Multimedia Retrieval Arjen P de Vries arjen acm org

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Multimedia Retrieval Arjen P. de Vries arjen@acm. org (Note: a lot of input from Thijs Westerveld) Centrum voor Wiskunde en Informatica

Outline • Overview Indexing Multimedia • Generative Models & MMIR – Probabilistic Retrieval – Language models, GMMs • Experiments – Corel experiments

Indexing Multimedia

A Wealth of Information Speech Audio Images Temporal composition Database

Associated Information gender name country Player Profile history Biography id picture

User Interaction Poses a query Gives examples Views Evaluates query text video segments results feedback Database

Indexing Multimedia • Manually added descriptions – ‘Metadata’ • Analysis of associated data – Speech, captions, OCR, auto-cues, … • Content-based retrieval – Approximate retrieval – Domain-specific techniques

Limitations of Metadata • Vocabulary problem – Dark vs. somber • Different people describe different aspects – Dark vs. evening

Limitations of Metadata • Encoding Specificity Problem – A single person describes different aspects in different situations • Many aspects of multimedia simply cannot be expressed unambiguously – Processes in left (analytic, verbal) vs. right brain (aesthetics, synthetic, nonverbal)

Approximate Retrieval • Based on similarity – Find all objects that are similar to this one – Distance function – Representations capture some (syntactic) meaning of the object • ‘Query by Example’ paradigm

Feature extraction N-dimensional space Ranking Display

Low-level Features

Query image

So, … Retrieval? !

IR is about satisfying vague information needs provided by users, (imprecisely specified in ambiguous natural language) by satisfying them approximately against information provided by authors (specified in the same ambiguous natural language) Smeaton

No ‘Exact’ Science! • Evaluation is not done analytically, but experimentally – real users (specifying requests) – test collections (real document collections) – benchmarks (TREC: text retrieval conference) – Precision – Recall –. . .

Known Item

Query Query

Results …

Query Query

Results …

Observation • Automatic approaches are successful under two conditions: – the query example is derived from the same source as the target objects – a domain-specific detector is at hand

Semantic gap… concepts ? features raw multimedia data

1. Generic Detectors

Retrieval Process Database Query Parsing Query type Nouns Adjectives Detector / Feature selection Filtering Ranking Camera operations Invariant People, Names color spaces Natural/physical objects Monologues . .

Parameterized detectors Example Topic 41 ‘Shots with at least 8 people’ People detector <1, 2, 3, many> Results

Detectors The universe and everything F O C U S Camera operations (pan, zoom, tilt, …) People (face based) Names (Video. OCR) Natural objects (color space selection) Physical objects (color space selection) Monologues (specifically designed) Press conferences (specifically designed) Interviews (specifically designed) Domain specific detectors

2. Domain knowledge

Player Segmentation Original image Initial segmentation Final segmentation

Advanced Queries Show clips from tennis matches, starring Sampras, playing close to the net;

3. Get to know your users

Mirror Approach • Gather User’s Knowledge – Introduce semi-automatic processes for selection and combination of feature models • Local Information – Relevance feedback from a user • Global Information – Thesauri constructed from all users

Feature extraction N-dimensional space Clustering Ranking Concepts Display Thesauri

Low-level Features

Identify Groups

Representation • Groups of feature vectors are conceptually equivalent to words in text retrieval • So, techniques from text retrieval can now be applied to multimedia data as if these were text!

Query Formulation • Clusters are internal representations, not suited for user interaction • Use automatic query formulation based on global information (thesaurus) and local information (user feedback)

Interactive Query Process • Select relevant clusters from thesaurus • Search collection • Improve results by adapting query – Remove clusters occuring in irrelevant images – Add clusters occuring in relevant images

Assign Semantics

Visual Thesaurus Glcm_47 Correct cluster representing ‘Tree’, ‘Forest’ ‘Incoherent’ cluster Fractal_23 Mis-labeled cluster Gabor_20

Learning • Short-term: Adapt query to better reflect this user’s information need • Long-term: Adapt thesaurus and clustering to improve system for all users

Thesaurus Only After Feedback

4. Nobody is unique!

Collaborative Filtering • Also: social information filtering – Compare user judgments – Recommend differences between similar users • People’s tastes are not randomly distributed • You are what you buy (Amazon)

Collaborative Filtering • Benefits over content-based approach – Overcomes problems with finding suitable features to represent e. g. art, music – Serendipity – Implicit mechanism for qualitative aspects like style • Problems: large groups, broad domains

5. Ask for help

Query Articulation Feature extraction N-dimensional space How to articulate the query?

What is the query semantics ?

Details matter

Problem Statement • Feature vectors capture ‘global’ aspects of the whole image • Overall image characteristics dominate the feature-vectors • Hypothesis: users are interested in details

Irrelevant Background Query Result

Image Spots • Image-spots articulate desired image details – Foreground/background colors – Colors forming ‘shapes’ – Enclosure of shapes by background colors • Multi-spot queries define the spatial relations between a number of spots

Query Images Results Hist 16 Hist Spot+Hist 5968 6274 6098 5953 6612 6563 7062 7107 6888 7034 192 2 4 3 1 14 2 4 3 1

A: Simple Spot Query `Black sky’

B: Articulated Multi-Spot Query `Black sky’ above `Monochrome ground’

C: Histogram Search in `Black Sky’ images 2 -4: 14:

Complicating Factors • What are Good Feature Models? • What are Good Ranking Functions? • Queries are Subjective!

Probabilistic Approaches

Generative Models LM abstract video of Bayesian model to that present the disclosure can a on for retrieval in have is probabilistic still of for of using this In that is to only queries visual combines visual information look search video the retrieval based search. Both get decision (a visual generic results (a difficult We visual we still needs, search. talk what to do this for with retrieval still specific retrieval information a as model still

Generative Models… • A statistical model for generating data – Probability distribution over samples in a given ‘language’aka M ‘Language( Modelling’ P( |M) =P |M) P ( | M, ) © Victor Lavrenko, Aug. 2002

‘Language Modelling’ • Not just ‘English’ • But also, the language of – – author newspaper text document image • Hiemstra or Robertson? • ‘Parsimonious language models explicitly address the relation between levels of language models that are typically used for smoothing. ’

‘Language Modelling’ • Not just ‘English’ • But also, the language of – – author newspaper text document image • Guardian or Times?

‘Language Modelling’ • Not just English! • But also, the language of – – author newspaper text document image • or ?

Unigram and higher-order models P( ) =P( )P( | ) P( | • Unigram Models P( )P( ) • N-gram Models P( )P( | ) • Other Models – Grammar-based models, etc. – Mixture models © Victor Lavrenko, Aug. 2002 )

The fundamental problem • Usually we don’t know the model M – But have a sample representative of that model P( |M( )) • First estimate a model from a sample • Then compute the observation probability M © Victor Lavrenko, Aug. 2002

Indexing: determine models Docs Models • Indexing – Estimate Gaussian Mixture Models from images using EM – Based on feature vector with colour, texture and position information from pixel blocks – Fixed number of components

Retrieval: use query likelihood • Query: • Which of the models is most likely to generate these 24 samples?

Probabilistic Image Retrieval ?

Rank by P(Q|M) P(Q|M 1) P(Q|M 2) P(Q|M 3) P(Q|M 4) Query

Probabilistic Retrieval Model • Text – Rank using probability of drawing query terms from document models • Images – Rank using probability of drawing query blocks from document models • Multi-modal – Rank using joint probability of drawing query samples from document models

Text Models • Unigram Language Models (LM) – Urn metaphor • P( © Victor Lavrenko, Aug. 2002 )~P( )P( ) = 4/9 * 2/9 * 4/9 * 3/9

Generative Models and IR • Rank models (documents) by probability of generating the query • Q: • P( | ) = 4/9 * 2/9 * 4/9 * 3/9 = 96/94 • P( | ) = 3/9 * 3/9 = 81/94 • P( | ) = 2/9 * 3/9 * 2/9 * 4/9 = 48/94 • P( | ) = 2/9 * 5/9 * 2/9 = 40/94

Image Models • Urn metaphor not useful – Drawing pixels useless • Pixels carry no semantics – Drawing pixel blocks not effective • chances of drawing exact query blocks from document slim • Use Gaussian Mixture Models (GMM) – Fixed number of Gaussian components/clusters/concepts

Key-frame representation split Y colour channels Cb Cr Take samples position DCT coefficients Query model 675 661 668 665 669 9 7 -7 10 -5 12 13 13 11 18 11 5 3 2 7 1 -5 -3 4 -3 9 11 0 5 1 4 3 -1 2 -5 1517 1536 1534 -9 2 0 0 0 -3 -4 -5 -5 -5 0 0 0 0 1 0 0 0 1 0 0 850 844 837 829 833 EM algorithm 15 5 3 0 -5 4 4 3 3 4 0 -2 -3 -1 -1 1 0 0 4 1 -2 0 3 -2 -2 1 0 -1 1 1 1 1 2 3 4 5

Image Models ? • Expectation-Maximisation (EM) algorithm – iteratively • estimate component assignments • re-estimate component parameters

Expectation Maximization E M Component 1 Component 2 Component 3

Expectation Maximization animation E M Component 1 Component 2 Component 3

Corel Experiments

Testing the Model on Corel • 39 classes, ~100 images each • Build models from all images • Use each image as query – Rank full collection – Compute MAP (mean average precision) • AP=average of precision values after each relevant image is retrieved • MAP is mean of AP over multiple queries – Relevant from query class

Query: Top 5: Example results

MAP per Class (mean: . 12) • • • • English Pub Signs English Country Gardens Arabian Horses. 31 Dawn & Dusk. 21 Tropical Plants. 19 Land of the Pyramids Canadian Rockies Lost Tribes. 17 Elephants. 17 Tigers. 16 Tropical Sea Life Exotic Tropical Flowers Lions. 15 Indigenous People Nesting Birds. 13 … . 36. 33 . 19. 18 . 16. 15 • • • • … Sweden. 07 Ireland. 07 Wildlife of the Galapagos Hawaii. 07 Rural France. 07 Zimbabwe. 07 Images of Death Valley Nepal. 07 Foxes & Coyotes North American Deer California Coasts North American Wildlife Peru. 05 Alaskan Wildlife. 05 Namibia. 05 . 07. 06. 06

Class confusion • • Query from class A Relevant from class B Queries retrieve images from own class Interesting mix-ups – Beaches – Greek islands – Indigenous people – Lost tribes – English country gardens – Tropical plants – Arabian Horses • Similar backgrounds

Background Matching Query: Top 5:

Care for more? T. Westerveld and A. P. de Vries, Generative probabilistic models for multimedia retrieval: query generation versus document generation, in IEE Vision, Image and Signal Processing. http: //www. cwi. nl/~arjen/ arjen@acm. org