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					Part 1: Bag-of-words models
by Li Fei-Fei (UIUC)

Related works
• Early “bag of words” models: mostly texture recognition
– Cula et al. 2001; Leung et al. 2001; Schmid 2001; Varma et al. 2002, 2003; Lazebnik et al. 2003

• Hierarchical Bayesian models for documents (pLSA, LDA, etc.)
– Hoffman 1999; Blei et al, 2004; Teh et al. 2004

• Object categorization
– Dorko et al. 2004; Csurka et al. 2003; Sivic et al. 2005; Sudderth et al. 2005;

• Natural scene categorization
– Fei-Fei et al. 2005

Object

Bag of ‘words’

Analogy to documents
Of all the sensory impressions proceeding to the brain, the visual experiences are the dominant ones. Our perception of the world around us is based essentially on the messages that reach the brain from our eyes. For a long time it was thought that the retinal sensory, by point image was transmitted pointbrain, to visual centers in the brain; the cerebral cortex was a visual, perception, movie screen, so to speak, upon which the retinal, cerebral cortex, image in the eye was projected. Through the discoveries of Hubelcell, optical eye, and Wiesel we now know that behind the origin of the visual nerve, image perception in the brain there is a considerably more complicated course of events. By Hubel, Wiesel following the visual impulses along their path to the various cell layers of the optical cortex, Hubel and Wiesel have been able to demonstrate that the message about the image falling on the retina undergoes a stepwise analysis in a system of nerve cells stored in columns. In this system each cell has its specific function and is responsible for a specific detail in the pattern of the retinal image. China is forecasting a trade surplus of $90bn (£51bn) to $100bn this year, a threefold increase on 2004's $32bn. The Commerce Ministry said the surplus would be created by a predicted 30% jump in exports to $750bn, compared with a 18% rise in imports to China, trade, $660bn. The figures are likely to further annoy the US, which has long argued that surplus, commerce, China's exports are unfairly helped by a exports, imports, US, deliberately undervalued yuan. Beijing agrees the surplus is too high, but says the yuan, bank, domestic, yuan is only one factor. Bank of China foreign, increase, governor Zhou Xiaochuan said the country also needed to do more tovalue trade, boost domestic demand so more goods stayed within the country. China increased the value of the yuan against the dollar by 2.1% in July and permitted it to trade within a narrow band, but the US wants the yuan to be allowed to trade freely. However, Beijing has made it clear that it will take its time and tread carefully before allowing the yuan to rise further in value.

learning

recognition

feature detection & representation
image representation

codewords dictionary

category models (and/or) classifiers

category decision

Representation

2.

1. 3.

feature detection & representation

codewords dictionary

image representation

1.Feature detection and representation

1.Feature detection and representation
• Regular grid
– Vogel et al. 2003 – Fei-Fei et al. 2005

1.Feature detection and representation
• Regular grid
– Vogel et al. 2003 – Fei-Fei et al. 2005

• Interest point detector
– Csurka et al. 2004 – Fei-Fei et al. 2005 – Sivic et al. 2005

1.Feature detection and representation
• Regular grid
– Vogel et al. 2003 – Fei-Fei et al. 2005

• Interest point detector
– Csurka et al. 2004 – Fei-Fei et al. 2005 – Sivic et al. 2005

• Other methods
– Random sampling (Ullman et al. 2002) – Segmentation based patches (Barnard et al. 2003)

1.Feature detection and representation

Compute SIFT descriptor
[Lowe‟99]

Normalize patch

Detect patches
[Mikojaczyk and Schmid ‟02] [Matas et al. ‟02] [Sivic et al. ‟03]

Slide credit: Josef Sivic

1.Feature detection and representation
…

2. Codewords dictionary formation
…

2. Codewords dictionary formation
…

Vector quantization

Slide credit: Josef Sivic

2. Codewords dictionary formation

Fei-Fei et al. 2005

Image patch examples of codewords

Sivic et al. 2005

3. Image representation

frequency

…..
codewords

Representation

2.

1. 3.

feature detection & representation

codewords dictionary

image representation

Learning and Recognition

codewords dictionary

category models (and/or) classifiers

category decision

2 case studies
1. Naïve Bayes classifier
– Csurka et al. 2004

2. Hierarchical Bayesian text models (pLSA and LDA)
– – – Background: Hoffman 2001, Blei et al. 2004 Object categorization: Sivic et al. 2005, Sudderth et al. 2005 Natural scene categorization: Fei-Fei et al. 2005

First, some notations
• wn: each patch in an image
– wn = [0,0,…1,…,0,0]T

• w: a collection of all N patches in an image
– w = [w1,w2,…,wN]

• dj: the jth image in an image collection • c: category of the image • z: theme or topic of the patch

Case #1: the Naïve Bayes model

c

w
N

c  arg max
c

p(c | w)  p(c) p(w | c)  p (c) p ( wn | c)
n 1

N

Object class decision

Prior prob. of the object classes

Image likelihood given the class

Csurka et al. 2004

Csurka et al. 2004

Csurka et al. 2004

Case #2: Hierarchical Bayesian text models
Probabilistic Latent Semantic Analysis (pLSA)

d
D

z

w
N
Hoffman, 2001

Latent Dirichlet Allocation (LDA)

c
D


N

z

w
Blei et al., 2001

Case #2: Hierarchical Bayesian text models
Probabilistic Latent Semantic Analysis (pLSA)

d
D

z

w
N

“face”
Sivic et al. ICCV 2005

Case #2: Hierarchical Bayesian text models

“beach”

Latent Dirichlet Allocation (LDA)

c
D


N

z

w

Fei-Fei et al. ICCV 2005

d
D

z

w
N

Case #2: the pLSA model

d
D

z

w
N

Case #2: the pLSA model
K

p( wi | d j )   p( wi | z k ) p ( z k | d j )
k 1

Observed codeword distributions

Codeword distributions per theme (topic)

Theme distributions per image
Slide credit: Josef Sivic

Case #2: Recognition using pLSA

z  arg max p( z | d )
z



Slide credit: Josef Sivic

Case #2: Learning the pLSA parameters
Observed counts of word i in document j

Maximize likelihood of data using EM
M … number of codewords N … number of images
Slide credit: Josef Sivic

Demo
• Course website

task: face detection – no labeling

Demo: feature detection
• Output of crude feature detector
– Find edges – Draw points randomly from edge set – Draw from uniform distribution to get scale

Demo: learnt parameters
• Learning the model: do_plsa(„config_file_1‟) • Evaluate and visualize the model: do_plsa_evaluation(„config_file_1‟)

Codeword distributions per theme (topic)

Theme distributions per image

p( w | z )

p( z | d )

Demo: recognition examples

Demo: categorization results
• Performance of each theme

Demo: naïve Bayes
• Learning the model: do_naive_bayes(„config_file_2‟) • Evaluate and visualize the model: do_naive_bayes_evaluation(„config_file_2‟)

Learning and Recognition

codewords dictionary

category models (and/or) classifiers

category decision

Invariance issues
• Scale and rotation
– Implicit – Detectors and descriptors

Kadir and Brady. 2003

Invariance issues
• Scale and rotation • Occlusion
– Implicit in the models – Codeword distribution: small variations – (In theory) Theme (z) distribution: different occlusion patterns

Invariance issues
• Scale and rotation • Occlusion • Translation
– Encode (relative) location information

Sudderth et al. 2005

Invariance issues
• • • • Scale and rotation Occlusion Translation View point (in theory)
– Codewords: detector and descriptor – Theme distributions: different view points

Fergus et al. 2005

Model properties
Of all the sensory impressions proceeding to the brain, the visual experiences are the dominant ones. Our perception of the world around us is based essentially on the messages that reach the brain from our eyes. For a long time it was thought that the retinal sensory, by point image was transmitted pointbrain, to visual centers in the brain; the cerebral cortex was a visual, perception, movie screen, so to speak, upon which the retinal, cerebral cortex, image in the eye was projected. Through the discoveries of Hubelcell, optical eye, and Wiesel we now know that behind the origin of the visual nerve, image perception in the brain there is a considerably more complicated course of events. By Hubel, Wiesel following the visual impulses along their path to the various cell layers of the optical cortex, Hubel and Wiesel have been able to demonstrate that the message about the image falling on the retina undergoes a stepwise analysis in a system of nerve cells stored in columns. In this system each cell has its specific function and is responsible for a specific detail in the pattern of the retinal image.

• Intuitive
– Analogy to documents

Model properties

• Intuitive • (Could use) generative models
– Convenient for weaklyor un-supervised training – Prior information – Hierarchical Bayesian framework

Sivic et al., 2005, Sudderth et al., 2005

Model properties

• Intuitive • (Could use) generative models • Learning and recognition relatively fast
– Compare to other methods

Weakness of the model
• No rigorous geometric information of the object components • It‟s intuitive to most of us that objects are made of parts – no such information • Not extensively tested yet for
– View point invariance – Scale invariance

• Segmentation and localization unclear


				
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