Object Recognition as Machine Translation Matching Words and Pictures Heather Dunlop : Advanced Perception April 17, 2006

Machine Translation Altavista’s Babel Fish: –There are three more weeks of classes! –Il y a seulement trois semaines supplémentaires de classes! –¡Hay solamente tres más semanas de clases! –Ci sono soltanto tre nuove settimane dei codici categoria! –Es gibt nur drei weitere Wochen Kategorien!

Statistical Machine Translation Statistically link words in one language to words in another Requires aligned bitext –eg. Hansard for Canadian parliament

Statistical Machine Translation Assuming an unknown one-one correspondence between words, come up with a joint probability distribution linking words in the two languages Missing data problem: solution is EM Given the translation probabilities, estimate the correspondences Given the correspondences, estimate the translation probabilities

Multimedia Translation Data: –Words are associated with images, but correspondences are unknown sun sea sky

Auto-Annotation Predicting words for the images tiger grass cat

Region Naming Can also be applied to object recognition Requires a large data set

Browsing

Auto-Illustration Moby Dick

Data Sets of Annotated Images Corel data set Museum image collections News photos (with captions)

First Paper Object Recognition as Machine Translation: Learning a Lexicon for a Fixed Image Vocabulary by Pinar Duygulu, Kobus Barnard, Nando de Freitas, David Forsyth –A simple model for annotation and correspondence

Overview

Input Representation Segment with Normalized Cuts: Only use regions larger than a threshold (typically 5-10 per image) Form vector representation of each region Cluster regions with k-means to form blob tokens sun sky waves sea word tokens

Input Representation Represent each region with a feature vector –Size: portion of the image covered by the region –Position: coordinates of center of mass –Color: avg. and std. dev. of (R,G,B), (L,a,b) and (r=R/(R+G+B),g=G/(R+G+B)) –Texture: avg. and variance of 16 filter responses –Shape: area / perimeter 2, moment of inertia, region area / area of convex hull

Tokenization

Assignments Each word is predicted with some probability by each blob

Expectation Maximization Select word with highest probability to assign to each blob probability that blob b ni translates to word w nj probability of obtaining word w nj given instance of blob b ni # of images # of words # of blobs

Expectation Maximization Initialize to blob-word co-occurrences: Iterate: Given the translation probabilities, estimate the correspondences Given the correspondences, estimate the translation probabilities

Word Prediction On a new image: –Segment –For each region: Extract features Find the corresponding blob token using nearest neighbor Use the word posterior probabilities to predict words

Refusing to Predict Require: p(word|blob) > threshold –ie. Assign a null word to any blob whose best predicted word lies below the threshold Prunes vocabulary, so fit new lexicon

Indistinguishable Words Visually indistinguishable: –cat and tiger, train and locomotive Indistinguishable with our features: –eagle and jet Entangled correspondence: –polar – bear –mare/foals – horse Solution: cluster similar words –Obtain similarity matrix –Compare words with symmetrised KL divergence –Apply N-Cuts on matrix to get clusters –Replace word with its cluster label

Experiments Train with 4500 Corel images –4-5 words for each image –371 words in vocabulary –5-10 regions per image –500 blobs Test on 500 images

Auto-Annotation Determine most likely word for each blob If probability of word is greater than some threshold, use in annotation

Measuring Performance Do we predict the right words?

Region Naming / Correspondence

Measuring Performance Do we predict the right words? Are they on the right blobs? Difficult to measure because data set contains no correspondence information Must be done by hand on a smaller data set Not practical to count false negatives

Successful Results

Unsuccessful Results

Refusing to Predict

Clustering

Merging Regions

Results light bar = average number of times blob predicts word in correct place dark bar = average number of times blob predicts word which is in the image

Second paper Matching Words and Pictures by Kobus Barnard, Pinar Duygulu, Nando de Freitas, David Forsyth, David Blei, Michael I. Jordan –Comparing lots of different models for annotation and correspondence

Annotation Models Multi-modal hierarchical aspect models Mixture of multi-modal LDA

Multi-Model Hierarchical Aspect Model cluster = a path from a leaf to the root

Multi-Model Hierarchical Aspect Model All observations are produced independent of one another I-0: as above I-1: cluster dependent level structure –p(l|d) replaced with p(l|c,d) I-2: generative model –p(l|d) replaced with p(l|c) –allows prediction for documents not in training set document observations clusterslevels normalization Gaussian frequency tables

Multi-Model Hierarchical Aspect Model Model fitting is done with EM Word prediction: set of observed blobs

Mixture of Multi-Modal LDA multinomial Dirichlet multinomial multivariate Gaussian mixture component and hidden factor

Mixture of Multi-Modal LDA Distribution parameters estimated with EM Word prediction: posterior over mixture components posterior Dirichlet

Correspondence Models Discrete translation Hierarchical clustering Linking word and region emission probabilities Paired word and region emission

Discrete Translation Similar to first paper Use k-means to vector-quantize the set of features representing an image region Construct a joint probability table linking word tokens to blob tokens Data set doesn’t provide explicit correspondences –Missing data problem => EM

Hierarchical Clustering Again, using vector-quantized image regions Word prediction:

Linking Word and Region Emission Words emitted conditioned on observed blobs D-O: as above (D for dependent) D-1: cluster dependent level distributions –Replace p(l|c,d) with p(l|d) D-2: generative model –Replace p(l|d) with p(l) B U W

Paired Word and Region Emission at Nodes Observed words and regions are emitted in pairs: D={(w,b)} C-0: as above (C for correspondence) C-1: cluster dependent level structure –p(l|d) replaced with p(l|c,d) C-2: generative model –p(l|d) replaced with p(l|c)

Wow, That’s a Lot of models! Multi-modal hierarchical: I-0, I-1, I-2 Multi-modal LDA Discrete translation Hierarchical clustering Linked word and region emission: D-0, D-1, D-2 Paired word and region emission: C-0, C-1, C-2 Count = 12 Why so many?

Evaluation Methods Annotation performance measures: –KL divergence between predicted and target distributions: –Word prediction measure: n = # of words in image r = # of words predicted correctly # of words predicted is set to # of actual keywords –Normalized classification score: w = # of words predicted incorrectly N = vocabulary size

Results Methods using clustering are very reliant on having images that are close to the training data MoM-LDA has strong resistance to over-fitting D-0 (linked word and region emission) appears to give best results, taking all measures and data sets into consideration

Successful Results

Unsuccessful Results good annotation, poor correspondence complete failure

N-cuts vs. Blobworld Normalized Cuts Blobworld

N-cuts vs. Blobworld

Browsing Results Clustering by text onlyClustering by image features only

Browsing Results Clustering by both text and image features only

Search Results query: tiger, river tiger, cat, water, grass tiger, cat, grass, trees tiger, cat, water, grass tiger, cat, grass, forest tiger, cat, water, grass

Auto-Illustration Results Passage from Moby Dick: –“The large importance attached to the harpooneer's vocation is evinced by the fact, that originally in the old Dutch Fishery, two centuries and more ago, the command of a whale-ship!…” Words extracted from the passage using natural language processing tools –large importance attached fact old dutch century more command whale ship was per son was divided officer word means fat cutter time made days was general vessel whale hunting concern british title old dutch official present rank such more good american officer boat night watch ground command ship deck grand political sea men mast

Auto-Illustration Results Top-ranked images retrieved using all extracted words:

Conclusions Lots of different models developed –Hard to tell which is best Can be used with any set of features Numerous applications: –Auto-annotation –Region naming (aka object recognition) –Browsing –Searching –Auto-illustration Improvements in translation from visual to semantic representations lead to improvements in image access