Image processing and features

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1 Image processing and features Gabriele Bleser Thanks to Harald Wuest, Folker Wientapper and Marc Pollefeys

2 Introduction Previous lectures: geometry Pose estimation Epipolar geometry What is the input to these geometric computations? Today: (point) correspondence problem 2

3 We always need some image processing Camera calibration Camera pose estimation Homography estimation F matrix estimation Triangulation 3

4 Introduction First approaches: marker based Marker (also fiducial): Artificial landmark Distinctive pattern (unique code) Easy detection in images 4

5 Introduction Today: marker less image processing using natural landmarks/features CAD model Line model 5

6 Motivation (Kanade Lucas point tracker) 6

7 Formalisation The correspondence problem can consist in deciding which primitive in an image corresponds to which landmark in 3D space which primitive in one image corresponds to which primitive in another image (in the sense that they are both images of the same landmark in 3D space) Obvious problems?? 7

8 Nomenclature Feature: Primitive (point, region, line, curve, etc.) combined with a descriptor describing its appearance Associated to a natural or synthetic landmark in 3D space Corresponding features form the basis for pose and structure estimation Here: focus on rectangular markers and point features 8

9 Outline Marker based tracking (overview) Point of interest detection Feature matching Feature tracking Need for wide baseline matching 9

10 Marker based tracking Representation of a rectangular marker: Marker code 3D coordinates of corner points 0 1 <Marker> <Code Line1="1111" 3 2 Line2="0100" Line3="0100" 10 Line4="0000"/> <Points nb="4"> <Point x="0" y="0" z="0"/> <Point x="10" y="0" z="0"/> <Point x="10" y="10" z="0"/> <Point x="0" y="10" z="0"/> </Points> </Marker> 10

11 Marker based tracking: workflow 1. Closed contour extraction 2. Square detection 3. Identification (marker code) 4. Establishing 2D/3D correspondences 5. Homography estimation (previous lecture) 6. Camera pose extraction (previous lecture) 11

12 Marker based tracking 1. Closed contour extraction: Compute image gradient (convolution with 1 st order gradient filter) How do we start? 12

13 Explanation: image convolution Definition: discrete 2D convolution Properties: Commutative: Associative: Distributive: Derivation: x y y x H y y x x I y x H I ), ( ), ( ), ( ) ), ( ), ( ( dx dy y y x x I y x H I H H I G H I G H I ) ( ) ( ) ( ) ( ) ( G I H I G H I ) ( ) ( ) ( H I H I H I x x x 13 E.g. Gaussian smoothing, mean filter, etc.,

14 Explanation: 1 st order gradient filters Image derivative change in pixel intensity Better: Gaussian smoothing + differentiation Discrete approximation convolution with Sobel kernel ), ( ˆ y x M x ), ( ˆ y x M y 1 1 ), ( 1 1 ), ( y x D y x D y x Example filter masks: Example filter masks (Sobel): Example of approximated partial derivate of Gaussian kernel

15 Explanation: 1 st order gradient filters Image gradients are the basis for edge and corner detection Input image Vertical derivative: θ Edge strength: Horizontal derivative: Gradient orientation: θ 2, 15

16 1. Closed contour extraction: Marker based tracking Compute image gradient (convolution with 1 st order gradient filter) Find edges (e.g. Canny operator) Non maxima suppression Hysteresis threshold Find closed contours How does this work? RGB greyscale Binary (edge/non egde) 16

17 Marker based tracking 2. Square detection: Approximate closed contours as polygons Check for squares: 4 corners, minimal/maximal area, convexity, approx. 90 degree angles Find corners 17

18 Marker based tracking 3. Marker identification: assign known markers to the detected squares? 18

19 Marker based tracking 3. Marker identification: for each detected square 1. Estimate homography 2. Sample and compare code in all four directions 3. Find the best overall fit (use any similarity measure) 19

20 Marker based tracking Advantages Reliable detection Few outliers Disadvantages: Modification/preparation of natural scene required All markers must be given in one coordinate system hard to measure Registration of natural objects in marker coordinate system required (for augmentations) Therefore: marker less tracking 20

21 Marker less tracking Principle: natural features are detected in the images and are then either tracked or matched in subsequent frames. Possible feature descriptors: Surrounding texture patch Colour histograms Gradient histogram (SIFT) Topics: Point of interest (POI) detection Feature matching Feature tracking 21

22 Point of interest detection What are good point features? Significant greyscale changes in all image directions corners Stable across views homogeneous region: no change edge: change in one direction corner: significant change in all directions 22

23 Point of interest detection Find points that differ as much as possible from all neighbouring points. homogeneous edge corner 23

24 Point of interest detection How do we detect corners? Change of intensity under small displacement Sum of squared differences over area with shift :, displaced intensity intensity For corners, the SSD is big for all 24

25 Point of interest detection Approximation for small displacements (first order Taylor expansion):,, is a symmetric matrix called structure tensor, which describes the intensity changes of respective image region. 25

26 Point of interest detection homogeneous edge corner Find points for which the minimum change at some shift, is still big, for i.e. maximize smallest eigenvalue of M 26

27 Point of interest detection Geometric interpretation of the structure tensor: eigenvalues and eigenvectors describe the gradient distribution. Direction of smallest change Direction of largest change

28 Point of interest detection Classification of image pixels based on the eigenvalues of the structure tensor: 2 Edge 2 >> 1 Corner 1 and 2 big, 1 ~ 2 ; intensity change in all directions 1 and 2 small, constant intensity in all directions Homogenious region Edge 1 >>

29 Point of interest detection Image Interest operator (e.g. Harris, KLT) Interest map Non maxima suppression POI list 29

30 Point of interest detection: properties Rotation invariant Not scale invariant Now we have extracted suitable points. How do we follow them across images? 30

31 Feature matching vs. tracking Extract features independently and then match by comparing descriptors Often when searching correspondences between two different images/views, or without prior pose information (e.g. initialisation, loop closing) Problem: outliers Extract features in first images and then try to find same feature back in next view Often when searching correspondences within a continuous image sequence, with good pose prediction Frame to frame tracking Problem: drift 31

32 Simple feature matching For each corner in image 1 find the corner in image 2 that is most similar and vice versa Only compare geometrically compatible points Keep mutual best matches 32

33 Simple feature matching: example What are the problems? Non distinctive features Perspective distortions/illumination changes 33

34 Simple feature matching Comparing image regions: compare intensities pixel by pixel I(x,y) J(x,y) Dissimilarity measures Sum of Squared Differences 34

35 Simple feature matching Comparing image regions: compare intensities pixel by pixel I(x,y) J(x,y) Similarity measures Zero mean Normalized Cross Correlation,,, What does this compute?,,, 35

36 Feature tracking Direct registration/tracking of features in their local neighbourhood Simple method block matching : For each corner in frame A, find the displacement in frame B by trying out all positions in a block with fixed size around the previous position and searching the best fit. Frame A Frame B y, yv x xu 36

37 Optical flow Alternative method: Kanade Lucas tracker (KLT) Iterative minimisation of SSD Intensity of previous image Intensity of transformed current image Frame A Frame B Model with parameters 37

38 Optical flow Method: Gauss Newton like minimisation Set derivative with respect to to zero First order Taylor expansion of, Solve resulting linear equation system iteratively: recompute, until the change is small enough A nice derivation is given in: [J. Y. Bouguet. Pyramidal Implementation of the Lucas Kanade Feature Tracker: Description of the algorithm. Technical report, Intel Corporation, Microprocessor Research Labs, 2002] 38

39 Optical flow: translational model Assumption: small feature displacements Estimate pure translation: Problems (as before): Big displacement Frame A Frame B Perspective distortions Illumination changes y, yv Drift! x xu 39

40 Optical flow: multi scale extension The registration may not converge, if the feature displacements are big (little/not enough overlap) Solution: coarse to fine registration (image pyramid) 40

41 Optical flow: affine model Model and estimate affine geometric and photometric transformations contrast brightness + Advantages: Handles affine distortions Handles affine illumination changes Problem: Converges only with good initial estimate 41

42 Optical flow tracking 42

43 Usually two stage approach: Advanced optical flow algorithm Stage 1: Estimate pure translation from frame to frame (coarse to fine) Stage 2: Estimate the affine transformation and illumination parameters to the initial feature patch (use previous estimate as initial guess),,,,,,,,,, Remove drift! 43

44 Wide baseline matching,,,,,,,,,, More about this in the next but one lecture 44

45 References Marker tracking: H. Kato and M. Billinghurst. Marker Tracking and HMD Calibration for a video based Augmented Reality Conferencing System. In International Workshop on Augmented Reality, page 85 94, San Francisco, USA, October Optical flow: B. Lucas and T. Kanade. An Iterative Image Registration Technique with an Application to Stereo Vision. In International Joint Conference on Artificial Intelligence (IJCAI), pages , April J. Y. Bouguet. Pyramidal Implementation of the Lucas Kanade Feature Tracker: Description of the algorithm. Technical report, Intel Corporation, Microprocessor Research Labs, T. Zinßer, C. Gräßl, and H. Niemann. Efficient Feature Tracking for Long Video Sequences. In Deutsche Arbeitsgemeinschaft Mustererkennung (DAGM), pages , Tübingen, Germany, August

46 Source code ARToolKit (marker tracking) OpenCV 46

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