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Object Recognizing

Object Recognizing. Recognition. Features Classifiers Example ‘winning’ system. Object Classes. Individual Recognition. Object parts Automatic, or query-driven. Window. Mirror. Window. Door knob. Headlight. Back wheel. Bumper. Front wheel. Headlight. Class Non-class .

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Object Recognizing

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  1. Object Recognizing

  2. Recognition • Features • Classifiers • Example ‘winning’ system

  3. Object Classes

  4. Individual Recognition

  5. Object partsAutomatic, or query-driven Window Mirror Window Door knob Headlight Back wheel Bumper Front wheel Headlight

  6. Class Non-class

  7. Class Non-class

  8. Features and Classifiers Same features with different classifiers Same classifier with different features

  9. Generic Features Simple (wavelets) Complex (Geons)

  10. Class-specific Features: Common Building Blocks

  11. Optimal Class Components? • Large features are too rare • Small features are found everywhere Find features that carry the highest amount of information

  12. Mutual information H(C) F=0 F=1 H(C) when F=1 H(C) when F=0 I(C;F) = H(C) – H(C/F)

  13. Mutual Information I(C,F) Class: 1 1 0 1 0 1 0 0 Feature: 1 0 0 1 1 1 0 0 I(F,C) = H(C) – H(C|F)

  14. Optimal classification features • Theoretically: maximizing delivered information minimizes classification error • In practice: informative object components can be identified in training images

  15. KL Classification Error P(C,F) determines the best classification error: p(F|C) p(C) C F p(C|F)

  16. Selecting Fragments

  17. Adding a New Fragment(max-min selection) MIΔ ? MI = MI [Δ ; class] - MI [ ; class ] Select: Maxi MinkΔMI (Fi, Fk) (Min. over existing fragments, Max. over the entire pool)

  18. Horse-class features Car-class features Pictorial features Learned from examples

  19. Fragments with positions On all detected fragments within their regions

  20. Variability of Airplanes Detected

  21. Class-fragments and Activation Malach et al 2008

  22. Bag of words

  23. – Bag of visual words A large collection of image patches

  24. Generate a dictionary using K-means clustering

  25. – – – Each class has its words historgram Limited or no Geometry Simple and popular, no longer state-of-the art.

  26. Class II

  27. HoG Descriptor Dallal, N & Triggs, B. Histograms of Oriented Gradients for Human Detection

  28. SIFT: Scale-invariant Feature Transform • MSER: Maximally Stable Extremal Regions • SURF: Speeded-up Robust Features • Cross correlation • …. • HoG and SIFT are the most widely used.

  29. SVM – linear separation in feature space

  30. Optimal Separation SVM Perceptron The Nature of Statistical Learning Theory, 1995 Rosenblatt, Principles of Neurodynamics 1962. Find a separating plane such that the closest points are as far as possible

  31. +1 The Margin -1 0 Separating line: w ∙ x + b = 0 Far line: w ∙ x + b = +1 Their distance: w ∙ ∆x = +1 Separation: |∆x| = 1/|w| Margin: 2/|w|

  32. Max Margin Classification The examples are vectors xi The labels yi are +1 for class, -1 for non-class (Equivalently, usually used How to solve such constraint optimization?

  33. Using Lagrange multipliers: Using Lagrange multipliers: Minimize LP = With αi > 0 the Lagrange multipliers

  34. Minimizing the Lagrangian Minimize Lp : Set all derivatives to 0: Also for the derivative w.r.t. αi Dual formulation: Maximize the Lagrangian w.r.t. the αi and the above two conditions.

  35. Dual formulation Mathematically equivalent formulation: Can maximize the Lagrangian with respect to the αi After manipulations – concise matrix form:

  36. SVM: in simple matrix form We first find the α. From this we can find: w, b, and the support vectors. The matrix H is a simple ‘data matrix’: Hij = yiyj <xi∙xj> Final classification: w∙x + b ∑αi yi <xi x> + b Because w = ∑αi yi xi Only <xi x> with support vectors are used

  37. Full story – separable case Classification of a new data point x: sgn ( ∑ [αi yi <xi x> + b] )

  38. Quadratic Programming QP Minimize (with respect to x) Subject to one or more constraints of the form: Ax < b (inequality constraints) Ex = d (equality constraints) The problem can be solved in polynomial time of Pos. def. Q. (NP-hard otherwise)

  39. Full story: separable case Non- C ≥ Classification of a new data point x: sgn ( ∑ [αi yi <xi x> + b] )

  40. Kernel Classification

  41. Full story – Kernal case Hij = K(xi,xj) Classification of a new data point x: sgn ( ∑ [αi yi <xi x> + b] )

  42. Felzenszwalb Algorithm • Felzenszwalb, McAllester, Ramanan CVPR 2008. A Discriminatively Trained, Multiscale, Deformable Part Model • Many implementation details, will describe the main points.

  43. Using patches with HoG descriptors and classification by SVM Person model: HoG

  44. Object model using HoG A bicycle and its ‘root filter’ The root filter is a patch of HoG descriptor Image is partitioned into 8x8 pixel cells In each block we compute a histogram of gradient orientations

  45. Dealing with scale: multi-scale analysis The filter is searched on a pyramid of HoG descriptors, to deal with unknown scale

  46. Adding Parts A part Pi = (Fi, vi, si, ai, bi). Fi is filter for the i-th part, vi is the center for a box of possible positions for part i relative to the root position, si the size of this box ai and bi are two-dimensional vectors specifying coefficients of a quadratic function measuring a score for each possible placement of the i-th part. That is, ai and bi are two numbers each, and the penalty for deviation ∆x, ∆y from the expected location is a1 ∆x + a2 ∆y+ b1 ∆x2 + b2 ∆y2

  47. Bicycle model: root, parts, spatial map Person model

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