Transcript Self-similarity - VideoLectures.NET

Global and Efficient Self-Similarity for
Object Classification and Detection
Thomas Deselaers and Vittorio Ferrari
CALVIN group
Computer Vision Laboratory
ETH Zurich
Switzerland
CVPR 2010
Conventional Image Descriptors
Measure direct image properties
gradients
colors
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Self-Similarity vs Conventional Descriptors
Assumption of conventional image descriptors
• There is a direct visual property shared by images of objects of the same class
(e.g. colors, gradients, …).
• This property can be used to compare images.
Self-similarity:
• Indirect property: geometric layout of repeating patches within an image
• More general property
[Shechtman, Irani CVPR 07]
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Local Self-Similarity Descriptors
[Shechtman, Irani CVPR 07]
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Using Local Self-Similarity Descriptors
Applications: object recognition, image retrieval, action recognition
• Ensemble matching [Shechtman CVPR 07]
• Nearest neighbor matching [Boiman CVPR 08]
• Bag of local self-similarities
[Gehler ICCV09, Vedaldi ICCV09, Hörster ACMM08, Lampert CVPR09, Chatfield
ICCV09 WS]
1. Compute LSS descriptors for an image
2. Assign the LSS descriptors to a codebook
3. Represent the image as a histogram of LSS descriptors
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Self-Similarity goes Global
Capture long-range self-similarities and their spatial arrangement
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Self-Similarity goes Global
Capture long-range self-similarities and their spatial arrangement
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Global Self-Similarity Tensor
compute self-similarity
between all pairs of
pixels
4D self-similarity tensor
Note: local self-similarities included
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Problems with the GSS Tensor
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11
300
500
• Computation time:
• Memory requirement:
∼ 20h
∼ 80GB
Aim: Reduce both
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Outline
• Efficient global self-similarity tensor
• Global self-similarity descriptors
– Bag of correlation surfaces
– Self-similarity hypercubes
• Detection with self-similarity hypercubes
– Efficient sliding window
– Efficient subwindow search
• Experiments
– Global self-similarity better than local self-similarity
– Complementary to conventional descriptors
– Object detection possible
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Efficient Global Self-Similarity Tensor
Find an efficient approximation
Quantize patches
to
according to codebook
If two patches are assigned to the same prototype, they are similar
Reduces runtime to
speedup: 750
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Efficient Global Self-Similarity
Two patches are only similar if they are assigned to the same prototype
Reduces memory to
reduction:
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Patch Prototype Codebooks
Remember: Self-similarity encodes image content indirectly
Image-specific codebooks can be smaller than conventional ones
see paper for more
generic codebooks and
extensive evaluation
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Global Self-Similarity Descriptors
So far:
• Compact GSS computed efficiently
Now:
• Descriptors that can be used in machine learning classifiers
• Fixed dimensionality
• Compact representation
• Self-similarity hypercubes: now
• Bag of correlation surfaces: only in the paper
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Self-Similarity Hybercubes
SSH of size
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SSHs for Detection
• Computing SSH naïvely requires
operations
• Sliding windows has to evaluate many windows
operations
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Efficient Computation of SSHs
Compute integral self-similarity tensor:
can be obtained using 16 lookups in
 160000
operations to compute SSH
for an image window
 ∼5000x speedup
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Efficient Subwindow Search for SSH
•
Derive an upper bound on
the score of a set of
windows
•
Section 5.2 in our paper
•
Similar to [Lampert PAMI09]
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Experiments: Object classification
PASCAL 07 objects
– 9608 cropped images of objects from PASCAL 07
– 20 classes
Task: Classify each test image into one of 20 classes
Model: Linear SVM
Train: train+val
Test: test
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classification accuracy [%]
Classification on the PASCAL 07 objects set
+ GSS outperform LSS
+ Self-Similarity is truly complementary to conventional descriptors
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Experiments: Object detection
ETHZ Shape Classes
– 255 images
– 5 classes (apple logos, bottles, giraffes, mugs, swans)
Task: Detect objects in images
Detector: Linear SVM, sliding windows
e.g. [Ferrari CVPR07, Maji CVPR09]
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Detection Results
DR at 0.5 PASCAL overlap
bottles
giraffes
swans
mug
s
FPPI 0.4
apple logos
DR at FPPI 0.4
}
}
SSH
BoLSS
BoLSS
SSH
apple logos
10.0
80.0
bottles
10.7
96.4
giraffes
23.4
85.1
mugs
6.5
67.7
swans
17.6
70.6
Average
13.6
80.0
Comparison results (avg):
[Ferrari CVPR07]: 71.9
[Maji CVPR09]: 93.2
… many more
+ SSH outperforms BOLSS
+ it is possible to use GSS for detection with good results
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Runtimes for Computing Descriptors
• 200x200 image
• GSS tensor
– directly: 5512s (∼1.5 hours)
– using our method: 81s (∼1.5 minutes)
• Computing descriptors: few seconds
• Our method: 70x speedup
• For Reference:
– GIST: 0.4s
– BOLSS: 0.7s
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Runtimes for Detection
Given the prototype assignment map (80s) (once only)
SSH sliding window: 30s/img (once per class)
For Comparison
– Computing direct GSS tensor for 25000 windows: 4 years/img
Speedup: ∼1 million
⇒ Using our methods, GSS can be used for object detection
For Reference:
– Felzenszwalb PAMI 09: 5s.
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Global and Feasible
Efficient Self-Similarity for
Object Classification and Detection
Thomas Deselaers and Vittorio Ferrari
CALVIN group
Computer Vision Laboratory
ETH Zurich
Switzerland
CVPR 2010
Conclusion
• self-similarity should be considered globally
– Global self-similarity performs better than local self-similarity
• truly complementary to conventional descriptors
• global self-similarity is feasible
– efficient computation of self-similarity
– two descriptors based on self-similarity
• global self-similarity for detection
• code will be available soon
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Thank you for your attention
Thomas Deselaers and Vittorio Ferrari
Global and Efficient Self-Similarity for
Object Classification and Detection
Code will be available
http://www.vision.ee.ethz.ch/~calvin
Thank you for your attention
Thomas Deselaers and Vittorio Ferrari
Global and Efficient Self-Similarity for
Object Classification and Detection
Code will be available
http://www.vision.ee.ethz.ch/~calvin