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Introduction Morphology Linear filters Detection Evaluation Summary Seminar: Medical Image Processing A robust approach for automatic detection and segmentation of cracks in underground pipeline images Tim Niemueller < tim@niemueller.de

  1. Introduction Morphology Linear filters Detection Evaluation Summary 1 Introduction 2 Morphology What is mathematical morphology? Morphology operations Specific parameters for crack detection 3 Linear filters 4 Detection 5 Evaluation 6 Summary Tim Niemueller Crack Detection and Segmentation 9 / 41

  2. Introduction Morphology Linear filters Detection Evaluation Summary What is mathematical morphology? Introduction Mathematical morphology (MM) developed by Matheron and Serra at the Ecole des Mines in Paris Tim Niemueller Crack Detection and Segmentation 10 / 41

  3. Introduction Morphology Linear filters Detection Evaluation Summary What is mathematical morphology? Introduction Mathematical morphology (MM) developed by Matheron and Serra at the Ecole des Mines in Paris Extract features based on a priori knowledge about object geometry Tim Niemueller Crack Detection and Segmentation 10 / 41

  4. Introduction Morphology Linear filters Detection Evaluation Summary What is mathematical morphology? Introduction Mathematical morphology (MM) developed by Matheron and Serra at the Ecole des Mines in Paris Extract features based on a priori knowledge about object geometry Set-theoretic method providing a quantitative description of geometric structures Tim Niemueller Crack Detection and Segmentation 10 / 41

  5. Introduction Morphology Linear filters Detection Evaluation Summary What is mathematical morphology? Introduction Mathematical morphology (MM) developed by Matheron and Serra at the Ecole des Mines in Paris Extract features based on a priori knowledge about object geometry Set-theoretic method providing a quantitative description of geometric structures Based on expanding and shrinking operations with regard to a given structuring element (knowledge about object) Tim Niemueller Crack Detection and Segmentation 10 / 41

  6. Introduction Morphology Linear filters Detection Evaluation Summary What is mathematical morphology? Introduction Mathematical morphology (MM) developed by Matheron and Serra at the Ecole des Mines in Paris Extract features based on a priori knowledge about object geometry Set-theoretic method providing a quantitative description of geometric structures Based on expanding and shrinking operations with regard to a given structuring element (knowledge about object) Originally for B/W images, extended for gray images (interesting case here) Tim Niemueller Crack Detection and Segmentation 10 / 41

  7. Introduction Morphology Linear filters Detection Evaluation Summary What is mathematical morphology? Definitions Images are defined as a function mapping from points to intensity values (here: grayscale, I min = 0 and I max = 255): F : Z 2 �→ [ I min , I max ] Tim Niemueller Crack Detection and Segmentation 11 / 41

  8. Introduction Morphology Linear filters Detection Evaluation Summary What is mathematical morphology? Definitions Images are defined as a function mapping from points to intensity values (here: grayscale, I min = 0 and I max = 255): F : Z 2 �→ [ I min , I max ] Binary structuring elements (SE) are defined as a function: B : Z 2 �→ [0 , 1] Tim Niemueller Crack Detection and Segmentation 11 / 41

  9. Introduction Morphology Linear filters Detection Evaluation Summary What is mathematical morphology? Definitions Images are defined as a function mapping from points to intensity values (here: grayscale, I min = 0 and I max = 255): F : Z 2 �→ [ I min , I max ] Binary structuring elements (SE) are defined as a function: B : Z 2 �→ [0 , 1] Tim Niemueller Crack Detection and Segmentation 11 / 41

  10. Introduction Morphology Linear filters Detection Evaluation Summary What is mathematical morphology? Important notation specialties for crack detection General MM: Crack detection MM: Foreground: white Foreground: black Background: black Background: white Tim Niemueller Crack Detection and Segmentation 12 / 41

  11. Introduction Morphology Linear filters Detection Evaluation Summary What is mathematical morphology? Important notation specialties for crack detection General MM: Crack detection MM: Foreground: white Foreground: black Background: black Background: white Some items change meaning: Hole Object Object Hole Tim Niemueller Crack Detection and Segmentation 12 / 41

  12. Introduction Morphology Linear filters Detection Evaluation Summary What is mathematical morphology? Important notation specialties for crack detection General MM: Crack detection MM: Foreground: white Foreground: black Background: black Background: white Some items change meaning: Hole Object Object Hole Some operations change meaning: Expanding Shrinking Shrinking Expanding Tim Niemueller Crack Detection and Segmentation 12 / 41

  13. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Dilation Structuring element (SE) B δ e B ( F )( P 0 ) = max P ∈ P 0 ∪ e · B ( P 0 ) ( F ( P )) SE dimension scaling factor e (default: e = 1) F Image P 0 Point in image (repeat for every point) Basic expanding operation. Tim Niemueller Crack Detection and Segmentation 13 / 41

  14. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Dilation Structuring element (SE) B δ e B ( F )( P 0 ) = max P ∈ P 0 ∪ e · B ( P 0 ) ( F ( P )) SE dimension scaling factor e (default: e = 1) F Image P 0 Point in image (repeat for every point) Basic expanding operation. A A ⊕ B B Tim Niemueller Crack Detection and Segmentation 13 / 41

  15. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Dilation Structuring element (SE) B δ e B ( F )( P 0 ) = max P ∈ P 0 ∪ e · B ( P 0 ) ( F ( P )) SE dimension scaling factor e (default: e = 1) F Image P 0 Point in image (repeat for every point) Basic expanding operation. (general, black background, white foreground) Tim Niemueller Crack Detection and Segmentation 13 / 41

  16. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Dilation Structuring element (SE) B δ e B ( F )( P 0 ) = max P ∈ P 0 ∪ e · B ( P 0 ) ( F ( P )) SE dimension scaling factor e (default: e = 1) F Image P 0 Point in image (repeat for every point) Basic expanding operation. (crack detection, white background, black foreground) Tim Niemueller Crack Detection and Segmentation 13 / 41

  17. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Erosion Structuring element (SE) B ε e B ( F )( P 0 ) = min P ∈ P 0 ∪ e · B ( P 0 ) ( F ( P )) SE dimension scaling factor e (default: e = 1) F Image P 0 Point in image (repeat for every point) Basic shrinking operation. Tim Niemueller Crack Detection and Segmentation 14 / 41

  18. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Erosion Structuring element (SE) B ε e B ( F )( P 0 ) = min P ∈ P 0 ∪ e · B ( P 0 ) ( F ( P )) SE dimension scaling factor e (default: e = 1) F Image P 0 Point in image (repeat for every point) Basic shrinking operation. A A ⊖ B B Tim Niemueller Crack Detection and Segmentation 14 / 41

  19. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Erosion Structuring element (SE) B ε e B ( F )( P 0 ) = min P ∈ P 0 ∪ e · B ( P 0 ) ( F ( P )) SE dimension scaling factor e (default: e = 1) F Image P 0 Point in image (repeat for every point) Basic shrinking operation. (general, black background, white foreground) Tim Niemueller Crack Detection and Segmentation 14 / 41

  20. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Erosion Structuring element (SE) B ε e B ( F )( P 0 ) = min P ∈ P 0 ∪ e · B ( P 0 ) ( F ( P )) SE dimension scaling factor e (default: e = 1) F Image P 0 Point in image (repeat for every point) Basic shrinking operation. (crack detection, white background, black foreground) Tim Niemueller Crack Detection and Segmentation 14 / 41

  21. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Opening B Structuring element (SE) γ e B ( F ) = δ e B ( ε e e SE dimension scaling factor (default: e = 1) B ( F )) F Image Dilation of the erosion Tim Niemueller Crack Detection and Segmentation 15 / 41

  22. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Opening B Structuring element (SE) γ e B ( F ) = δ e B ( ε e e SE dimension scaling factor (default: e = 1) B ( F )) F Image Dilation of the erosion Removes small objects Tim Niemueller Crack Detection and Segmentation 15 / 41

  23. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Opening B Structuring element (SE) γ e B ( F ) = δ e B ( ε e e SE dimension scaling factor (default: e = 1) B ( F )) F Image Dilation of the erosion Removes small objects (basic opening by 3 × 3 square SE: original image) Tim Niemueller Crack Detection and Segmentation 15 / 41

  24. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Opening B Structuring element (SE) γ e B ( F ) = δ e B ( ε e e SE dimension scaling factor (default: e = 1) B ( F )) F Image Dilation of the erosion Removes small objects (basic opening by 3 × 3 square SE: eroded) Tim Niemueller Crack Detection and Segmentation 15 / 41

  25. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Opening B Structuring element (SE) γ e B ( F ) = δ e B ( ε e e SE dimension scaling factor (default: e = 1) B ( F )) F Image Dilation of the erosion Removes small objects (basic opening by 3 × 3 square SE: eroded and dilated) Tim Niemueller Crack Detection and Segmentation 15 / 41

  26. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Closing B Structuring element (SE) φ e B ( F ) = ε e B ( δ e e SE dimension scaling factor (default: e = 1) B ( F )) F Image Erosion of the dilation Tim Niemueller Crack Detection and Segmentation 16 / 41

  27. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Closing B Structuring element (SE) φ e B ( F ) = ε e B ( δ e e SE dimension scaling factor (default: e = 1) B ( F )) F Image Erosion of the dilation Removes small holes Tim Niemueller Crack Detection and Segmentation 16 / 41

  28. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Closing B Structuring element (SE) φ e B ( F ) = ε e B ( δ e e SE dimension scaling factor (default: e = 1) B ( F )) F Image Erosion of the dilation Removes small holes (basic closing by 3 × 3 square SE: original image) Tim Niemueller Crack Detection and Segmentation 16 / 41

  29. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Closing B Structuring element (SE) φ e B ( F ) = ε e B ( δ e e SE dimension scaling factor (default: e = 1) B ( F )) F Image Erosion of the dilation Removes small holes (basic closing by 3 × 3 square SE: dilated) Tim Niemueller Crack Detection and Segmentation 16 / 41

  30. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Closing B Structuring element (SE) φ e B ( F ) = ε e B ( δ e e SE dimension scaling factor (default: e = 1) B ( F )) F Image Erosion of the dilation Removes small holes (basic closing by 3 × 3 square SE: dilated and eroded) Tim Niemueller Crack Detection and Segmentation 16 / 41

  31. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Top-hat B Structuring element (SE) τ e B ( F ) = F − γ e e SE dimension scaling factor (default: e = 1) B ( F ) F Image Removes a particular feature from the image Tim Niemueller Crack Detection and Segmentation 17 / 41

  32. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Top-hat B Structuring element (SE) τ e B ( F ) = F − γ e e SE dimension scaling factor (default: e = 1) B ( F ) F Image Removes a particular feature from the image Example: edge detection using top-hat filter (edge detection by top-hat: original image) Tim Niemueller Crack Detection and Segmentation 17 / 41

  33. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Top-hat B Structuring element (SE) τ e B ( F ) = F − γ e e SE dimension scaling factor (default: e = 1) B ( F ) F Image Removes a particular feature from the image Example: edge detection using top-hat filter (edge detection by top-hat: erosion by 3 × 3 square) Tim Niemueller Crack Detection and Segmentation 17 / 41

  34. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Top-hat B Structuring element (SE) τ e B ( F ) = F − γ e e SE dimension scaling factor (default: e = 1) B ( F ) F Image Removes a particular feature from the image Example: edge detection using top-hat filter (edge detection by top-hat: opening by 3 × 3 square) Tim Niemueller Crack Detection and Segmentation 17 / 41

  35. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Top-hat B Structuring element (SE) τ e B ( F ) = F − γ e e SE dimension scaling factor (default: e = 1) B ( F ) F Image Removes a particular feature from the image Example: edge detection using top-hat filter (edge detection by top-hat: top-hat with original image) Tim Niemueller Crack Detection and Segmentation 17 / 41

  36. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Top-hat B Structuring element (SE) τ e B ( F ) = F − γ e e SE dimension scaling factor (default: e = 1) B ( F ) F Image Removes a particular feature from the image Example: edge detection using top-hat filter (edge detection by top-hat: inverted result) Tim Niemueller Crack Detection and Segmentation 17 / 41

  37. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Geodesic reconstruction One of the most common MM techniques Tim Niemueller Crack Detection and Segmentation 18 / 41

  38. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Geodesic reconstruction One of the most common MM techniques Instead of one image and a SE now two images are used Tim Niemueller Crack Detection and Segmentation 18 / 41

  39. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Geodesic reconstruction One of the most common MM techniques Instead of one image and a SE now two images are used Marker image is source image, mask image is max. or min. image (depending on operation) Tim Niemueller Crack Detection and Segmentation 18 / 41

  40. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Geodesic reconstruction One of the most common MM techniques Instead of one image and a SE now two images are used Marker image is source image, mask image is max. or min. image (depending on operation) Geodesic: Extracts connected components based on distance Tim Niemueller Crack Detection and Segmentation 18 / 41

  41. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Geodesic reconstruction One of the most common MM techniques Instead of one image and a SE now two images are used Marker image is source image, mask image is max. or min. image (depending on operation) Geodesic: Extracts connected components based on distance Can be used with different morphological operations Tim Niemueller Crack Detection and Segmentation 18 / 41

  42. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Geodesic reconstruction by erosion (geodesic closing) B Isotropic structuring element F Image (Marker) G Image (Mask) ε Erosion � � Φ( F , G ) = ε ( n ) G , ε ( n − 1) ε (0) G ( F ) = max ( ε B ( F )) B , G ( F ) = F G number of iterations until n stability has been reached ( ε ( n ) B , G ( F ) = ε ( n +1) B , G ( F ) holds) Tim Niemueller Crack Detection and Segmentation 19 / 41

  43. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Geodesic reconstruction by erosion (geodesic closing) B Isotropic structuring element F Image (Marker) G Image (Mask) ε Erosion � � Φ( F , G ) = ε ( n ) G , ε ( n − 1) ε (0) G ( F ) = max ( ε B ( F )) B , G ( F ) = F G number of iterations until n stability has been reached ( ε ( n ) B , G ( F ) = ε ( n +1) B , G ( F ) holds) 1 Erode marker image Tim Niemueller Crack Detection and Segmentation 19 / 41

  44. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Geodesic reconstruction by erosion (geodesic closing) B Isotropic structuring element F Image (Marker) G Image (Mask) ε Erosion � � Φ( F , G ) = ε ( n ) G , ε ( n − 1) ε (0) G ( F ) = max ( ε B ( F )) B , G ( F ) = F G number of iterations until n stability has been reached ( ε ( n ) B , G ( F ) = ε ( n +1) B , G ( F ) holds) 1 Erode marker image 2 Take maximum of eroded image and mask image Tim Niemueller Crack Detection and Segmentation 19 / 41

  45. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Geodesic reconstruction by erosion (geodesic closing) B Isotropic structuring element F Image (Marker) G Image (Mask) ε Erosion � � Φ( F , G ) = ε ( n ) G , ε ( n − 1) ε (0) G ( F ) = max ( ε B ( F )) B , G ( F ) = F G number of iterations until n stability has been reached ( ε ( n ) B , G ( F ) = ε ( n +1) B , G ( F ) holds) 1 Erode marker image 2 Take maximum of eroded image and mask image 3 If image has been changed in this iteration goto 1 Tim Niemueller Crack Detection and Segmentation 19 / 41

  46. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Geodesic reconstruction by erosion (geodesic closing) B Isotropic structuring element F Image (Marker) G Image (Mask) ε Erosion � � Φ( F , G ) = ε ( n ) G , ε ( n − 1) ε (0) G ( F ) = max ( ε B ( F )) B , G ( F ) = F G number of iterations until n stability has been reached ( ε ( n ) B , G ( F ) = ε ( n +1) B , G ( F ) holds) (segment 1 and 4: original image) Tim Niemueller Crack Detection and Segmentation 19 / 41

  47. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Geodesic reconstruction by erosion (geodesic closing) B Isotropic structuring element F Image (Marker) G Image (Mask) ε Erosion � � Φ( F , G ) = ε ( n ) G , ε ( n − 1) ε (0) G ( F ) = max ( ε B ( F )) B , G ( F ) = F G number of iterations until n stability has been reached ( ε ( n ) B , G ( F ) = ε ( n +1) B , G ( F ) holds) (segment 1 and 4: dilation by linear SE, length = 45 pixel, vertical) Tim Niemueller Crack Detection and Segmentation 19 / 41

  48. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Geodesic reconstruction by erosion (geodesic closing) B Isotropic structuring element F Image (Marker) G Image (Mask) ε Erosion � � Φ( F , G ) = ε ( n ) G , ε ( n − 1) ε (0) G ( F ) = max ( ε B ( F )) B , G ( F ) = F G number of iterations until n stability has been reached ( ε ( n ) B , G ( F ) = ε ( n +1) B , G ( F ) holds) (segment 1 and 4: marked dilation result) Tim Niemueller Crack Detection and Segmentation 19 / 41

  49. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Geodesic reconstruction by erosion (geodesic closing) B Isotropic structuring element F Image (Marker) G Image (Mask) ε Erosion � � Φ( F , G ) = ε ( n ) G , ε ( n − 1) ε (0) G ( F ) = max ( ε B ( F )) B , G ( F ) = F G number of iterations until n stability has been reached ( ε ( n ) B , G ( F ) = ε ( n +1) B , G ( F ) holds) (segment 1 and 4: dilation by linear SE, length = 7 , horizontal) Tim Niemueller Crack Detection and Segmentation 19 / 41

  50. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Geodesic reconstruction by erosion (geodesic closing) B Isotropic structuring element F Image (Marker) G Image (Mask) ε Erosion � � Φ( F , G ) = ε ( n ) G , ε ( n − 1) ε (0) G ( F ) = max ( ε B ( F )) B , G ( F ) = F G number of iterations until n stability has been reached ( ε ( n ) B , G ( F ) = ε ( n +1) B , G ( F ) holds) (segment 1 and 4: un-marked dilation result) Tim Niemueller Crack Detection and Segmentation 19 / 41

  51. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Geodesic reconstruction by erosion (geodesic closing) B Isotropic structuring element F Image (Marker) G Image (Mask) ε Erosion � � Φ( F , G ) = ε ( n ) G , ε ( n − 1) ε (0) G ( F ) = max ( ε B ( F )) B , G ( F ) = F G number of iterations until n stability has been reached ( ε ( n ) B , G ( F ) = ε ( n +1) B , G ( F ) holds) (segment 1 and 4: geodesic reconstruction with original as mask) Tim Niemueller Crack Detection and Segmentation 19 / 41

  52. Introduction Morphology Linear filters Detection Evaluation Summary Morphology operations Geodesic reconstruction by dilation (geodesic opening) B Isotropic structuring element F Image (Marker) G Image (Mask) δ Dilation Γ( F , G ) = δ ( n ) � G , δ ( n − 1) � δ (0) G ( F ) = min ( δ B ( F )) B , G ( F ) = F G n number of iterations until stability has been reached ( δ ( n ) B , G ( F ) = δ ( n +1) B , G ( F ) holds) 1 Dilate marker image 2 Take minimum of dilated image and mask image 3 If image has been changed in this iteration goto 1 Tim Niemueller Crack Detection and Segmentation 20 / 41

  53. Introduction Morphology Linear filters Detection Evaluation Summary Specific parameters for crack detection Structuring elements Based on observation of cracks specific SEs are chosen Tim Niemueller Crack Detection and Segmentation 21 / 41

  54. Introduction Morphology Linear filters Detection Evaluation Summary Specific parameters for crack detection Structuring elements Based on observation of cracks specific SEs are chosen Linear SE Tim Niemueller Crack Detection and Segmentation 21 / 41

  55. Introduction Morphology Linear filters Detection Evaluation Summary Specific parameters for crack detection Structuring elements Based on observation of cracks specific SEs are chosen Linear SE SE length: 12 pixel Tim Niemueller Crack Detection and Segmentation 21 / 41

  56. Introduction Morphology Linear filters Detection Evaluation Summary Specific parameters for crack detection Structuring elements Based on observation of cracks specific SEs are chosen Linear SE SE length: 12 pixel Degree of rotation: every 10 ◦ from 0 ◦ to 180 ◦ Tim Niemueller Crack Detection and Segmentation 21 / 41

  57. Introduction Morphology Linear filters Detection Evaluation Summary Specific parameters for crack detection Structuring elements Based on observation of cracks specific SEs are chosen Linear SE SE length: 12 pixel Degree of rotation: every 10 ◦ from 0 ◦ to 180 ◦ Tim Niemueller Crack Detection and Segmentation 21 / 41

  58. Introduction Morphology Linear filters Detection Evaluation Summary Specific parameters for crack detection Structuring elements Based on observation of cracks specific SEs are chosen Linear SE SE length: 12 pixel Degree of rotation: every 10 ◦ from 0 ◦ to 180 ◦ Filters have been chosen for dark features Tim Niemueller Crack Detection and Segmentation 21 / 41

  59. Introduction Morphology Linear filters Detection Evaluation Summary 1 Introduction 2 Morphology 3 Linear filters What are linear filters? Filters used for crack detection 4 Detection 5 Evaluation 6 Summary Tim Niemueller Crack Detection and Segmentation 22 / 41

  60. Introduction Morphology Linear filters Detection Evaluation Summary What are linear filters? Linear filters Pictures of zebras and dalmatians both have black and white pixels Tim Niemueller Crack Detection and Segmentation 23 / 41

  61. Introduction Morphology Linear filters Detection Evaluation Summary What are linear filters? Linear filters Pictures of zebras and dalmatians both have black and white pixels They appear in about the same amount Tim Niemueller Crack Detection and Segmentation 23 / 41

  62. Introduction Morphology Linear filters Detection Evaluation Summary What are linear filters? Linear filters Pictures of zebras and dalmatians both have black and white pixels They appear in about the same amount Difference in order and characteristic appearance of groups Tim Niemueller Crack Detection and Segmentation 23 / 41

  63. Introduction Morphology Linear filters Detection Evaluation Summary What are linear filters? Linear filters Pictures of zebras and dalmatians both have black and white pixels They appear in about the same amount Difference in order and characteristic appearance of groups Linear filters are means to detect these specific characteristics Tim Niemueller Crack Detection and Segmentation 23 / 41

  64. Introduction Morphology Linear filters Detection Evaluation Summary What are linear filters? Linear filters Pictures of zebras and dalmatians both have black and white pixels They appear in about the same amount Difference in order and characteristic appearance of groups Linear filters are means to detect these specific characteristics Each pixel is set to a weighted sum of its and its neighbours’ values (convolution) Tim Niemueller Crack Detection and Segmentation 23 / 41

  65. Introduction Morphology Linear filters Detection Evaluation Summary What are linear filters? Linear filters Pictures of zebras and dalmatians both have black and white pixels They appear in about the same amount Difference in order and characteristic appearance of groups Linear filters are means to detect these specific characteristics Each pixel is set to a weighted sum of its and its neighbours’ values (convolution) Weights defined as matrix (kernel) Tim Niemueller Crack Detection and Segmentation 23 / 41

  66. Introduction Morphology Linear filters Detection Evaluation Summary What are linear filters? Linear filters Pictures of zebras and dalmatians both have black and white pixels They appear in about the same amount Difference in order and characteristic appearance of groups Linear filters are means to detect these specific characteristics Each pixel is set to a weighted sum of its and its neighbours’ values (convolution) Weights defined as matrix (kernel) Here: edge detection Tim Niemueller Crack Detection and Segmentation 23 / 41

  67. Introduction Morphology Linear filters Detection Evaluation Summary Filters used for crack detection Gaussian Gaussian kernel Smoothing an image 0.16 0.14 0.16 0.12 0.1 0.14 0.08 0.12 0.06 0.1 0.04 0.08 0.02 0.06 0 0.04 0.02 4 0 2 -4 0 -2 -2 0 2 -4 4 „ − ( x 2+ y 2) « 1 G σ ( x , y ) = 2 πσ 2 exp 2 σ 2 Tim Niemueller Crack Detection and Segmentation 24 / 41

  68. Introduction Morphology Linear filters Detection Evaluation Summary Filters used for crack detection Gaussian Gaussian kernel Smoothing an image 0.16 Discrete Gaussian kernel from 0.14 0.16 0.12 0.1 0.14 0.08 0.12 0.06 Gaussian function 0.1 0.04 0.08 0.02 0.06 0 0.04 0.02 4 0 2 -4 0 -2 -2 0 2 -4 4 „ − ( x 2+ y 2) « 1 G σ ( x , y ) = 2 πσ 2 exp 2 σ 2 2 1 2 1 3 16 16 16 2 4 2 G 1 = 6 7 16 16 16 4 5 1 2 1 16 16 16 Tim Niemueller Crack Detection and Segmentation 24 / 41

  69. Introduction Morphology Linear filters Detection Evaluation Summary Filters used for crack detection Gaussian Gaussian kernel Smoothing an image 0.16 Discrete Gaussian kernel from 0.14 0.16 0.12 0.1 0.14 0.08 0.12 0.06 Gaussian function 0.1 0.04 0.08 0.02 0.06 0 0.04 0.02 4 0 2 -4 0 -2 -2 0 2 -4 4 „ − ( x 2+ y 2) « 1 G σ ( x , y ) = 2 πσ 2 exp 2 σ 2 2 1 2 1 3 16 16 16 2 4 2 G 1 = 6 7 (a) Original (b) Gaussian 16 16 16 4 5 1 2 1 16 16 16 Tim Niemueller Crack Detection and Segmentation 24 / 41

  70. Introduction Morphology Linear filters Detection Evaluation Summary Filters used for crack detection Laplacian of Gaussian Laplacian of Gaussian kernel Classic method for edge detection Tim Niemueller Crack Detection and Segmentation 25 / 41

  71. Introduction Morphology Linear filters Detection Evaluation Summary Filters used for crack detection Laplacian of Gaussian Laplacian of Gaussian kernel Classic method for edge detection Laplacian operator: ( ∇ 2 f )( x , y ) = ∂ 2 f ∂ x 2 + ∂ 2 f ∂ y 2 Tim Niemueller Crack Detection and Segmentation 25 / 41

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