of 3D points provided by a SaM reconstruction pipeline.


Plane fitting

Plane fitting


Questions

Questions?


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