# 2D Image Processing. Edge and Corner Detection. Prof. Didier Stricker Dr. Alain Pagani

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1 2D Image Processing Edge and Corner Detection Prof. Didier Stricker Dr. Alain Pagani Kaiserlautern University DFKI Deutsches Forschungszentrum für Künstliche Intelligenz 1

2 Goals Where is the information in an image? How is an object characterized? How can I find measurements in the image? The correct Good Features are essential for tracking! 2

3 Outline Edge detection Canny edge detector Point extraction 3 Some slides from Lazebnik

4 Edge detection Goal: Identify sudden changes (discontinuities) in an image Intuitively, most semantic and shape information from the image can be encoded in the edges More compact than pixels Ideal: artist s line drawing (but artist is also using object-level knowledge) 4 Source: D. Lowe and Debis

5 Edge Detection 5

6 What Causes Intensity Changes? Geometric events surface orientation (boundary) discontinuities depth discontinuities color and texture discontinuities Non-geometric events illumination changes specularities shadows inter-reflections surface normal discontinuity depth discontinuity color discontinuity illumination discontinuity 6

7 Why is Edge Detection Useful? Important features can be extracted from the edges of an image (e.g., corners, lines, curves). These features are used by higher-level computer vision algorithms (e.g., recognition). 7

8 Effect of Illumination 8

9 Edge Descriptors Edge direction: perpendicular to the direction of maximum intensity change (i.e., edge normal) Edge strength: related to the local image contrast along the normal. Edge position: the image position at which the edge is located. 9

10 Characterizing edges An edge is a place of rapid change in the image intensity function image intensity function (along horizontal scanline) first derivative edges correspond to extrema of derivative 10

11 Image gradient The gradient of an image: The gradient points in the direction of most rapid increase in intensity The gradient direction is given by How does this relate to the direction of the edge? The edge strength is given by the gradient magnitude 11 Source: Steve Seitz

12 Differentiation and convolution Recall, for 2D function, f(x,y): We could approximate this as: f f (x + ε, y) f (x, y ) = lim ε 0 x ε ε f f (xn+1, y ) f (xn, y) x Δx (which is obviously a convolution) -1 1 Check! 12 Source: D. Forsyth, D. Lowe

13 Finite differences: example Which one is the gradient in the x-direction (resp. y-direction)? 13

14 Effects of noise Consider a single row or column of the image Plotting intensity as a function of position gives a signal Where is the edge? 14 Source: S. Seitz

15 Effects of noise Finite difference filters respond strongly to noise Image noise results in pixels that look very different from their neighbors Generally, the larger the noise the stronger the response What is to be done? Smoothing the image should help, by forcing pixels different to their neighbors (=noise pixels?) to look more like neighbors 15 Source: D. Forsyth

16 Solution: smooth first f g f*g d ( f g) dx d To find edges, look for peaks in ( f g ) dx 16 Source: S. Seitz

17 Derivative theorem of convolution Differentiation and convolution both linear operators: they commute d df dg dx ( f g)= dx g = f dx This saves us one operation: f d g dx f d g dx 17 Source: S. Seitz

18 Derivative of Gaussian filter * [1-1] = This filter is separable 18

19 Derivative of Gaussian filter x- direc?on y- direc?on 19

20 Tradeoff between smoothing and localization 1 pixel 3 pixels 7 pixels Smoothed derivative removes noise, but blurs edge. Also finds edges at different scales. 20 Source: D. Forsyth

21 Finite difference filters Other approxima?ons of deriva?ve ﬁlters exist: 21 Source: K. Grauman

22 Implementation issues The gradient magnitude is large along a thick trail or ridge, so how do we identify the actual edge points? How do we link the edge points to form curves? 22 Source: D. Forsyth

23 Designing an edge detector Criteria for an optimal edge detector: Good detection: the optimal detector must minimize the probability of false positives (detecting spurious edges caused by noise), as well as that of false negatives (missing real edges) Good localization: the edges detected must be as close as possible to the true edges Single response: the detector must return one point only for each true edge point; that is, minimize the number of local maxima around the true edge 23 Source: L. Fei- Fei

24 Canny edge detector This is probably the most widely used edge detector in computer vision Theoretical model: step-edges corrupted by additive Gaussian noise Canny has shown that the first derivative of the Gaussian closely approximates the operator that optimizes the product of signal-to-noise ratio and localization J. Canny, A Computa*onal Approach To Edge Detec*on, IEEE Trans. on PaOern Analysis and Machine Intelligence, 8: , Source: L. Fei- Fei

25 Canny edge detector 1. Filter image with derivative of Gaussian 2. Find magnitude and orientation of gradient 3. Non-maximum suppression: Thin multi-pixel wide ridges down to single pixel width 4. Linking and thresholding (hysteresis): Define two thresholds: low and high Use the high threshold to start edge curves and the low threshold to continue them MATLAB: edge(image, canny ) 25 Source: D. Lowe, L. Fei- Fei

26 Example original image (Lena) 26

27 Example norm of the gradient 27

28 Example thresholding 28

29 Example thinning (non-maximum suppression) 29

30 Non-maximum suppression At q, we have a maximum if the value is larger than those at both p and at r. Interpolate to get these values. Source: D. Forsyth 30

31 Edge linking Assume the marked point is an edge point. Then we construct the tangent to the edge curve (which is normal to the gradient at that point) and use this to predict the next points (here either r or s). Source: D. Forsyth 31

32 Hysteresis thresholding Check that maximum value of gradient value is sufficiently large drop-outs? use hysteresis use a high threshold to start edge curves and a low threshold to continue them. 32 Source: S. Seitz

33 Hysteresis thresholding original image high threshold (strong edges) low threshold (weak edges) hysteresis threshold 33 Source: L. Fei- Fei

34 Effect of σ (Gaussian kernel spread/size) original Canny with Canny with The choice of σ depends on desired behavior large σ detects large scale edges small σ detects fine features 34 Source: S. Seitz

35 Edge detection is just the beginning Berkeley segmentation database: image human segmentation gradient magnitude 35

36 Features Some slides from S. Seitz 36

37 Image Matching 37

38 Image Matching 38

39 Invariant local features Find features that are invariant to transformations geometric invariance: translation, rotation, scale photometric invariance: brightness, exposure, Feature Descriptors 39

40 Advantages of local features Locality features are local, so robust to occlusion and clutter Distinctiveness can differentiate a large database of objects Quantity hundreds or thousands in a single image Efficiency real-time performance achievable Generality exploit different types of features in different situations 40

41 More motivation Feature points are used for: Image alignment (e.g., mosaics) 3D reconstruction Motion tracking Object recognition Indexing and database retrieval Robot navigation other 41

42 Interest point candidates auto-correlation 42

43 Steps in Corner Detection 1. For each pixel, the corner operator is applied to obtain a cornerness measure for this pixel. 2. Threshold cornerness map to eliminate weak corners. 3. Apply non-maximal suppression to eliminate points whose cornerness measure is not larger than the cornerness values of all points within a certain distance. 43

44 Steps in Corner Detection (cont d) 44

45 Local measures of uniqueness Suppose we only consider a small window of pixels What defines whether a feature is a good or bad candidate? Slide adapted from Darya Frolova, Denis Simakov, Weizmann Institute. 45

46 Feature detection Local measure of feature uniqueness How does the window change when you shift it? Shifting the window in any direction causes a big change flat region: no change in all directions edge : no change along the edge direction corner : significant change in all directions Slide adapted from Darya Frolova, Denis Simakov, Weizmann Institute. 46

47 Feature detection: the math Consider shifting the window W by (u,v) how do the pixels in W change? compare each pixel before and after by summing up the squared differences (SSD) W this defines an SSD error of E(u,v): 47

48 Small motion assumption Taylor Series expansion of I: If the motion (u,v) is small, then first order approximation is good Plugging this into the formula on the previous slide 48

49 Feature detection: the math Consider shifting the window W by (u,v) how do the pixels in W change? compare each pixel before and after by summing up the squared differences this defines an error of E(u,v): W 49

50 Feature detection: the math This can be rewritten: For the example above You can move the center of the green window to anywhere on the blue unit circle Which directions will result in the largest and smallest E values? We will show that we can find these directions by looking at the eigenvectors of H 50

51 Feature detection: the error function Ø A new corner measurement by investigating the shape of the error function T u Hu represents a quadratic function; Thus, we can analyze E s shape by looking at the property of H 51

52 Feature detection: the error function High-level idea: what shape of the error function will we prefer for features? Das Bild kann nicht angezeigt werden. Dieser Computer verfügt möglicherweise über zu wenig Arbeitsspeicher, um das Bild Das zu Bild kann nicht angezeigt werden. Dieser Computer verfügt möglicherweise über zu wenig Arbeitsspeicher, um das Bild zu Das Bild kann nicht angezeigt werden. Dieser Computer verfügt möglicherweise über zu wenig Arbeitsspeicher, um das Bild zu öffnen, oder das Bild ist beschädigt. Starten Sie den Computer neu, und öffnen Sie dann erneut die Datei. Wenn weiterhin öffnen, das rote oder das Bild ist beschädigt. Starten Sie den Computer neu, und öffnen Sie dann erneut die Datei. Wenn weiterhin das rote öffnen, oder das Bild ist beschädigt. Starten Sie den Computer neu, und öffnen Sie dann erneut die Datei. Wenn weiterhin das rote x angezeigt wird, müssen Sie das Bild möglicherweise löschen und dann erneut einfügen. x angezeigt wird, müssen Sie das Bild möglicherweise löschen und dann erneut einfügen. x angezeigt wird, müssen Sie das Bild möglicherweise löschen und dann erneut einfügen. flat edge corner 52

53 Quadratic forms Quadratic form (homogeneous polynomial of degree two) of n variables xi Examples = 53

54 Symmetric matrices Quadratic forms can be represented by a real symmetric matrix A where 54

55 Eigenvalues of symmetric matrices 55 Brad Osgood

56 Eigenvectors of symmetric matrices where Q is an orthogonal matrix (the columns of which are eigenvectors of A), and Λ is real and diagonal (having the eigenvalues of A on the diagonal). 56

57 Eigenvectors of symmetric matrices x T Ax T T T =x QΛQ x ( T T ) ( λ2 q2 T = Q x ΛQ x = y T Λy ( ) (Λ y ) 1 2 = Λ y = ztz T 1 2 z z =1 λ1 q1 ) T x x =1 57

58 Harris corner detector Intensity change in shifting window: eigenvalue analysis u E (u, v ) [u, v ] H λ-, λ+ eigenvalues of H v We can visualize H as an ellipse with axis lengths and directions determined by its eigenvalues and eigenvectors. direction of the slowest change (λmin)1/2 Ellipse E(u,v) = const (λmax)1/2 direction of the fastest change 58

59 Visualize quadratic functions A = = T 59

60 Visualize quadratic functions A = = T 60

61 Visualize quadratic functions A = = T 61

62 Visualize quadratic functions A = = T

63 Feature detection: the math This can be rewritten: xx+ Eigenvalues and eigenvectors of H Define shifts with the smallest and largest change (E value) x+ = direction of largest increase in E. λ+ = amount of increase in direction x+ x- = direction of smallest increase in E. λ- = amount of increase in direction x+ 63

64 Feature detection: the math How are λ+, x+, λ-, and x+ relevant for feature detection? What s our feature scoring function? Want E(u,v) to be large for small shifts in all directions the minimum of E(u,v) should be large, over all unit vectors [u v] this minimum is given by the smaller eigenvalue (λ-) of H 64

65 Feature detection summary (KanadeTomasi) Here s what you do Compute the gradient at each point in the image Create the H matrix from the entries in the gradient Compute the eigenvalues Find points with large response (λ- > threshold) Choose those points where λ- is a local maximum as features J. Shi and C. Tomasi (June 1994). "Good Features to Track". 9th IEEE Conference on Computer Vision and Pattern Recognition. Springer. 65

66 Feature detection summary Here s what you do Compute the gradient at each point in the image Create the H matrix from the entries in the gradient Compute the eigenvalues Find points with large response (λ- > threshold) Choose those points where λ- is a local maximum as features 66

67 The Harris operator λ- is a variant of the Harris operator for feature detection (λ- = λ1 ; λ+ = λ2) The trace is the sum of the diagonals, i.e., trace(h) = h11 + h22 Very similar to λ- but less expensive (no square root)* Called the Harris Corner Detector or Harris Operator Lots of other detectors, this is one of the most popular * 67

68 The Harris operator Measure of corner response (Harris): 2 R = deth k (trace H ) With: det H = λ1λ2 trace H = λ1 + λ2 (k empirical constant, k = ) 68

69 The Harris operator Harris operator 69

70 Harris detector example 70

71 f value (red high, blue low) 71

72 Threshold (f > value) 72

73 Find local maxima of f 73

74 Harris features (in red) 74

75 Harris detector: Steps 1. Compute Gaussian derivatives at each pixel 2. Compute second moment matrix H in a Gaussian window around each pixel 3. Compute corner response function R 4. Threshold R 5. Find local maxima of response function (nonmaximum suppression) 2 R = det( H ) α trace( H ) = λ1λ2 α (λ1 + λ2 ) 2 α: constant (0.04 to 0.06) C. Harris and M. Stephens. "A Combined Corner and Edge Detector. Proceedings of the 4th Alvey Vision Conference: pages ,

76 Thank you! 76

77 Quick review: eigenvalue/eigenvector The eigenvectors of a matrix A are the vectors x that satisfy: The scalar λ is the eigenvalue corresponding to x The eigenvalues are found by solving: In our case, A = H is a 2x2 matrix, so we have The solution: Once you know λ, you find x by solving 77

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