combining detection, recognition and segmentationkemelmi/combining detection... · •testimage is...
TRANSCRIPT
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Combining Detection, Recognition and Segmentation
Presented by:
Ira Kemelmacher-Shlizerman Sharon Alpert
For the Advanced Topics in Computer Vision courseSpring 2006
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Words Matching in Ascii
Find the word sleepingsleeping in the string:
“I have never taken any exercise except sleeping and resting”
Mark Twain
Simply go over the string and match each substring of length 8.
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But what if we want to detect a face?
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Detection/recognition challenges (partial list)
• Partial occlusions.
• Variation in pose.
• intra-class variations
• Lighting, scale, deformations, background clutter etc.
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Try to find the face in this image
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Segmentation
Group homogeneous areas
Use low-level cues:
IntensityColorTexture
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Segmentation - where is the edge?
Low-level local cues are not enough to get meaningful segmentation
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Segmentation & Recognition
• Segmentation recognition
Segmentation helps recognitionSegmentation helps recognition
Recognized objects constrain Recognized objects constrain segmentationsegmentation
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Labeling & Segmentation
Knowing the image type or the labeling
coast buildings
Can constraint the segmentation
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Labeling & Recognition Unlikely:Unlikely: TOASTERTOASTERLikely:Likely: CARCAR
highway
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We will talk about• Recognition/Detection
Scene categories recognition S. Lazebnik et al.
Given an explicit image of the model V. Ferrari et al.
• Segmentation with Recognition E. Borenstein and S. UllmanE. Borenstein , E. Sharon and S. UllmanA. Levin and Y. Weiss
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Scene and context categorization• outdoor• city• traffic• …
(From Iccv 2005 tutorial)
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Scene and context categorization
•Buildings•Urban•…
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The idea
• Represent images as “bags of features”• Spatial relations between features• Training:
1. Get features from labeled images 2. Histogram the features3. Use the histograms to train a classifier
• Testing1. Use classifier to label new image
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Training: Extract featuresLabel: coast
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Extract features
1. Extract features from labeled images
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Count the features
• Level 0
Bag of Features
11 8 12
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Multi-scale bag of featuresLazebnik etal, CVPR06
• Level 0• Level 1
3 2
2 4
2
2
2
2
2
2
4
4S. Lazebnik, C. Schmid, and J. Ponce. Beyond Bags of Features: Spatial Pyramid Matching for Recognizing Natural Scene Categories, CVPR 2006, to appear.
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Multi-scale bag of features
• Level 0• Level 1• Level 2
0 00
00 1 2 01 1 11 1 1
1 1 2 11
10 0 0 03 1 10 0 1
0 10 11
00 1 0 21 0 00 1 0
1 0
S. Lazebnik, C. Schmid, and J. Ponce. Beyond Bags of Features: Spatial Pyramid Matching for Recognizing Natural Scene Categories, CVPR 2006, to appear.
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Matching between 2 images• For each feature type • For each pyramid level L = 0,1,2
• Note: coarser level matches include the finer level ones
3 22 4
FeatureLevel = 1
0 22 1
0 48 1
Image 1 Image 2
min( , ) = =
S. Lazebnik, C. Schmid, and J. Ponce. Beyond Bags of Features: Spatial Pyramid Matching for Recognizing Natural Scene Categories, CVPR 2006, to appear.
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Matching between 2 images
NEW matches
Weight
Classification function
•Weight = penalize matches in larger cells.• The resulting kernel is Mercer kernel.• Use for the classification
S. Lazebnik, C. Schmid, and J. Ponce. Beyond Bags of Features: Spatial Pyramid Matching for Recognizing Natural Scene Categories, CVPR 2006, to appear.
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Types of Features
Oriented edge points 8 directions * 2 scales = 16 types
SIFT descriptors
Cluster features to a vocabulary = 200 typesS. Lazebnik, C. Schmid, and J. Ponce. Beyond Bags of Features: Spatial Pyramid Matching for Recognizing Natural Scene Categories, CVPR 2006, to appear.
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Classification
• By SVM (support vector machine)
• Train classifier by applying the kernel on labeled images
• Test image is assigned the label of the classifier with the highest response
CLASS
NON-class
S. Lazebnik, C. Schmid, and J. Ponce. Beyond Bags of Features: Spatial Pyramid Matching for Recognizing Natural Scene Categories, CVPR 2006, to appear.
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Main contributionMulti-scale representation to the “bag of features” approach
Considers spatial relation between features
64.6%64.6% using multiscale, L=2 41.2% with L=0
53.9%53.9% State-of-the-art (Zhang etal.)
Caltech 101:
S. Lazebnik, C. Schmid, and J. Ponce. Beyond Bags of Features: Spatial Pyramid Matching for Recognizing Natural Scene Categories, CVPR 2006, to appear.
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Results
Inside city
kitchen
office
store
mountain forest
city
Tallbldg
city
street
S. Lazebnik, C. Schmid, and J. Ponce. Beyond Bags of Features: Spatial Pyramid Matching for Recognizing Natural Scene Categories, CVPR 2006, to appear.
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PerformanceHigh performance Poor performance
Textureless animals, camouflageCoherent scenes, little clutter
S. Lazebnik, C. Schmid, and J. Ponce. Beyond Bags of Features: Spatial Pyramid Matching for Recognizing Natural Scene Categories, CVPR 2006, to appear.
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Recognize this particular model:
in this input image
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Model image
Recognition
Find the location of the model+ get segmentation
V. Ferrari, T. Tuytelaars, and L. Van Gool. Simultaneous Object Recognition and Segmentation by Image Exploration. In Proc. ECCV, 2004.
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Why is it hard?
• Clutter• Occlusions• Scale • Viewpoint• Non-rigid
deformations
Matching solely by affine invariant features is not robust
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The Idea Ferrari etal., ECCV 2004
1. Find initial set of invariant features
2. Expansion - Look around them to construct more matching features
3. Contraction - Leave the correct, and remove mismatches
4. Iteratively construct more and more matches, increasingly farther from the initial ones.
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Different stages of the algV. Ferrari, T. Tuytelaars, and L. Van Gool. Simultaneous Object Recognition and Segmentation by Image Exploration. In Proc. ECCV, 2004.
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Soft Matching
• Initial feature extraction in model and image
t
I t If t
I t I dtd
t
0
00
( )( )
( )max( , )
−=
−∫
4. Calculate moments to get internal ellipse
5. Double ellipse size
T. Tuytelaars and L. Van Gool Wide Baseline Stereo based on Local, Affinely invariant Regions In Brit. Mach. Vis. Conf., pp. 412-422, 2000.
1. Choose point by non-max suppression
2. Transfer rays 3. Local extremum along the rays
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Initial Matching-cont.
• Every test region matched to 0-3 model regions by thresholding the:
– Mahalanobis distance (on color moments)
– Similarity measure (NCC on gray levels + Euclidean distance in RGB space)
– Geometric refinement – find affine transf. that maximized the similarity
V. Ferrari, T. Tuytelaars, and L. Van Gool. Simultaneous Object Recognition and Segmentation by Image Exploration. In Proc. ECCV, 2004.
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Early expansion
• Coverage of the model image
•Each feature in model gives support for a part of the covering
V. Ferrari, T. Tuytelaars, and L. Van Gool. Simultaneous Object Recognition and Segmentation by Image Exploration. In Proc. ECCV, 2004.
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Early expansion
1. Map a region from the model to image using the transf. defined by the initial matching.
2. Refine the transformation.
3. Keep the matching with the best similarity (which also above a detection threshold).
4. Discard all matches that did not succeed in propagating any region.
V. Ferrari, T. Tuytelaars, and L. Van Gool. Simultaneous Object Recognition and Segmentation by Image Exploration. In Proc. ECCV, 2004.
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Result of the expansion
Test ImageModel Image
V. Ferrari, T. Tuytelaars, and L. Van Gool. Simultaneous Object Recognition and Segmentation by Image Exploration. In Proc. ECCV, 2004.
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Early Contraction
Model Image Test
Image
Correct matching = same intersections in test & model
Match is removed if:
V. Ferrari, T. Tuytelaars, and L. Van Gool. Simultaneous Object Recognition and Segmentation by Image Exploration. In Proc. ECCV, 2004.
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Main expansion
• Matches from pervious step added to support group.
• Follow similar steps as early expansion.
• Refinement is applied after we picked a new matching region.
V. Ferrari, T. Tuytelaars, and L. Van Gool. Simultaneous Object Recognition and Segmentation by Image Exploration. In Proc. ECCV, 2004.
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Main ContractionSidedness Constraint
1 2 3m m mside(R ,R ,R ) =
-1 right side
1 left side
• Test each region Ri with every two other regions Rj, Rk
• Correct match will give 0
V. Ferrari, T. Tuytelaars, and L. Van Gool. Simultaneous Object Recognition and Segmentation by Image Exploration. In Proc. ECCV, 2004.
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Main contraction –cont.The filtering algorithm:
sidedness similarity
Finally, iterate between contraction & expansion...V. Ferrari, T. Tuytelaars, and L. Van Gool. Simultaneous Object Recognition and Segmentation by Image Exploration. In Proc. ECCV, 2004.
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Illustration: Soft matches
0%10%20%30%40%50%60%70%80%90%
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soft m
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early
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1st m
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Correct matches
Mismatches
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Illustration: Early expansion
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Correct matches
Mismatches
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Illustration: Early contraction
0%10%20%30%40%50%60%70%80%90%
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Correct matches
Mismatches
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Illustration: 1st Main Expansion
0%10%20%30%40%50%60%70%80%90%
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Correct matches
Mismatches
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Illustration: 1st Main Contraction
0%10%20%30%40%50%60%70%80%90%
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Correct matches
Mismatches
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Illustration: 2nd Main Expansion
0%10%20%30%40%50%60%70%80%90%
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soft m
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Correct matches
Mismatches
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Illustration: 2nd Main Contraction
0%10%20%30%40%50%60%70%80%90%
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Correct matches
Mismatches
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Results
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Results
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Results
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Results
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Segmentation & Recognition
• Segmentation recognition
Segmentation helps recognitionSegmentation helps recognition
Recognized objects constrain Recognized objects constrain segmentationsegmentation
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Bottom – Up segmentation
• Goal aggregate pixels with similar low level features.
• E. Sharon et al. method : construct a 4-connected graph
Node=Pixel
Edge weights:Similar intensity=high weight
Eitan Sharon, Achi Brandt, and Ronen Basri, Segmentation and boundary detection using multiscale intensity measurements., CVPR, 2001, pp. 469–476.
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Bottom – Up segmentation
• Apply a fast multi level solver called Algebraic MultiGrid (AMG), for graph coarsening.
Eitan Sharon, Achi Brandt, and Ronen Basri, Segmentation and boundary detection using multiscale intensity measurements., CVPR, 2001, pp. 469–476.
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Bottom – Up segmentation
Eitan Sharon, Achi Brandt, and Ronen Basri, Segmentation and boundary detection using multiscale intensity measurements., CVPR, 2001, pp. 469–476.
1. Calculate coarse scale node.
2. Create inter scale weights and calculate new features.
3. Update coarse scale weights by averaging the fine scale weights.
4. Repeat the above process. Segments emerges as node at some coarse scale.
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Difficulties in bottom-up segmentation
Scale IIIScale III
Scale IIScale II
Scale IScale I
InputInput
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How to constrain the segmentation?
Think on an horse for example:Think on an horse for example:
Shape is an important cueShape is an important cue
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Introducing shape Into the segmentation
• Instead of Bottom up use Top-down
Learn how to segment this type of images
Result: Binary segmentation Background / foreground
Training images
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Top-Down - Borenstein & Ullman. Randomly collect a large set of candid. fragments
class non-class
Select subset of informative fragments
Figure-Background Labeling
Detection and segmentation
E. Borenstein, S. Ullman. Learning to Segment. ECCV (3) 2004: 315-328
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Selecting Informative Fragments
• Informative = likely to be detected in class compared with non-class images
• Fragments are added to maximize the gain in mutual information
• Highly overlapping
• Well-distributed over the figure
E. Borenstein, S. Ullman. Learning to Segment. ECCV (3) 2004: 315-328
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Segmenting Informative FragmentsSegmentation ?Segmentation ?
Informative Informative fragmentsfragments
BottomBottom--Up Up segmentationsegmentation Degree of Degree of
covercover
E. Borenstein, S. Ullman. Learning to Segment. ECCV (3) 2004: 315-328
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Segmenting Informative FragmentsSegmentation ?Segmentation ?
Overlap with the average Overlap with the average Edges Edges D(x,yD(x,y) of fragment ) of fragment in all the training images. in all the training images.
E. Borenstein, S. Ullman. Learning to Segment. ECCV (3) 2004: 315-328
Likelihood determined by Likelihood determined by the degree of coverthe degree of cover
++ ==
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Segmenting a test image• Each fragment votes for the
classification of pixels it covers
• Count the number of votes for figure and background
• Check consistency test between the received labeling and labeling of each fragment using NCC
detection rateweight
false alarms rate=
E. Borenstein, S. Ullman. Learning to Segment. ECCV (3) 2004: 315-328
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Top-Down - Borenstein & Ullman.Main contributions
• Top down approach
• Automatic fragments labeling
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ResultsThis Alg. figure-groundlabeling
Manual figure-groundlabeling
Bottom-up seg.
E. Borenstein, S. Ullman. Learning to Segment. ECCV (3) 2004: 315-328
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Zoom-in to the resultsTop-down Bottom-up
The boundaries are exact
We did not get the horse as one piece
The shape of the horse is captured
The boundaries are not exact
Need to make a combination of Top-Down & Bottom-Up approaches
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Bottom up & Top down segmentations
Test Image Training set
The “correct” segmentation is some how a compromisecompromisebetween Top-down & bottom up.
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How to combine?
• Approach 1 – Do separate segmentations are combine them together.
E. Borenstein, E. Sharon, S. Ullman (2004)
• Approach 2 – Combine them in one pass.A. Levin and Y. Weiss (2006)
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Combining Top-Down and Bottom-Up Segmentation- Borenstein et al.
• Use the hierarchy formed by the bottom-up segmentation.
•• Each segment at each scaleEach segment at each scalegets figure/background label.
scale
E. Borenstein, E. Sharon, S. Ullman, Combining Top-Down and Bottom-Up Segmentation, Proceedings IEEE workshop on Perceptual Organization in Computer Vision, IEEE CVPR Washington, DC, June 2004.
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Combining Top-Down and Bottom-Up segmentation
• Each father-descendentcost depends on their labels.
fatherfather--descendent costdescendent costTotalTotal TreeTree costcost
}1,1{, −∈−i
ssiFatherFatherlabel label
Descendent Descendent label label
),(),...,( 1−∑= i
iiiN ssfssTCost
if
E. Borenstein, E. Sharon, S. Ullman, Combining Top-Down and Bottom-Up Segmentation, Proceedings IEEE workshop on Perceptual Organization in Computer Vision, IEEE CVPR Washington, DC, June 2004.
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Combining Top-Down and Bottom-Up : father-descendent cost
• Each father-descendent energy has two terms:
BottomBottom--up termup termTopTop––down termdown term
Sum of SSD from the Sum of SSD from the TopTop--Down segmentation Down segmentation (only on leafs)(only on leafs)
DescendentDescendent more salient more salient --penalty for different fatherpenalty for different father--descendent labels decreases. descendent labels decreases.
),()(),( −− += iiiiiiii ssbstssfImage
segmentation tree
E. Borenstein, E. Sharon, S. Ullman, Combining Top-Down and Bottom-Up Segmentation, Proceedings IEEE workshop on Perceptual Organization in Computer Vision, IEEE CVPR Washington, DC, June 2004.
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Combining Top-Down and Bottom-Up Segmentation
• Since the cost function factorizes:
• Sum product algorithm is used (similar to BP)
fatherfather--descendent costdescendent costTotalTotal TreeTree costcost
),(),...,( 1−∑= i
iiiN ssfssTCost
E. Borenstein, E. Sharon, S. Ullman, Combining Top-Down and Bottom-Up Segmentation, Proceedings IEEE workshop on Perceptual Organization in Computer Vision, IEEE CVPR Washington, DC, June 2004.
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Combining Top-Down and Bottom-Up Segmentation
• Requires only one passone pass in each direction.
• A local computation in each segments results in:
)),...,,1,,...,(min()1( 111 Niis ssssTcostTcosti +−=
)),...,,1,,...,(min()1( 111 Niis ssssTcostTcosti +− −=−
E. Borenstein, E. Sharon, S. Ullman, Combining Top-Down and Bottom-Up Segmentation, Proceedings IEEE workshop on Perceptual Organization in Computer Vision, IEEE CVPR Washington, DC, June 2004.
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Combining Top-Down and Bottom-Up Segmentation
• Confidence map of the segmentation is given by:
Smm
ii SS )1()1( −−
E. Borenstein, E. Sharon, S. Ullman, Combining Top-Down and Bottom-Up Segmentation, Proceedings IEEE workshop on Perceptual Organization in Computer Vision, IEEE CVPR Washington, DC, June 2004.
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ResultsBottomBottom--Up Up
segmentationsegmentation
TopTop--Down segmentationDown segmentation
E. Borenstein, E. Sharon, S. Ullman, Combining Top-Down and Bottom-Up Segmentation, Proceedings IEEE workshop on Perceptual Organization in Computer Vision, IEEE CVPR Washington, DC, June 2004.
Combined segmentationCombined segmentation
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ResultsBottomBottom--Up Up
segmentationsegmentation
TopTop--Down segmentationDown segmentation
Combined segmentationCombined segmentation
E. Borenstein, E. Sharon, S. Ullman, Combining Top-Down and Bottom-Up Segmentation, Proceedings IEEE workshop on Perceptual Organization in Computer Vision, IEEE CVPR Washington, DC, June 2004.
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Combining Top-Down and Bottom-Up Segmentation- A. Levin et al.
• Combine in one segmentation process.
• Use only few fragments.
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Markov Random Field
• Joint distribution of labels and local features
),(),(1),(,
, iii
ijiji
ji yxxxZ
yxp ∏∏= φψ
iφ
ji ,ψ
One hidden -- one observation
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Conditional Random Field• Conditional distribution of labels given whole image
)y;()y;,(y)(
1)y|(,
, ii
ijiji
ji xxxZ
xp ∏∏= φψ
Lafferty et al. (2001), Kumar and Hebert (2003)
•Models the conditional and not the joint probability.
•CRF globally conditionedon the observation Y – long long range dependences range dependences
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Log notation
)y;()y;,(y)(
1)y|(,
, ii
ijiji
ji xxxZ
xp ∏∏= φψ
))y;,(log()y;,( ,, jijijiji xxxx ψψ −=
))y;(log()y;( iiii xx φφ −=
∑∑=
+−ji
iiji
jiji xxx
eZ
xp ,,, )y;()y;,(
y)(1)y|(
φψ
Take the log:
Easier to work with summations
)y;(xE=
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How this helps the segmentation?
ix jx
II
II
Low-level term(Intensity)
Local energy term derived from image
fragments
A. Levin and Y. Weiss. Learning to Combine Bottom-Up and Top-Down Segmentation. Proc. of the European Conference on Computer Vision (ECCV), Graz, Austria, May 2006.
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Learning
Labeling discontinuities == image discontinuities
2ijd - RGB distance
We want to learn:
1, λλ =Frr
2λ+ ...+2.A. Levin and Y. Weiss. Learning to Combine Bottom-Up and Top-Down Segmentation. Proc. of the European Conference on Computer Vision (ECCV), Graz, Austria, May 2006.
ν1. - power of the low level term
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Learning
+);,(1 Frr
l νλ );,(2 Frr
l νλ
Given a set of segmented training images
Switch the roll of hidden & observation to get likelihood of the labels x conditioned on the image I.
A. Levin and Y. Weiss. Learning to Combine Bottom-Up and Top-Down Segmentation. Proc. of the European Conference on Computer Vision (ECCV), Graz, Austria, May 2006.
Find the parameters that maximize the sum of the log-likelihood :
∑= tt FF );,();,(
rrl
rrl νλνλ
);,,(log);,,;(
);,;|(log);,(
FIZFIxE
FIxpF
ttt
ttt
rrrr
rrrrl
νλνλ
νλνλ
−−
==Where:
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Learning
• The CRF log likelihood is convex with respect to the weighting parameters
• On the other hand - exact computation of derivatives and Z(I) is in general intractable.
• Use approximations
νλ ,kLafferty et al. (2001)
A. Levin and Y. Weiss. Learning to Combine Bottom-Up and Top-Down Segmentation. Proc. of the European Conference on Computer Vision (ECCV), Graz, Austria, May 2006.
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Learning- selection fragments
Out of a pool of fragments at random sizes & locations we want to select a few informative ones.
Straightforward computation of the likelihood improvement is not practical since each iteration will require inference for each .F
rr,λ
How to do it efficiently ?A. Levin and Y. Weiss. Learning to Combine Bottom-Up and Top-Down Segmentation. Proc. of the European Conference on Computer Vision (ECCV), Graz, Austria, May 2006.
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Learning- selection fragments
∑∑ −+−=k
IFkji
kxxjxixvIxE ,
,)()();( λRecall the energy term:
∑ −=ji
jxixvIxE,
0 )()();(If no fragments are used:
IFji
xxjxixvIxE ,1,
1 1)()();( −+−= ∑ λIf we add one fragment:
Energy changeby adding F1
1
11001
);();(|);(1 λ
λλ ∂∂
+≈=IxEIxEIxELinear approximation:
A. Levin and Y. Weiss. Learning to Combine Bottom-Up and Top-Down Segmentation. Proc. of the European Conference on Computer Vision (ECCV), Graz, Austria, May 2006.
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Learning- selection fragments
∑ −=ji
jxixvIxE,
0 )()();(Start without any Fragments
IFji
xxjxixvIxE ,1,
1 1)()();( −+−= ∑ λIf we add one fragment:
1
11001
);();(|);(1 λ
λλ ∂∂
+≈=IxEIxEIxE
Energy change by adding F1
Choose the fragment with large change.
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Learning- selection fragments
• Continue in a greedy approach.
• Add a predefined number of fragments.
• After all the fragments are added, improve the accuracy of .kλ
A. Levin and Y. Weiss. Learning to Combine Bottom-Up and Top-Down Segmentation. Proc. of the European Conference on Computer Vision (ECCV), Graz, Austria, May 2006.
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Segmentation
• Given a new image the segmentation is the assignment x that maximizes the probability:
• Where E(x;Inew) is the learned energy term.
);(
)(1)|( newIxE
newnew e
IZIxp −=
A. Levin and Y. Weiss. Learning to Combine Bottom-Up and Top-Down Segmentation. Proc. of the European Conference on Computer Vision (ECCV), Graz, Austria, May 2006.
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Results
Fragments used:
MAP segmentation
Overlaid segmentation
One fragment One fragment Two fragments Two fragments ThreeThree fragmentsfragments
A. Levin and Y. Weiss. Learning to Combine Bottom-Up and Top-Down Segmentation. Proc. of the European Conference on Computer Vision (ECCV), Graz, Austria, May 2006.
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Results
Original image
Fragments used:
Overlaid segmentation
A. Levin and Y. Weiss. Learning to Combine Bottom-Up and Top-Down Segmentation. Proc. of the European Conference on Computer Vision (ECCV), Graz, Austria, May 2006.
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Summary
• Scene Labeling
• Recognition given explicit model
• Segmentation & recognition
• outdoor• city• traffic• …
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Thanks!