learning visual representations for unfamiliar environments kate saenko, brian kulis, trevor darrell...
TRANSCRIPT
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Learning visual representations for unfamiliar
environments
Kate Saenko, Brian Kulis,
Trevor Darrell
UC Berkeley EECS & ICSI
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The challenge of large scale visual interaction
?Last decade has proven the superiority of models learned from data vs. hand engineered structures!
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• “Unsupervised”: Learn models from “found data”; often exploit multiple modalities (text+image)
Large-scale learning
… The Tote is the perfect example of two handbag design principles that ... The lines of this tote are incredibly sleek, but ... The semi buckles that form the handle attachments are ...
WikipediaFlickr
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E.g., finding visual senses
4
Artifact sense: “telephone”DICTIONARY
1: (n) telephone, phone, telephone set (electronic equipment that converts sound into electrical signals that can be transmitted over distances and then converts received signals back into sounds)
2: (n) telephone, telephony (transmitting speech at a distance)
[Saenko and Darrell ’09]
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• “Unsupervised”: Learn models from “found data”; often exploit multiple modalities (text+image)
• Supervised: Crowdsource labels (e.g., ImageNet)
Large-scale Learning
… The Tote is the perfect example of two handbag design principles that ... The lines of this tote are incredibly sleek, but ... The semi buckles that form the handle attachments are ...
WikipediaFlickr
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Yet…• Even the best collection of images from the web
and strong machine learning methods can often yield poor classifiers on in-situ data!
• Supervised learning assumption: training distribution == test distribution
• Unsupervised learning assumption: joint distribution is stationary w.r.t. online world and real world
Almost never true!6
?
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“What You Saw Is Not What You Get”
The models fail due to domain shift
SVM:54%NBNN:61%
SVM:20%NBNN:19%
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Close-up Far-away
amazon.com Consumer imagesFLICKR CCTV
Examples of visual domain shifts
digital SLR webcam
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Examples of domain shift: change in camera, feature type, dimension
digital SLR webcam
SURF
VQ to 300
SIFT
VQ to 1000
Different dimensions
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Solutions?
• Do nothing (poor performance)
• Collect all types of data (impossible)
• Find out what changed (impractical)
• Learn what changed
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Prior Work on Domain Adaptation
• Pre-process the data [Daumé ’07] : replicate features to also create source- and domain-specific versions; re-train learner on new features
• SVM-based methods [Yang’07], [Jiang’08], [Duan’09], [Duan’10] : adapt SVM parameters
• Kernel mean matching [Gretton’09] : re-weight training data to match test data distribution
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Our paradigm: Transform-based Domain Adaptation
Previous methods’ drawbacks
• cannot transfer learned shift to new categories
• cannot handle new features
We can do both by learning domain transformations*
Example: “green” and “blue” domains
W
* Saenko, Kulis, Fritz, and Darrell. Adapting visual category models to new domains. ECCV, 2010
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Symmetric assumption fails!
Limitations of symmetric transforms
Saenko et al. ECCV10 used metric learning:
• symmetric transforms
• same features
How do we learn more
general shifts?
W
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Asymmetric transform (rotation)
Latest approach*: asymmetric transforms
• Metric learning model no longer applicable
• We propose to learn asymmetric transforms
– Map from target to source
– Handle different dimensions
*Kulis, Saenko, and Darrell, What You Saw is Not What You Get: Domain Adaptation Using Asymmetric Kernel Transforms, CVPR 2011
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Asymmetric transform (rotation)
W
Latest approach: asymmetric transforms
• Metric learning model no longer applicable
• We propose to learn asymmetric transforms
– Map from target to source
– Handle different dimensions
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Model Details
• Learn a linear transformation to map points from one domain to another
– Call this transformation W
– Matrices of source and target:
W
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Loss Functions
Choose a point x from the source and y from the target, and consider inner product:
Should be “large” for similar objects and “small” for dissimilar objects
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Loss Functions
• Input to problem includes a collection of m loss functions
• General assumption: loss functions depend on data only through inner product matrix
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Regularized Objective Function
• Minimize a linear combination of sum of loss functions and a regularizer:
• We use squared Frobenius norm as a regularizer
– Not restricted to this choice
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The Model Has Drawbacks
• A linear transformation may be insufficient
• Cost of optimization grows as the product of the dimensionalities of the source and target data
• What to do?
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Kernelization
• Main idea: run in kernel space
– Use a non-linear kernel function (e.g., RBF kernel) to learn non-linear transformations in input space
– Resulting optimization is independent of input dimensionality
– Additional assumption necessary: regularizer is a spectral function
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Kernelization
Original Transformation Learning Problem
Kernel matrices for source and target
New Kernel Problem
Relationship between original and new problems at optimality
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Summary of approach
Input space
Input space
1. Multi-Domain Data 2. Generate Constraints, Learn W
3. Map via W 4. Apply to New Categories
Tra
in T
ime
Test
Tim
e
Test point
Test pointy1 y2
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Multi-domain dataset
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Experimental Setup
• Utilized a standard bag-of-words model
• Also utilize different features in the target domain
– SURF vs SIFT
– Different visual word dictionaries
• Baseline for comparing such data: KCCA
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Novel-class experiments
• Test method’s ability to transfer domain shift to unseen classes
• Train transform on half of the classes, test on the other half
Our Method (linear)Our Method
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Extreme shift example
Nearest neighbors in source using transformation
Query from target Nearest neighbors in source using KCCA+KNN
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Conclusion
• Should not rely on hand-engineered features any more than we rely on hand engineered models!
• Learn feature transformation across domains
• Developed a domain adaptation method based on regularized non-linear transforms– Asymmetric transform achieves best results on
more extreme shifts
– Saenko et al ECCV 2010 and Kulis et al CVPR 2011; journal version forthcoming