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Object Category Recognition
Computer Vision
CS 543 / ECE 549
University of Illinois
Derek Hoiem
03/16/10
(Plus leftover material from image categorization)
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Today’s class: categorization
• More about classifiers
• Overview of object category detection
• More about categorization in general
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Image Categorization
Training Labels
Training Images
Classifier Training
Training
Image Features
Image Features
Testing
Test Image
Trained Classifier
Trained Classifier Outdoor
Prediction
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Many classifiers to choose from
• SVM
• Neural networks
• Naïve Bayes
• Bayesian network
• Logistic regression
• Randomized Forests
• Boosted Decision Trees
• K-nearest neighbor
• RBMs
• Etc.
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No Free Lunch Theorem
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Classifiers: Linear SVM
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Classifiers: Linear SVM
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Classifiers: Linear SVM
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Classifiers: Kernelized SVM
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Using SVMs• Good general purpose classifier
– Generalization depends on margin, so works well with many weak features
– No feature selection– Usually requires some parameter tuning
• Choosing kernel– Linear: fast training/testing – start here– RBF: related to neural networks, nearest neighbor– Chi-squared, histogram intersection: good for histograms
(but slower, esp. chi-squared)– Can learn a kernel function
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Classifiers: Decision Trees
x x
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Boosted Decision Trees
…
Gray?
High inImage?
Many LongLines?
Yes
No
NoNo
No
Yes Yes
Yes
Very High Vanishing
Point?
High in Image?
Smooth? Green?
Blue?
Yes
No
NoNo
No
Yes Yes
Yes
Ground Vertical Sky
[Collins et al. 2002]
P(label | good segment, data)
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Using Boosted Decision Trees
• Flexible: can deal with both continuous and categorical variables
• How to control bias/variance trade-off– Size of trees
– Number of trees
• Boosting trees often works best with a small number of well-designed features
• Boosting “stubs” can give a fast classifier
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K-nearest neighbor
x x
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1-nearest neighbor
x x
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3-nearest neighbor
x x
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5-nearest neighbor
x x
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Using K-NN
• Simple, so another good one to try first
• With infinite examples, 1-NN provably has error that is at most twice Bayes optimal error
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What to remember about classifiers
• No free lunch: machine learning algorithms are tools, each well-suited to some purposes but not others
• Try simple classifiers first
• Better to have smart features and simple classifiers than simple features and smart classifiers
• Use increasingly powerful classifiers with more training data (bias-variance tradeoff)
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Some Machine Learning References
• General– Tom Mitchell, Machine Learning, McGraw Hill, 1997
– Christopher Bishop, Neural Networks for Pattern Recognition, Oxford University Press, 1995
• Adaboost– Friedman, Hastie, and Tibshirani, “Additive logistic
regression: a statistical view of boosting”, Annals of Statistics, 2000
• SVMs– http://www.support-vector.net/icml-tutorial.pdf
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Moving on…
Object Categories
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Object category detection in computer vision
Goal: detect all pedestrians, cars, monkeys, etc in image
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General Process of Object Recognition
Specify Object Model
Generate Hypotheses
Score Hypotheses
Resolution
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Specifying an object model
1. Statistical Template in Bounding Box– Object is some (x,y,w,h) in image
– Features defined wrt bounding box coordinates
Image Template Visualization
Images from Felzenszwalb
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Specifying an object model
2. Articulated parts model– Object is configuration of parts
– Each part is detectable
Images from Felzenszwalb
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Specifying an object model
3. Hybrid template/parts model
Detections
Template Visualization
Felzenszwalb et al. 2008
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Specifying an object model
4. 3D-ish model
• Object is collection of 3D planar patches under affine transformation
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General Process of Object Recognition
Specify Object Model
Generate Hypotheses
Score Hypotheses
Resolution
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Generating hypotheses
1. Sliding window– Test patch at each location and scale
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Generating hypotheses
1. Sliding window– Test patch at each location and scale
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Generating hypotheses
2. Voting from patches/keypoints
Interest PointsMatched Codebook
EntriesProbabilistic
Voting
3D Voting Space(continuous)
x
y
s
ISM model by Leibe et al.
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Generating hypotheses
3. Region-based proposal
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General Process of Object Recognition
Specify Object Model
Generate Hypotheses
Score Hypotheses
Resolution
Many types of classifiers, features
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Resolving detection scores
1. Non-max suppression
Score = 0.1
Score = 0.8 Score = 0.8
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Resolving detection scores
2. Context/reasoning
meters
met
ers
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More thoughts on categories…
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Object Categorization
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Object Categorization
?
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An example of categorical perception
Continuous perception: graded response
Many perceptual phenomena are a mixture of the two: categorical at an everyday level of magnification, but continuous at a more microscopic level.
Categorical perception: “sharp” boundaries
Slide Credit: Torralba
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Categorical perception: “sharp” boundaries
happinessfear
• Identification Task
% id
enti
ficat
ion
AngerFearHappiness
Red vs. yellow vs. green vs. blue
Slide Credit: Torralba
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Continuous perception: graded response
20-24 25-29 30-34 35-39 40-44 45-49 50-54
Young vs. Old
Dark vs. Light
Slide Credit: Torralba
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Why do we care about categories?
Perception of function: We can perceive the 3D shape, texture, material properties, without knowing about objects. But, the concept of category encapsulates also information about what can we do with those objects.
“We therefore include the perception of function as a proper – indeed, crucial –subject for vision science”, from Vision Science, chapter 9, Palmer.
Slide Credit: Torralba
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Why do we care about categories?
When we recognize an object we can make predictions about its behavior in the future, beyond of what is immediately perceived.
Slide Credit: Torralba
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The perception of function
• Direct perception (affordances): GibsonFlat surfaceHorizontalKnee-high…
Sittableupon
Chair Chair
Chair?
Flat surfaceHorizontalKnee-high…
Sittableupon
Chair
• Mediated perception (Categorization)
Slide Credit: Torralba
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Direct perception
Some aspects of an object function can be perceived directly– Functional form: Some forms clearly indicate to a
function (“sittable-upon”, container, cutting device, …)
Sittable-upon Sittable-upon
Sittable-upon
It does not seem easyto sit-upon this…
Slide Credit: Torralba
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Direct perception
Some aspects of an object function can be perceived directly– Observer relativity: Function is observer
dependent
From http://lastchancerescueflint.org
Slide Credit: Torralba
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Limitations of Direct Perception
Objects of similar structure might have very different functions
Not all functions seem to be available from direct visual information only.
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Limitations of Direct Perception
Propulsion system
Strong protective surface
Something that looks like a door
Sure, I can travel to space on this object
Visual appearance might be a very weak cue to function
Slide Credit: Torralba
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So categorize or not?
“It seems exceedingly unlikely (though logically possible) that we categorize everything in our visual fields”, Palmer.
Hypothesis: we categorize the objects that are relevant for a specific task that we have at hand, but we only extract affordances from the others.
Slide Credit: Torralba
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How many categories?
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Slide by Aude OlivaMany
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How many object categories are there?
Biederman 1987
Slide Credit: Torralba
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How many categories?
• Probably this question is not even specific enough to have an answer
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Which level of categorization is the right one?
Car is an object composed of: a few doors, four wheels (not all visible at all times), a roof, front lights, windshield
?
Slide Credit: Torralba
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How do you define a category?
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Prototype or Sum of Exemplars ?
Prototype Model Exemplars Model
Category judgments are madeby comparing a new exemplarto the prototype.
Category judgments are madeby comparing a new exemplarto all the old exemplars of a categoryor to the exemplar that is the mostappropriate
Slide Credit: Torralba
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How do you define a category?
In computer vision:
LAB Histogram
Textons
Bag of SIFT
HOG
xxx x
xx
x
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ooooo = Object
Definition
Examples Image Features Classifier+ +
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Levels of Categorization
BASIC-LEVEL CATEGORIES
SUPERORDINATE LEVEL CATEGORIES
SUBORDINATE LEVEL CATEGORIES
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Rosch’s Levels of Categorization
Definition of Basic Level:
• Similar shape: Basic level categories are the highest-level category for which their members have similar shapes.
• Similar motor interactions: … for which people interact with its members using similar motor sequences.
• Common attributes: … there are a significant number of attributes in common between pairs of members.
Sub Basic Superordinate
similarity
Similarity declines only slightlygoing from subordinate tobasic level, and then dropsdramatically.
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• Rosch et al (1976) found that when people are shown pictures of objects, they identify objects at a basic level more quickly than they identified objects at higher or lower levels.
• Objects appear to be recognized first at their basic level, and only afterwards they are classified in terms of higher or lowers level categories
Levels of Categorization
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Typicality effects
• Typicality: how good or common an item is a member of a given category.
• The typical exemplar is like a representation of the average (or central tendency)
• But, the representation of a category varies with experience, so does the “typical” exemplar
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Entry-level categories(Jolicoeur, Gluck, Kosslyn 1984)
• Typical member of a basic-level category are categorized at the expected level
• Atypical members tend to be classified at a subordinate level.
A birdAn ostrich
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We do not need to recognize the exact category
A new class can borrow information from similar categories
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Next class
• Sliding window detectors