CS 556: Computer Vision. Lecture 13

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1 CS 556: Computer Vision Lecture 1 Prof. Sinisa Todorovic sinisa@eecs.oregonstate.edu 1

2 Outline Perceptual grouping Low-level segmentation Ncuts

3 Perceptual Grouping

4 What do you see? 4

5 What do you see? Rorschach test 5

6 Gestalt Laws Arise from constraints of the real world Similarity Proximity Closure Good continuation Common fate Figure-ground Symmetry Periodicity 6

7 Gestalt Laws -- Similarity We group image parts with similar properties (color, shape, texture) Because same matter in the real world often yields same image properties 7

8 Gestalt Laws -- Proximity We tend to group nearby image parts Because matter is cohesive resulting in meaningful configurations of nearby objects 8

9 Gestalt Laws -- Closure We tend to ignore gaps and hallucinate complete, closed contours Because objects have closed surfaces, and so their image projections should have closed boundaries 9

10 Gestalt Laws -- Good Continuation We prefer to see configurations forming smooth contours Because objects have locally smooth surfaces 10

11 Gestalt Laws -- Common Fate We tend to see distinct image parts with same motion as a unit Because parts of a moving object move coherently in the same manner 11

12 Gestalt Laws -- Figure-Ground We see certain image areas as foreground or figure and the remaining areas as background or ground Because of a target oriented nature of human vision 1

13 Gestalt Laws -- Symmetry We tend to see symmetric image parts as figure Because many objects are symmetric due to functionality/growth/reproduction processes 1

14 Gestalt Laws -- Periodicity We tend to see objects in image parts that spatially repeat Because many objects are spatially periodic due to functionality/growth/reproduction processes 14

15 Issues How to formalize Gestalt laws? What Gestalt law should we apply first? Different orderings Different groupings 15

16 Top-down Interpretation from Context Context and feedback from higher levels may resolve low-level ambiguities Because of the context we may not be even aware of alternatives 16

17 Top-down Interpretation from Context Context and feedback from higher levels may resolve low-level ambiguities Because of the context we may not be even aware of alternatives 16

18 Top-down Interpretation from Context Context and feedback from higher levels may resolve low-level ambiguities Because of the context we may not be even aware of alternatives 16

19 Top-down Interpretation from Context Context and feedback from higher levels may resolve low-level ambiguities Because of the context we may not be even aware of alternatives 16

20 Top-down Interpretation from Context Context and feedback from higher levels may resolve low-level ambiguities Because of the context we may not be even aware of alternatives 16

21 Clustering as a Formalism of Perceptual Grouping Group image elements that are close/neighbors/similar Broad theory is absent at present Motivation: reducing the amount of information -- summarization more compact representation of data -- efficiency Should support application CLOSE/NEIGHBORS/SIMILAR... imply a model 17

22 Clustering as a Computational Framework of Perceptual Grouping 18

23 Example: Pixel Clustering segments or regions results of the Ncuts algorithm 19

24 Graph-based Clustering 0

25 Graph-based Clustering Image elements are represented by a graph 0

26 Graph-based Clustering Image elements are represented by a graph Clustering = Graph partitioning into subgraphs 0

27 Major Goal: Graph Partitioning

28 Major Goal: Graph Partitioning

29 Example: Pixel Clustering segments or regions results of the Ncuts algorithm

30 Graph Terminology Adjacency matrix Node degree Volume Cut Association

31 Adjacency Matrix j A = i a ij a ij =exp 1 σ SIFT i SIFT j 4

32 Adjacency Matrix j A = i a ij Affinities between pairs of nodes (i,j) in the graph a ij =exp 1 σ SIFT i SIFT j 4

33 Adjacency Matrix j A = i a ij Affinities between pairs of nodes (i,j) in the graph Example: Nodes = Pixels a ij =exp 1 σ SIFT i SIFT j 4

34 Node Degree d i = d i = j a ij 5

35 Node Degree D = 0.1 d d d N 1 1 i = i 0 d i = 1 T i D 1 i 6

36 Volume of a Subgraph G 1 G vol(g 1 )= i G 1 d i 7

37 Volume of a Subgraph D = d d d N 1 G1 = G 1 0 vol(g 1 )=1 T G 1 D 1 G1 8

38 Association within a Subgraph G 1 G assoc(g 1,G 1 )= i,j G 1 a ij 9

39 Association within a Subgraph j adjacency matrix A = i a ij assoc(g 1,G 1 )=1 T G 1 A 1 G1 0

40 Cut between Two Graph Partitions G G G 1 G G 1 G = cut(g 1,G )= i G 1,j G a ij 1

41 Cut between Two Graph Partitions G G G 1 G G 1 G = cut(g,g 1 )= i G,j G 1 a ij

42 Cut between Two Graph Partitions G G G 1 G G 1 G = cut(g 1,G ) = cut(g,g 1 ) in general

43 Cut between Two Graph Partitions cut(g 1,G ) = vol(g 1,G ) assoc(g 1,G ) = 1 T G 1 D 1 G1 1 T G 1 A 1 G1 = 1 T G 1 (D A) 1 G1 4

44 Cut between Two Graph Partitions cut(g,g 1 )=1 T G (D A) 1 G =(1 1 G1 ) T (D A) (1 1 G1 ) 5

45 Normalized-Cut Clustering Find two partitions G1 and G of graph G So that the following criterion is minimized: min Ncut(G 1,G )= cut(g 1,G ) G 1,G vol(g 1 ) + cut(g,g 1 ) vol(g ) 6

46 Normalized-Cut Objective Function cut(g 1,G ) min G 1,G vol(g 1 ) + cut(g,g 1 ) vol(g ) 7

47 Normalized-Cut Objective Function cut(g 1,G ) min G 1,G vol(g 1 ) + cut(g,g 1 ) vol(g ) 1 T (D A)1 G min 1 G1 G 1,G 1 T + 1T (D A)1 G G D1 1 G T D1 1 G1 G G 7

48 Normalized-Cut Objective Function cut(g 1,G ) min G 1,G vol(g 1 ) + cut(g,g 1 ) vol(g ) 1 T (D A)1 G min 1 G1 G 1,G 1 T + 1T (D A)1 G G D1 1 G T D1 1 G1 G G 1 T A 1 G max 1 G1 G 1,G 1 T + 1T A 1 G G D 1 1 G T D 1 1 G1 G G 7

49 Normalized-Cut Objective Function 1 T A 1 G max 1 G1 G 1,G 1 T + 1T A 1 G G D 1 1 G T D 1 1 G1 G G 1 T D G 1 A 1 max 1 G1 G 1,G 1 T + 1T D G 1 A 1 G 1 1 G T 1 1 G1 G G 8

50 Normalized-Cut Objective Function 1 T A 1 G max 1 G1 G 1,G 1 T + 1T A 1 G G D 1 1 G T D 1 1 G1 G G normalized Laplacian 1 T D G 1 A 1 max 1 G1 G 1,G 1 T + 1T D G 1 A 1 G 1 1 G T 1 1 G1 G G 8

51 Normalized-Cut Objective Function 1 T A 1 G max 1 G1 G 1,G 1 T + 1T A 1 G G D 1 1 G T D 1 1 G1 G G 1 T D G 1 A 1 max 1 G1 G 1,G 1 T + 1T D G 1 A 1 G 1 1 G T 1 1 G1 G G normalized Laplacian 1 T L 1 G max 1 G1 G 1,G 1 T + 1T L 1 G G 1 1 G T 1 1 G1 G G 8

52 Normalized-Cut Linearization 1 T L 1 G max 1 G1 G 1,G 1 T + 1T L 1 G G 1 1 G T 1 1 G1 G G X =[ ] T indicator vector for nodes that belong to a subgraph 9

53 Normalized-Cut Linearization 1 T L 1 G max 1 G1 G 1,G 1 T + 1T L 1 G G 1 1 G T 1 1 G1 G G X T LX G max 1 G1 X G1,X G XG T + XT LX G G X 1 G1 X T G X G subject to: X G1 {0, 1} n X G {0, 1} n 40

54 Normalized-Cut Linearization 1 T L 1 G max 1 G1 G 1,G 1 T + 1T L 1 G G 1 1 G T 1 1 G1 G G X T LX G max 1 G1 X G1,X G XG T + XT LX G G X 1 G1 X T G X G subject to: X G1 {0, 1} n X G {0, 1} n X T G 1 X G =0 orthogonality 40

55 Normalized-Cut Relaxation X T LX G max 1 G1 X G1,X G XG T + XT LX G G X 1 G1 X T G X G subject to: X G1 [0, 1] n X G [0, 1] n X T G 1 X G =0 41

56 How to Solve Normalized-Cut? X T LX G max 1 G1 X G1,X G XG T + XT LX G G X 1 G1 X T G X G subject to: X G1 [0, 1] n X G [0, 1] n X T G 1 X G =0 4

57 How to Solve Normalized-Cut? X T LX G max 1 G1 X G1,X G XG T + XT LX G G X 1 G1 X T G X G subject to: X G1 [0, 1] n X G [0, 1] n X T G 1 X G =0 4

58 How to Solve Normalized-Cut? X T LX G max 1 G1 X G1,X G XG T + XT LX G G X 1 G1 X T G X G subject to: X G1 [0, 1] n X G [0, 1] n X = XG T X =0 G1 eigenvector of the 1 G largest eigenvalue of L 4

59 How to Solve Normalized-Cut? X T LX G max 1 G1 X G1,X G XG T + XT LX G G X 1 G1 X T G X G subject to: X G1 [0, 1] n X G [0, 1] n X T G 1 X G =0 X = XG T X =0 G1 eigenvector of the 1 G largest eigenvalue of L X G =? 44

60 How to Solve Normalized-Cut? X T LX G max 1 G1 X G1,X G XG T + XT LX G G X 1 G1 X T G X G subject to: X G1 [0, 1] n X G [0, 1] n X T G 1 X G =0 X = XG T X =0 G1 eigenvector of the 1 G largest eigenvalue of L X G = eigenvector of the second largest eigenvalue of L 45

61 Ncuts: General Case cut min G 1,G,...,G K K k=1 1 T G k (D A) 1 Gk 1 T G k D 1 Gk volume 46

62 Ncuts: General Case cut min G 1,G,...,G K K k=1 1 T G k (D A) 1 Gk 1 T G k D 1 Gk volume relaxation s.t. max X 1,...,X K K k=1 X T k D 1 AX k X T k X k X k [0, 1] n k, l, X T k X l =0 46

63 Ncuts: General Case Solution: K eigenvectors of D -1 A corresponding to the K largest eigenvalues 47

64 Experiments -- Ncuts as Image Segmentation input image segments Shi&Malik

65 Experiments -- Ncuts as Image Segmentation input image segments Shi&Malik

66 Experiments -- Ncuts as Image Segmentation source: Jianbo Shi 50

67 Experiments -- Ncuts as Image Segmentation source: Jianbo Shi 51

68 Summary Gestalt laws of perceptual grouping: Similarity Proximity Closure Good continuation Common fate Figure-ground Symmetry Periodicity 5

69 Summary Ncuts: Find K partitions of graph G So as to minimize: min G 1,...,G K k cut(g k,g G k ) vol(g k ) 5

70 Summary node degree matrix D = adjacency d d 0. i d N matrix A = a ij Ncuts: K eigenvectors of D -1 A corresponding to the K largest eigenvalues 54

71 Next Class Meanshift 55

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