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  1. TEMPORAL EVENT CLUSTERING FOR DIGITAL PHOTO COLLECTIONS Matthew Cooper, Jonathan Foote, Andreas Girgensohn, and Lynn Wilcox ACM Multimedia ACM Transactions on Multimedia Computing , Communications and Application

  2. OUTLINE • Introduction • Feature extraction • Clustering techniques • Supervised event clustering • Unsupervised event clustering • Clustering goodness criteria • Experimental result • Conclusion

  3. Introduction • Users navigate their photos • Temporal order • Visual content • Associate time and content with the notion of a specific “event” • Photos associated with an event often exhibit little coherence in terms of either low-level image features or visual similarity • photographs from the same event are taken in relatively close proximity in time

  4. Basic concepts--- Event • Events are naturally associated with specific times and places. • Birthday party • Vacation • Wedding

  5. Basic concepts--- EXIF & CBIR Metadata • Exchangeable Image File (EXIF): • Time, Location, Focal length, Flash, etc. • => Season, place, weather, indoor/outdoor,etc • Content-based Image Retrieval (CBIR): • Color, Texture, Shape, etc. • => Face & Fingerprint Recognition,etc

  6. FEATURE EXTRACTION • EXIF headers are processed to extract the timestamp • The N photos in the collection are then ordered in time so the resulting timestamps, {tn:n = 1, . . . , N},satisfy t1 ≤ t2 ≤ … ≤ tN • Time difference between indices (photos) is nonuniform t1 t2 t3 t4 t5 t6 ….. t

  7. FEATURE EXTRACTION • Computing similarity matrices SK temporal similarity matrix

  8. FEATURE EXTRACTION • Computing similarity matrix • low-frequency discrete cosine transform (DCT) coefficients from each photo using the cosine distance measure content-based similarity matrix

  9. FEATURE EXTRACTION • computing novelty scores peaks in the novelty scores = cluster boundaries between contiguous groups of similar photos K=1000 K=10000 K=100000

  10. CLUSTERING TECHNIQUES • Supervised event clustering • Based on LVQ • Unsupervised event clustering • Scale-space analysis of the raw timestamp data • Temporal Similarity Analysis • Combining Time and Content-Based Similarity

  11. Supervised event clustering • Let K take M values : K ≡ {K1, . . . , KM} • Define the M × N matrix N(j,i) = νKj (i) • , where • Based on LVQ (Learning Vector Quantization) [Kohonen 1989] • LVQ codebook discriminates between the two classes “event boundary” and “event interior.” • The codebook vectors for each class are used for nearest-neighbor classification of the novelty features for each photo in the test set.

  12. Supervised event clustering • In the training phase, a codebook is calculated using an iterative procedure • Each step • Nearest codebook vector to each training sample is determined • shifted toward or away the training sample If Nx and Mc are in the same class If Nx and Mc aren’t in the same class

  13. Supervised event clustering • ALGORITHM 1 (LVQ-BASED PHOTO CLUSTERING). • (1) Calculate novelty features from labeled sorted training data for each scale K : • (i) compute the similarity matrix SK • (ii) compute the novelty score νK • (2) Train LVQ using the iterative procedure • (3) Calculate novelty features for the testing data for each K • (i) compute the similarity matrix SK • (ii) compute the novelty score νK • (4) Classify each test sample’s novelty features Ni using the LVQ codebook and the nearest-neighbor rule.

  14. Unsupervised event clustering • scale-space analysis • operate on the raw timestamps • T0 = [t1, . . . , tN] so that T0(i) = ti • ALGORITHM 2 (SCALE-SPACE PHOTO CLUSTERING). • (1) Extract timestamp data from photo collection: {t1, . . . , tN}. • (2) For each σ in descending order: • (i) compute Tσ • (ii) detect peaks in Tσ , tracing peaks from larger to smaller scales (decreasing σ).

  15. UNSUPERVISED EVENT CLUSTERING • Temporal Similarity Analysis • Locate peaks at each scale by analysis of the first difference of each novelty scoresνK, proceeding from coarse scale to fine (decreasing K) • To build a hierarchical set of event boundaries, we include boundaries detected at coarse scales in the boundary lists for all finer scales. checkerboard kernel used to compute the novelty features

  16. UNSUPERVISED EVENT CLUSTERING • Combining Time and Content-Based Similarity • constructed a content-based matrix SC using low-frequency DCT features and the cosine distance if |ti-tj| > 48h others if |ti-tj| > 48h others

  17. CLUSTERING GOODNESS CRITERIA • Peak detection at each scale K results in a hierarchical set of candidate boundaries • Subset must be selected to define the final event clusters • Three different automatic approaches • Similarity-Based Confidence Score • Boundary Selection via Dynamic Programming • BIC-Based Boundary Selection

  18. Similarity-Based Confidence Score Detected boundaries at each level K, BK = {b1, . . . , bnK }, indexed by photo: BK ⊂ {1, . . . , N} average interclustersimilarity between photos in adjacentclusters average intracluster similarity between the photos within eachcluster

  19. Boundary Selection via Dynamic Programming Reduced complexity Begin with the set of peaks detected from the novelty features at all scales Cost of the cluster between photos bi and bj

  20. Boundary Selection via Dynamic Programming • Optimal partitions with m boundaries based on the optimal partition with m−1 boundaries • First, optimal partitions are computed with two clusters • EF (j,m) is the optimal partition of the photos with cardinality m

  21. Boundary Selection via Dynamic Programming • Number of clusters increases, the total cost of the partition decreases monotonically • Selecting the optimal number of clusters, M∗, based on the total partition cost

  22. BIC-Based Boundary Selection • This method is based on the Bayes information criterion (BIC) [Schwarz 1978] • Assumption • timestamps within an event are distributed normally around the event mean Log-likelihood of the single segment model and the penalty term log-likelihood of the two segment model λ is 2 ,since we describe each segment using the sample mean μ,and variance, σ2

  23. BIC-BASED BOUNDARY SELECTION • Employ the hierarchical coarse-to-fine approach • At each scale, we test only the newly detected boundaries (undetected at coarser scales) • Add the boundaries for which the left side exceeds the right side

  24. ALGORITHM 3 (SIMILARITY-BASED PHOTO CLUSTERING) • (1) Extract and sort photo timestamps, {t1, . . . , tn}. • (2) For each K in decreasing order • (i) compute the similarity matrix Sk • (ii) compute the novelty score νK • (iii) detect peaks in νK • (iv) form event boundary list using event boundaries from previous iterations and newly detected peaks • (3) Determine a final boundary subset of collected boundaries over all scales considered according to one of the methods : • (a) the confidence score • (b) the DP boundary selection approach • (c) the BIC boundary selection approach

  25. EXPERIMENTAL RESULT • Run Times for Different Size Photo Collections • The times are in seconds • No Conf. indicates times for Steps 1 and 2 • BIC peak selection (BIC) • Dynamic programming peak selection (DP) • similarity-based peak selection (Conf.) • Doubling the number of photos(N),the time for the segmentation step(No Conf.) increases linearly, while including the confidence measure (Conf.) incurs a polynomial cost.

  26. EXPERIMENTAL RESULT • Compare the event clustering performance of eleven systems on two separate photo collections • Collection I consists of 1036 photos taken over 15 months • Collection II consists of 413 photos taken over 13 months • The first four algorithms in the table are “hand-tuned” to maximize performance. • The remaining algorithms are fully automatic.

  27. EXPERIMENTAL RESULT • Precision indicates the proportion of falsely labeled boundaries: • Recall measures the proportion of true boundaries detected: • The F-score is a composite of precision and recall:

  28. EXPERIMENTAL RESULT

  29. EXPERIMENTAL RESULT • The adaptive-thresholding algorithms exhibited high recall and low precision on both test sets, even with manual tuning • Scale-space and the two similarity-based approaches demonstrated more consistent performance and traded off precision and recall more evenly

  30. CONCLUSION • Employed the automatic temporal similarity-based method • Does not rely on preset thresholds or restrictive assumptions • As photo collections with location information become available, we hope to extend our system to combine temporal similarity, content-based similarity, and location-based similarity. • The automatic methods’ performance exceeded that of manually tuned alternatives in our testing, and have been well received by users of our photo management application.