Computer Vision: A Modern Approach, 2nd edition

I IMAGE FORMATION 1
1 Geometric Camera Models 3
1.1 Image Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.1 Pinhole Perspective . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.2 Weak Perspective . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1.3 Cameras with Lenses . . . . . . . . . . . . . . . . . . . . . . . 8
1.1.4 The Human Eye . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.2 Intrinsic and Extrinsic Parameters . . . . . . . . . . . . . . . . . . . 14
1.2.1 Rigid Transformations and Homogeneous Coordinates . . . . 14
1.2.2 Intrinsic Parameters . . . . . . . . . . . . . . . . . . . . . . . 16
1.2.3 Extrinsic Parameters . . . . . . . . . . . . . . . . . . . . . . . 18
1.2.4 Perspective Projection Matrices . . . . . . . . . . . . . . . . . 19
1.2.5 Weak-Perspective Projection Matrices . . . . . . . . . . . . . 20
1.3 Geometric Camera Calibration . . . . . . . . . . . . . . . . . . . . . 22
1.3.1 ALinear Approach to Camera Calibration . . . . . . . . . . . 23
1.3.2 ANonlinear Approach to Camera Calibration . . . . . . . . . 27
1.4 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2 Light and Shading 32
2.1 Modelling Pixel Brightness . . . . . . . . . . . . . . . . . . . . . . . 32
2.1.1 Reflection at Surfaces . . . . . . . . . . . . . . . . . . . . . . 33
2.1.2 Sources and Their Effects . . . . . . . . . . . . . . . . . . . . 34
2.1.3 The Lambertian+Specular Model . . . . . . . . . . . . . . . . 36
2.1.4 Area Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
2.2 Inference from Shading . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.2.1 Radiometric Calibration and High Dynamic Range Images . . 38
2.2.2 The Shape of Specularities . . . . . . . . . . . . . . . . . . . 40
2.2.3 Inferring Lightness and Illumination . . . . . . . . . . . . . . 43
2.2.4 Photometric Stereo: Shape from Multiple Shaded Images . . 46
2.3 Modelling Interreflection . . . . . . . . . . . . . . . . . . . . . . . . . 52
2.3.1 The Illumination at a Patch Due to an Area Source . . . . . 52
2.3.2 Radiosity and Exitance . . . . . . . . . . . . . . . . . . . . . 54
2.3.3 An Interreflection Model . . . . . . . . . . . . . . . . . . . . . 55
2.3.4 Qualitative Properties of Interreflections . . . . . . . . . . . . 56
2.4 Shape from One Shaded Image . . . . . . . . . . . . . . . . . . . . . 59
2.5 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3 Color 68
3.1 Human Color Perception . . . . . . . . . . . . . . . . . . . . . . . . . 68
3.1.1 Color Matching . . . . . . . . . . . . . . . . . . . . . . . . . . 68
3.1.2 Color Receptors . . . . . . . . . . . . . . . . . . . . . . . . . 71
3.2 The Physics of Color . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
3.2.1 The Color of Light Sources . . . . . . . . . . . . . . . . . . . 73
3.2.2 The Color of Surfaces . . . . . . . . . . . . . . . . . . . . . . 76
3.3 Representing Color . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
3.3.1 Linear Color Spaces . . . . . . . . . . . . . . . . . . . . . . . 77
3.3.2 Non-linear Color Spaces . . . . . . . . . . . . . . . . . . . . . 83
3.4 AModel of Image Color . . . . . . . . . . . . . . . . . . . . . . . . . 86
3.4.1 The Diffuse Term . . . . . . . . . . . . . . . . . . . . . . . . . 88
3.4.2 The Specular Term . . . . . . . . . . . . . . . . . . . . . . . . 90
3.5 Inference from Color . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
3.5.1 Finding Specularities Using Color . . . . . . . . . . . . . . . 90
3.5.2 Shadow Removal Using Color . . . . . . . . . . . . . . . . . . 92
3.5.3 Color Constancy: Surface Color from Image Color . . . . . . 95
3.6 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
II EARLY VISION: JUST ONE IMAGE 105
4 Linear Filters 107
4.1 Linear Filters and Convolution . . . . . . . . . . . . . . . . . . . . . 107
4.1.1 Convolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
4.2 Shift Invariant Linear Systems . . . . . . . . . . . . . . . . . . . . . 112
4.2.1 Discrete Convolution . . . . . . . . . . . . . . . . . . . . . . . 113
4.2.2 Continuous Convolution . . . . . . . . . . . . . . . . . . . . . 115
4.2.3 Edge Effects in Discrete Convolutions . . . . . . . . . . . . . 118
4.3 Spatial Frequency and Fourier Transforms . . . . . . . . . . . . . . . 118
4.3.1 Fourier Transforms . . . . . . . . . . . . . . . . . . . . . . . . 119
4.4 Sampling and Aliasing . . . . . . . . . . . . . . . . . . . . . . . . . . 121
4.4.1 Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
4.4.2 Aliasing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
4.4.3 Smoothing and Resampling . . . . . . . . . . . . . . . . . . . 126
4.5 Filters as Templates . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
4.5.1 Convolution as a Dot Product . . . . . . . . . . . . . . . . . 131
4.5.2 Changing Basis . . . . . . . . . . . . . . . . . . . . . . . . . . 132
4.6 Technique: Normalized Correlation and Finding Patterns . . . . . . 132
4.6.1 Controlling the Television by Finding Hands by Normalized
Correlation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
4.7 Technique: Scale and Image Pyramids . . . . . . . . . . . . . . . . . 134
4.7.1 The Gaussian Pyramid . . . . . . . . . . . . . . . . . . . . . 135
4.7.2 Applications of Scaled Representations . . . . . . . . . . . . . 136
4.8 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
5 Local Image Features 141
5.1 Computing the Image Gradient . . . . . . . . . . . . . . . . . . . . . 141
5.1.1 Derivative of Gaussian Filters . . . . . . . . . . . . . . . . . . 142
5.2 Representing the Image Gradient . . . . . . . . . . . . . . . . . . . . 144
5.2.1 Gradient-Based Edge Detectors . . . . . . . . . . . . . . . . . 145
5.2.2 Orientations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
5.3 Finding Corners and Building Neighborhoods . . . . . . . . . . . . . 148
5.3.1 Finding Corners . . . . . . . . . . . . . . . . . . . . . . . . . 149
5.3.2 Using Scale and Orientation to Build a Neighborhood . . . . 151
5.4 Describing Neighborhoods with SIFT and HOG Features . . . . . . 155
5.4.1 SIFT Features . . . . . . . . . . . . . . . . . . . . . . . . . . 157
5.4.2 HOG Features . . . . . . . . . . . . . . . . . . . . . . . . . . 159
5.5 Computing Local Features in Practice . . . . . . . . . . . . . . . . . 160
5.6 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
6 Texture 164
6.1 Local Texture Representations Using Filters . . . . . . . . . . . . . . 166
6.1.1 Spots and Bars . . . . . . . . . . . . . . . . . . . . . . . . . . 167
6.1.2 From Filter Outputs to Texture Representation . . . . . . . . 168
6.1.3 Local Texture Representations in Practice . . . . . . . . . . . 170
6.2 Pooled Texture Representations by Discovering Textons . . . . . . . 171
6.2.1 Vector Quantization and Textons . . . . . . . . . . . . . . . . 172
6.2.2 K-means Clustering for Vector Quantization . . . . . . . . . . 172
6.3 Synthesizing Textures and Filling Holes in Images . . . . . . . . . . 176
6.3.1 Synthesis by Sampling Local Models . . . . . . . . . . . . . . 176
6.3.2 Filling in Holes in Images . . . . . . . . . . . . . . . . . . . . 179
6.4 Image Denoising . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
6.4.1 Non-local Means . . . . . . . . . . . . . . . . . . . . . . . . . 183
6.4.2 Block Matching 3D (BM3D) . . . . . . . . . . . . . . . . . . 183
6.4.3 Learned Sparse Coding . . . . . . . . . . . . . . . . . . . . . 184
6.4.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
6.5 Shape from Texture . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
6.5.1 Shape from Texture for Planes . . . . . . . . . . . . . . . . . 187
6.5.2 Shape from Texture for Curved Surfaces . . . . . . . . . . . . 190
6.6 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
III EARLY VISION: MULTIPLE IMAGES 195
7 Stereopsis 197
7.1 Binocular Camera Geometry and the Epipolar Constraint . . . . . . 198
7.1.1 Epipolar Geometry . . . . . . . . . . . . . . . . . . . . . . . . 198
7.1.2 The Essential Matrix . . . . . . . . . . . . . . . . . . . . . . . 200
7.1.3 The Fundamental Matrix . . . . . . . . . . . . . . . . . . . . 201
7.2 Binocular Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . 201
7.2.1 Image Rectification . . . . . . . . . . . . . . . . . . . . . . . . 202
7.3 Human Stereopsis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
7.4 Local Methods for Binocular Fusion . . . . . . . . . . . . . . . . . . 205
7.4.1 Correlation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
7.4.2 Multi-Scale Edge Matching . . . . . . . . . . . . . . . . . . . 207
7.5 Global Methods for Binocular Fusion . . . . . . . . . . . . . . . . . . 210
7.5.1 Ordering Constraints and Dynamic Programming . . . . . . . 210
7.5.2 Smoothness and Graphs . . . . . . . . . . . . . . . . . . . . . 211
7.6 Using More Cameras . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
7.7 Application: Robot Navigation . . . . . . . . . . . . . . . . . . . . . 215
7.8 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
8 Structure from Motion 221
8.1 Internally Calibrated Perspective Cameras . . . . . . . . . . . . . . . 221
8.1.1 Natural Ambiguity of the Problem . . . . . . . . . . . . . . . 223
8.1.2 Euclidean Structure and Motion from Two Images . . . . . . 224
8.1.3 Euclidean Structure and Motion from Multiple Images . . . . 228
8.2 Uncalibrated Weak-Perspective Cameras . . . . . . . . . . . . . . . . 230
8.2.1 Natural Ambiguity of the Problem . . . . . . . . . . . . . . . 231
8.2.2 Affine Structure and Motion from Two Images . . . . . . . . 233
8.2.3 Affine Structure and Motion from Multiple Images . . . . . . 237
8.2.4 From Affine to Euclidean Shape . . . . . . . . . . . . . . . . 238
8.3 Uncalibrated Perspective Cameras . . . . . . . . . . . . . . . . . . . 240
8.3.1 Natural Ambiguity of the Problem . . . . . . . . . . . . . . . 241
8.3.2 Projective Structure and Motion from Two Images . . . . . . 242
8.3.3 Projective Structure and Motion from Multiple Images . . . . 244
8.3.4 From Projective to Euclidean Shape . . . . . . . . . . . . . . 246
8.4 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
IV MID-LEVEL VISION 253
9 Segmentation by Clustering 255
9.1 Human Vision: Grouping and Gestalt . . . . . . . . . . . . . . . . . 256
9.2 Important Applications . . . . . . . . . . . . . . . . . . . . . . . . . 261
9.2.1 Background Subtraction . . . . . . . . . . . . . . . . . . . . . 261
9.2.2 Shot Boundary Detection . . . . . . . . . . . . . . . . . . . . 264
9.2.3 Interactive Segmentation . . . . . . . . . . . . . . . . . . . . 265
9.2.4 Forming Image Regions . . . . . . . . . . . . . . . . . . . . . 266
9.3 Image Segmentation by Clustering Pixels . . . . . . . . . . . . . . . 268
9.3.1 Basic Clustering Methods . . . . . . . . . . . . . . . . . . . . 269
9.3.2 The Watershed Algorithm . . . . . . . . . . . . . . . . . . . . 271
9.3.3 Segmentation Using K-means . . . . . . . . . . . . . . . . . . 272
9.3.4 Mean Shift: Finding Local Modes in Data . . . . . . . . . . . 273
9.3.5 Clustering and Segmentation with Mean Shift . . . . . . . . . 275
9.4 Segmentation, Clustering, and Graphs . . . . . . . . . . . . . . . . . 277
9.4.1 Terminology and Facts for Graphs . . . . . . . . . . . . . . . 277
9.4.2 Agglomerative Clustering with a Graph . . . . . . . . . . . . 279
9.4.3 Divisive Clustering with a Graph . . . . . . . . . . . . . . . . 281
9.4.4 Normalized Cuts . . . . . . . . . . . . . . . . . . . . . . . . . 284
9.5 Image Segmentation in Practice . . . . . . . . . . . . . . . . . . . . . 285
9.5.1 Evaluating Segmenters . . . . . . . . . . . . . . . . . . . . . . 286
9.6 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
10 Grouping and Model Fitting 290
10.1 The Hough Transform . . . . . . . . . . . . . . . . . . . . . . . . . . 290
10.1.1 Fitting Lines with the Hough Transform . . . . . . . . . . . . 290
10.1.2 Using the Hough Transform . . . . . . . . . . . . . . . . . . . 292
10.2 Fitting Lines and Planes . . . . . . . . . . . . . . . . . . . . . . . . . 293
10.2.1 Fitting a Single Line . . . . . . . . . . . . . . . . . . . . . . . 294
10.2.2 Fitting Planes . . . . . . . . . . . . . . . . . . . . . . . . . . 295
10.2.3 Fitting Multiple Lines . . . . . . . . . . . . . . . . . . . . . . 296
10.3 Fitting Curved Structures . . . . . . . . . . . . . . . . . . . . . . . . 297
10.4 Robustness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299
10.4.1 M-Estimators . . . . . . . . . . . . . . . . . . . . . . . . . . . 300
10.4.2 RANSAC: Searching for Good Points . . . . . . . . . . . . . 302
10.5 Fitting Using Probabilistic Models . . . . . . . . . . . . . . . . . . . 306
10.5.1 Missing Data Problems . . . . . . . . . . . . . . . . . . . . . 307
10.5.2 Mixture Models and Hidden Variables . . . . . . . . . . . . . 309
10.5.3 The EM Algorithm for Mixture Models . . . . . . . . . . . . 310
10.5.4 Difficulties with the EM Algorithm . . . . . . . . . . . . . . . 312
10.6 Motion Segmentation by Parameter Estimation . . . . . . . . . . . . 313
10.6.1 Optical Flow and Motion . . . . . . . . . . . . . . . . . . . . 315
10.6.2 Flow Models . . . . . . . . . . . . . . . . . . . . . . . . . . . 316
10.6.3 Motion Segmentation with Layers . . . . . . . . . . . . . . . 317
10.7 Model Selection: Which Model Is the Best Fit? . . . . . . . . . . . . 319
10.7.1 Model Selection Using Cross-Validation . . . . . . . . . . . . 322
10.8 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322
11 Tracking 326
11.1 Simple Tracking Strategies . . . . . . . . . . . . . . . . . . . . . . . . 327
11.1.1 Tracking by Detection . . . . . . . . . . . . . . . . . . . . . . 327
11.1.2 Tracking Translations by Matching . . . . . . . . . . . . . . . 330
11.1.3 Using Affine Transformations to Confirm a Match . . . . . . 332
11.2 Tracking Using Matching . . . . . . . . . . . . . . . . . . . . . . . . 334
11.2.1 Matching Summary Representations . . . . . . . . . . . . . . 335
11.2.2 Tracking Using Flow . . . . . . . . . . . . . . . . . . . . . . . 337
11.3 Tracking Linear Dynamical Models with Kalman Filters . . . . . . . 339
11.3.1 Linear Measurements and Linear Dynamics . . . . . . . . . . 340
11.3.2 The Kalman Filter . . . . . . . . . . . . . . . . . . . . . . . . 344
11.3.3 Forward-backward Smoothing . . . . . . . . . . . . . . . . . . 345
11.4 Data Association . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349
11.4.1 Linking Kalman Filters with Detection Methods . . . . . . . 349
11.4.2 Key Methods of Data Association . . . . . . . . . . . . . . . 350
11.5 Particle Filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350
11.5.1 Sampled Representations of Probability Distributions . . . . 351
11.5.2 The Simplest Particle Filter . . . . . . . . . . . . . . . . . . . 355
11.5.3 The Tracking Algorithm . . . . . . . . . . . . . . . . . . . . . 356
11.5.4 A Workable Particle Filter . . . . . . . . . . . . . . . . . . . . 358
11.5.5 Practical Issues in Particle Filters . . . . . . . . . . . . . . . 360
11.6 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362
V HIGH-LEVEL VISION 365
12 Registration 367
12.1 Registering Rigid Objects . . . . . . . . . . . . . . . . . . . . . . . . 368
12.1.1 Iterated Closest Points . . . . . . . . . . . . . . . . . . . . . . 368
12.1.2 Searching for Transformations via Correspondences . . . . . . 369
12.1.3 Application: Building Image Mosaics . . . . . . . . . . . . . . 370
12.2 Model-based Vision: Registering Rigid Objects with Projection . . . 375
12.2.1 Verification: Comparing Transformed and Rendered Source
to Target . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377
12.3 Registering Deformable Objects . . . . . . . . . . . . . . . . . . . . . 378
12.3.1 Deforming Texture with Active Appearance Models . . . . . 378
12.3.2 Active Appearance Models in Practice . . . . . . . . . . . . . 381
12.3.3 Application: Registration in Medical Imaging Systems . . . . 383
12.4 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388
13 Smooth Surfaces and Their Outlines 391
13.1 Elements of Differential Geometry . . . . . . . . . . . . . . . . . . . 393
13.1.1 Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393
13.1.2 Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397
13.2 Contour Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402
13.2.1 The Occluding Contour and the Image Contour . . . . . . . . 402
13.2.2 The Cusps and Inflections of the Image Contour . . . . . . . 403
13.2.3 Koenderink’s Theorem . . . . . . . . . . . . . . . . . . . . . . 404
13.3 Visual Events: More Differential Geometry . . . . . . . . . . . . . . 407
13.3.1 The Geometry of the Gauss Map . . . . . . . . . . . . . . . . 407
13.3.2 Asymptotic Curves . . . . . . . . . . . . . . . . . . . . . . . . 409
13.3.3 The Asymptotic Spherical Map . . . . . . . . . . . . . . . . . 410
13.3.4 Local Visual Events . . . . . . . . . . . . . . . . . . . . . . . 412
13.3.5 The Bitangent Ray Manifold . . . . . . . . . . . . . . . . . . 413
13.3.6 Multilocal Visual Events . . . . . . . . . . . . . . . . . . . . . 414
13.3.7 The Aspect Graph . . . . . . . . . . . . . . . . . . . . . . . . 416
13.4 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417
14 Range Data 422
14.1 Active Range Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . 422
14.2 Range Data Segmentation . . . . . . . . . . . . . . . . . . . . . . . . 424
14.2.1 Elements of Analytical Differential Geometry . . . . . . . . . 424
14.2.2 Finding Step and Roof Edges in Range Images . . . . . . . . 426
14.2.3 Segmenting Range Images into Planar Regions . . . . . . . . 431
14.3 Range Image Registration and Model Acquisition . . . . . . . . . . . 432
14.3.1 Quaternions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433
14.3.2 Registering Range Images . . . . . . . . . . . . . . . . . . . . 434
14.3.3 Fusing Multiple Range Images . . . . . . . . . . . . . . . . . 436
14.4 Object Recognition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438
14.4.1 Matching Using Interpretation Trees . . . . . . . . . . . . . . 438
14.4.2 Matching Free-Form Surfaces Using Spin Images . . . . . . . 441
14.5 Kinect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446
14.5.1 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447
14.5.2 Technique: Decision Trees and Random Forests . . . . . . . . 448
14.5.3 Labeling Pixels . . . . . . . . . . . . . . . . . . . . . . . . . . 450
14.5.4 Computing Joint Positions . . . . . . . . . . . . . . . . . . . 453
14.6 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453
15 Learning to Classify 457
15.1 Classification, Error, and Loss . . . . . . . . . . . . . . . . . . . . . . 457
15.1.1 Using Loss to Determine Decisions . . . . . . . . . . . . . . . 457
15.1.2 Training Error, Test Error, and Overfitting . . . . . . . . . . 459
15.1.3 Regularization . . . . . . . . . . . . . . . . . . . . . . . . . . 460
15.1.4 Error Rate and Cross-Validation . . . . . . . . . . . . . . . . 463
15.1.5 Receiver Operating Curves . . . . . . . . . . . . . . . . . . . 465
15.2 Major Classification Strategies . . . . . . . . . . . . . . . . . . . . . 467
15.2.1 Example: Mahalanobis Distance . . . . . . . . . . . . . . . . 467
15.2.2 Example: Class-Conditional Histograms and Naive Bayes . . 468
15.2.3 Example: Classification Using Nearest Neighbors . . . . . . . 469
15.2.4 Example: The Linear Support Vector Machine . . . . . . . . 470
15.2.5 Example: Kernel Machines . . . . . . . . . . . . . . . . . . . 473
15.2.6 Example: Boosting and Adaboost . . . . . . . . . . . . . . . 475
15.3 Practical Methods for Building Classifiers . . . . . . . . . . . . . . . 475
15.3.1 Manipulating Training Data to Improve Performance . . . . . 477
15.3.2 Building Multi-Class Classifiers Out of Binary Classifiers . . 479
15.3.3 Solving for SVMS and Kernel Machines . . . . . . . . . . . . 480
15.4 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481
16 Classifying Images 482
16.1 Building Good Image Features . . . . . . . . . . . . . . . . . . . . . 482
16.1.1 Example Applications . . . . . . . . . . . . . . . . . . . . . . 482
16.1.2 Encoding Layout with GIST Features . . . . . . . . . . . . . 485
16.1.3 Summarizing Images with Visual Words . . . . . . . . . . . . 487
16.1.4 The Spatial Pyramid Kernel . . . . . . . . . . . . . . . . . . . 489
16.1.5 Dimension Reduction with Principal Components . . . . . . . 493
16.1.6 Dimension Reduction with Canonical Variates . . . . . . . . 494
16.1.7 Example Application: Identifying Explicit Images . . . . . . 498
16.1.8 Example Application: Classifying Materials . . . . . . . . . . 502
16.1.9 Example Application: Classifying Scenes . . . . . . . . . . . . 502
16.2 Classifying Images of Single Objects . . . . . . . . . . . . . . . . . . 504
16.2.1 Image Classification Strategies . . . . . . . . . . . . . . . . . 505
16.2.2 Evaluating Image Classification Systems . . . . . . . . . . . . 505
16.2.3 Fixed Sets of Classes . . . . . . . . . . . . . . . . . . . . . . . 508
16.2.4 Large Numbers of Classes . . . . . . . . . . . . . . . . . . . . 509
16.2.5 Flowers, Leaves, and Birds: Some Specialized Problems . . . 511
16.3 Image Classification in Practice . . . . . . . . . . . . . . . . . . . . . 512
16.3.1 Codes for Image Features . . . . . . . . . . . . . . . . . . . . 513
16.3.2 Image Classification Datasets . . . . . . . . . . . . . . . . . . 513
16.3.3 Dataset Bias . . . . . . . . . . . . . . . . . . . . . . . . . . . 515
16.3.4 Crowdsourcing Dataset Collection . . . . . . . . . . . . . . . 515
16.4 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 517
17 Detecting Objects in Images 519
17.1 The Sliding Window Method . . . . . . . . . . . . . . . . . . . . . . 519
17.1.1 Face Detection . . . . . . . . . . . . . . . . . . . . . . . . . . 520
17.1.2 Detecting Humans . . . . . . . . . . . . . . . . . . . . . . . . 525
17.1.3 Detecting Boundaries . . . . . . . . . . . . . . . . . . . . . . 527
17.2 Detecting Deformable Objects . . . . . . . . . . . . . . . . . . . . . . 530
17.3 The State of the Art of Object Detection . . . . . . . . . . . . . . . 535
17.3.1 Datasets and Resources . . . . . . . . . . . . . . . . . . . . . 538
17.4 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 539
18 Topics in Object Recognition 540
18.1 What Should Object Recognition Do? . . . . . . . . . . . . . . . . . 540
18.1.1 What Should an Object Recognition System Do? . . . . . . . 540
18.1.2 Current Strategies for Object Recognition . . . . . . . . . . . 542
18.1.3 What Is Categorization? . . . . . . . . . . . . . . . . . . . . . 542
18.1.4 Selection: What Should Be Described? . . . . . . . . . . . . . 544
18.2 Feature Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544
18.2.1 Improving Current Image Features . . . . . . . . . . . . . . . 544
18.2.2 Other Kinds of Image Feature . . . . . . . . . . . . . . . . . . 546
18.3 Geometric Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . 547
18.4 Semantic Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 549
18.4.1 Attributes and the Unfamiliar . . . . . . . . . . . . . . . . . . 550
18.4.2 Parts, Poselets and Consistency . . . . . . . . . . . . . . . . . 551
18.4.3 Chunks of Meaning . . . . . . . . . . . . . . . . . . . . . . . . 554
VI APPLICATIONS AND TOPICS 557
19 Image-Based Modeling and Rendering 559
19.1 Visual Hulls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 559
19.1.1 Main Elements of the Visual Hull Model . . . . . . . . . . . . 561
19.1.2 Tracing Intersection Curves . . . . . . . . . . . . . . . . . . . 563
19.1.3 Clipping Intersection Curves . . . . . . . . . . . . . . . . . . 566
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
21.1.3 Types of Image Query . . . . . . . . . . . . . . . . . . . . . . 630
21.1.4 What Users Do with Image Collections . . . . . . . . . . . . 631
21.2 Basic Technologies from Information Retrieval . . . . . . . . . . . . . 632
21.2.1 Word Counts . . . . . . . . . . . . . . . . . . . . . . . . . . . 632
21.2.2 Smoothing Word Counts . . . . . . . . . . . . . . . . . . . . . 633
21.2.3 Approximate Nearest Neighbors and Hashing . . . . . . . . . 634
21.2.4 Ranking Documents . . . . . . . . . . . . . . . . . . . . . . . 638
21.3 Images as Documents . . . . . . . . . . . . . . . . . . . . . . . . . . 639
21.3.1 Matching Without Quantization . . . . . . . . . . . . . . . . 640
21.3.2 Ranking Image Search Results . . . . . . . . . . . . . . . . . 641
21.3.3 Browsing and Layout . . . . . . . . . . . . . . . . . . . . . . 643
21.3.4 Laying Out Images for Browsing . . . . . . . . . . . . . . . . 644
21.4 Predicting Annotations for Pictures . . . . . . . . . . . . . . . . . . 645
21.4.1 Annotations from Nearby Words . . . . . . . . . . . . . . . . 646
21.4.2 Annotations from the Whole Image . . . . . . . . . . . . . . 646
21.4.3 Predicting Correlated Words with Classifiers . . . . . . . . . 648
21.4.4 Names and Faces . . . . . . . . . . . . . . . . . . . . . . . . 649
21.4.5 Generating Tags with Segments . . . . . . . . . . . . . . . . . 651
21.5 The State of the Art of Word Prediction . . . . . . . . . . . . . . . . 654
21.5.1 Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 655
21.5.2 Comparing Methods . . . . . . . . . . . . . . . . . . . . . . . 655
21.5.3 Open Problems . . . . . . . . . . . . . . . . . . . . . . . . . . 656
21.6 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 659
VII BACKGROUND MATERIAL 661
22 Optimization Techniques 663
22.1 Linear Least-Squares Methods . . . . . . . . . . . . . . . . . . . . . . 663
22.1.1 Normal Equations and the Pseudoinverse . . . . . . . . . . . 664
22.1.2 Homogeneous Systems and Eigenvalue Problems . . . . . . . 665
22.1.3 Generalized Eigenvalues Problems . . . . . . . . . . . . . . . 666
22.1.4 An Example: Fitting a Line to Points in a Plane . . . . . . . 666
22.1.5 Singular Value Decomposition . . . . . . . . . . . . . . . . . . 667
22.2 Nonlinear Least-Squares Methods . . . . . . . . . . . . . . . . . . . . 669
22.2.1 Newton’s Method: Square Systems of Nonlinear Equations. . 670
22.2.2 Newton’s Method for Overconstrained Systems . . . . . . . . 670
22.2.3 The Gauss—Newton and Levenberg—Marquardt Algorithms . 671
22.3 Sparse Coding and Dictionary Learning . . . . . . . . . . . . . . . . 672
22.3.1 Sparse Coding . . . . . . . . . . . . . . . . . . . . . . . . . . 672
22.3.2 Dictionary Learning . . . . . . . . . . . . . . . . . . . . . . . 673
22.3.3 Supervised Dictionary Learning . . . . . . . . . . . . . . . . . 675
22.4 Min-Cut/Max-Flow Problems and Combinatorial Optimization . . . 675
22.4.1 Min-Cut Problems . . . . . . . . . . . . . . . . . . . . . . . . 676
22.4.2 Quadratic Pseudo-Boolean Functions . . . . . . . . . . . . . . 677
22.4.3 Generalization to Integer Variables . . . . . . . . . . . . . . . 679
22.5 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 682
Bibliography 684
Index 737
List of Algorithms 760