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19.1.4 Triangulating Cone Strips .. .. ... .. ... .. .. ... . 567
19.1.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 568
19.1.6 Going Further: Carved Visual Hulls . . . . . . . . . . . . . . 572
19.2 Patch-Based Multi-View Stereopsis . . . . . . . . . . . . . . . . . . . 573
19.2.1 Main Elements of the PMVS Model . . . ... .. .. ... . 575
19.2.2 Initial Feature Matching . . . . . . . . . . . . . . . . . . . . . 578
19.2.3 Expansion .. .. .. ... .. .. ... .. ... .. .. ... . 579
19.2.4 Filtering ... .. .. ... .. .. ... .. ... .. .. ... . 580
19.2.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 581
19.3 The Light Field ... .. .. ... .. .. ... .. ... .. .. ... . 584
19.4 Notes . . . . . . .. .. .. ... .. .. ... .. ... .. .. ... . 587

20 Looking at People 590
20.1 HMM’s, Dynamic Programming, and Tree-Structured Models . . . . 590
20.1.1 Hidden Markov Models . . . . . ... .. ... .. .. ... . 590
20.1.2 Inference for an HMM .. .. .. ... .. ... .. .. ... . 592
20.1.3 Fitting an HMM with EM . . . . . .. .. ... .. .. ... . 597
20.1.4 Tree-Structured Energy Models . . .. .. ... .. .. ... . 600
20.2 Parsing People in Images . ... .. .. ... .. ... .. .. ... . 602
20.2.1 Parsing with Pictorial Structure Models . ... .. .. ... . 602
20.2.2 Estimating the Appearance of Clothing . . . . . . . . . . . . 604
20.3 Tracking People ... .. .. ... .. .. ... .. ... .. .. ... . 606
20.3.1 Why Human Tracking Is Hard . ... .. ... .. .. ... . 606
20.3.2 Kinematic Tracking by Appearance . . . . . . . . . . . . . . . 608
20.3.3 Kinematic Human Tracking Using Templates . . . . . . . . . 609
20.4 3D from 2D: Lifting .. .. ... .. .. ... .. ... .. .. ... . 611
20.4.1 Reconstruction in an Orthographic View . . . . . . . . . . . . 611
20.4.2 Exploiting Appearance for Unambiguous Reconstructions . . 613
20.4.3 Exploiting Motion for Unambiguous Reconstructions . . . . . 615
20.5 Activity Recognition . . . . . .. .. .. ... .. ... .. .. ... . 617
20.5.1 Background: Human Motion Data . . . . . . . . . . . . . . . 617
20.5.2 Body Configuration and Activity Recognition . . . . . . . . . 621
20.5.3 Recognizing Human Activities with Appearance Features . . 622
20.5.4 Recognizing Human Activities with Compositional Models . . 624
20.6 Resources . . . . .. .. .. ... .. .. ... .. ... .. .. ... . 624
20.7 Notes . . . . . . .. .. .. ... .. .. ... .. ... .. .. ... . 626

21 Image Search and Retrieval 627
21.1 The Application Context . . . . . . . . . . . . . . . . . . . . . . . . . 627
21.1.1 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . 628
21.1.2 User Needs . .. .. ... .. .. ... .. ... .. .. ... . 629