Unfolding Manifolds: Locally Linear KIsomaps - PowerPoint PPT Presentation

Title: Unfolding Manifolds: Locally Linear KIsomaps


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Unfolding Manifolds Locally Linear K-Isomaps

• Ashutosh Saxena
• Abhinav Gupta

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Agenda

• Need for Unfolding Manifolds
• Tannenbaum Isomaps
• Local Linear Embedding
• Short-circuiting problem in Isomaps
• Proposed Locally Linear K-Isomaps
• Results

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Dimensionality Reduction

• Need to analyze large amounts multivariate data.
• Human Faces
• Handwritten characters
• Speech Waveforms
• Global Climate patterns
• Discover compact representations of high
  dimensional data.
• Visualization
• Compression
• Better Recognition
• Probably meaningful dimensions

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Types of structures in Multivariate Data

• Clusters.
• On or around low Dimensional Manifolds
• Linear
• NonLinear

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Concepts of Manifolds

• A manifold is a topological space which is
  locally Euclidean.
• Manifolds arise naturally whenever there is a
  smooth variation of parameters
• like pose of the face in previous example
• Curve parameters in handwritten characters
• The dimension of a manifold is the minimum
  integer number of co-ordinates necessary to
  identify each point in that manifold.

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Non-linear Manifolds
PCA and MDS see the Euclidean distance
A
What is important is the geodesic distance
Unroll the manifold
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Preserve the geodesic distance and not the
euclidean distance.
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Two Methods

• Tenenbaum et.als Isomap Algorithm 1
• Global approach.
• On a low dimensional embedding
• Nearby points should be nearby.
• Farway points should be faraway.
• Roweis and Sauls Locally Linear Embedding
• Local approach
• Nearby points nearby
• 1 Tenenbaum, et al, SCIENCE, Dec 2000 22 290.
• 2 Roweis, et al, SCIENCE, Dec 2000, 22, 290.

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Isomap

• Estimate the geodesic distance between faraway
  points.
• For neighboring points Euclidean distance is a
  good approximation to the geodesic distance.
• For farway points estimate the distance by a
  series of short hops between neighboring points.
• Find shortest paths in a graph with edges
  connecting neighboring data points

Once we have all pairwise geodesic distances use
classical metric MDS
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Isomap
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Isomap-Algorithm

• Determine the neighbors.
• All points in a fixed radius.
• K nearest neighbors
• Construct a neighborhood graph.
• Each point is connected to the other if it is a K
  nearest neighbor.
• Edge Length equals the Euclidean distance
• Compute the shortest paths between two nodes
• Construct a lower dimensional embedding.
• Classical Multi-Dimensional Scaling (MDS )

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Local Linear Embedding
Fit Locally , Think Globally
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Fit Locally
We expect each data point and its neighbours to
lie on or close to a locally linear patch of
the manifold.
Each point can be written as a linear combination
of its neighbors. The weights choosen to minimize
the reconstruction Error.
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Think Globally
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Tennenbaum Isomaps Short Circuit Problem

• How to choose neighborhoods.
• Susceptible to short-circuit errors if
  neighborhood is larger than the folds in the
  manifold.
• If small we get isolated patches.
• Noisy Data Short-Circuit problem surfaces

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Sparse Swiss-roll data used
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Original DataCorrect 2-D embedding
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Noisy dataShort-Circuit occurs
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Proposed Algorithm

• A better method for choosing neighborhood is
  proposed in Tennenbaum algorithm.
• We explicitly use the fact that Manifolds are
  locally linear.
• Manifolds arise due to smooth variation of
  parameters.
• LL-K-Isomaps
• KLL neighbors out K-nearest neighbors are chosen,
  based on how well they reconstruct the point
  linearly

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LL-K-Isomaps algorithm

• Determine the neighbors.
• K nearest possible neighbors
• KLLneighbors based on local linearity
• Construct a neighborhood graph.
• Each point is connected to the other if it is a K
  nearest neighbor.
• Edge Length equals the Euclidean distance
• Compute the shortest paths between two nodes
• Construct a lower dimensional embedding.
• Classical Multi-Dimensional Scaling (MDS )

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LL-K-Isomaps Results

• Noise was added to the Swiss-Roll Data
• Tennenbaum original algorithm suffered from
  short-circuiting
• LL-K-Isomaps were able to find correct 2-D
  embedding with Noisy data

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Resultscontd
Residual Variance
Tennenbaum
Residual Variance
Proposed
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Results contd (disconnected neighborhood)
Tennenbaum
Proposed
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Future Work

• Convergence of algorithm will be proved
  mathematically
• Will be tested on
• Swiss-roll and S-roll data-sets
• Synthetic faces pose variations
• Handwritten characters

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Thank you. Questions ?