# Observations. Basic iteration Line estimated from 2 inliers

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1 Line estimated from 2 inliers 3 Observations We need (in this case!) a minimum of 2 points to determine a line Given such a line l, we can determine how well any other point y fits the line l For example: distance between y and l If we pick 2 arbitrary points from the dataset: We can easily determine a line l l is the correct line with some probability p LINE p LINE is related to the chance of picking only inliers p LINE is larger the fewer points that are used to determine l In general: if l is the correct line there are more additional points that can be fitted to the line than if l is an incorrect line 2 Basic iteration 1. Pick 2 random points 2. Fit a line l to the points 3. Determine how many other points in the dataset that can be fitted to l with some minimal error ². ² This forms the consensus set C 4. If C is sufficiently large, then the fitted line is probably OK. Keep it 4 1

2 Basic algorithm RANSAC Iterate K times 1. Pick 2 random points 2. Fit a line l to the points 3. Form the consensus set C, together with Number of points in C Matching error ² C of the set C relative to the line 4. If the consensus set is sufficiently large, then the fitted line is OK. In particular if N and/or ² C is better than the last line that was OK. Then keep it. For each iteration, we increase p SUCCESS = the probability that the correct line has been determined We need to iterate sufficiently many time to raise p SUCCESS to a useful level An undeterministic algorithm Finds a line estimated from only inliers with a probability p after K iterations 1 - p = Pr(pick at least one outlier every time) = (1 w 2 ) K p = 1 - (1 w 2 ) K 5 RANSAC This algorithm is called RANSAC RANdom SAmple Consensus Published by Fischler & Bolles in 1981 "Random Sample Consensus: A Paradigm for Model Fitting with Applications to Image Analysis and Automated Cartography". Comm. of the ACM 24: Several extensions / variations in the literature Preemptive RANSAC PROSAC RANSAC If w is known, we can choose the number of iterations, K, to make p reasonably high Example w = 0.5 p ¼ 0.94 for K = 10 p ¼ 0.99 for K =

3 The correspondence problem Given a set of interest points in two images, we want to determine correspondences, i.e., pairs of points that correspond to the same 3D point If there is a small relative baseline: Use tracking (Lucas-Kanade, etc) Track POIs in image 1 to their corresponding positions in image 2 Can be applied to parts an image sequence A POI typically disappears after a while in a longer sequence Track-retrack Remove all POIs that cannot be tracked forward and backward in time over several images A chicken and egg problem We need corresponding points to estimate F Point correspondences can be determined if we know F Can we determine F and correspondences at the same time? March 31, 2017 TSBB15, Lecture The correspondence problem If there are large baseline between the two images, tracking performance degrades Another approach is needed Chicken and egg revisited Let there be two views with P 1 points in one view and P 2 points in the other view We don t know which points in the first view that correspond to which points in the other view There is a set S of P 1 P 2 possible correspondences, or tentative correspondences March 31, 2017 TSBB15, Lecture

4 Chicken and egg revisited Probabilities The correct correspondences can be fitted to F, i.e., they satisfy the epipolar constraint for some F that only depends on which two views are used They are the inliers The incorrect correspondences are outliers Let w be the fraction of inliers in S In each iteration we pick N points that are all inliers with probability w N (approximately) The probability of not all N points are inliers is then given by 1 w N The probability of not all N points are inliers in K iterations is (1 w N ) K The probability that in K iteration, at least once, all N points are inliers: p = 1 (1 w N ) K Solve for K: K = log(1 p) log(1 w N ) 13 March 31, 2017 TSBB15, Lecture Use RANSAC Pick 8 random points from S We don t know if they really correspond, but this can be tested: 1. Use the 8-point algorithm to estimate F 2. Check how well F matches each pair in S 3. Collect those that fit well into the consensus set C 4. If C is sufficiently large: F is OK: keep F and C Iterate K times The odds are against us From the outset, the set of all tentative correspondences between two images can be VERY large (= P 1 P 2 ) VERY few of these are inliers: w is VERY small Here N = 8 This means that K must be VERY 8 large in order to make p SUCCESS close to 1 Possible strategies for dealing with this problem? 14 March 31, 2017 TSBB15, Lecture

5 Given P 1 points in image 1 and P 2 points in image 2 Form a P 1 P 2 matching matrix Each entry (i,j) is a hypothetical correspondence between point i in image 1 and point j in image 2 Set entry (i,j) = a matching score between point i and point j based on visual appearance For each column or row: keep only the largest entry Reduces m while keeping m 0 constant w increases ) r decreases for fixed p Run RANSAC on remaining tentative pairs The matching score can be based on similarity of visual appearance or other a priori knowledge about the scene (rather than geometric properties) For example SIFT features [see previous lecture!] MSER [see previous lecture!] Color description Camera motions in relation to scene depth Tracking quality The resulting correspondences are referred to as Tentative correspondences Putative correspondences 31 March 2017 TSBB15, lecture March 2017 TSBB15, lecture Score matrix Each entry in the matching matrix describes how well a certain point in image 1 matches another point in image 2. For example: high score = good match Threshold the matching scores to remove highprobability outliers and to identify high-probability inliers (two thresholds!) Remove high-probability outliers High probability inliers means > 50% probability From the original set D of possible correspondences, we have form two sets D 1 and D 0 such that D 0 contains the high-probability inliers A.k.a. putative correspondences D 1 contains the remaining correspondences D that are not high-probability outliers D 0 ½ D 1 ½ D D 1 D0 31 March 2017 TSBB15, lecture March 2017 TSBB15, lecture

6 Other ways to reduce K Correct correspondence High-probability correspondence Low-probability correspondence S 0 Medium-probability correspondence S 1 Try work with models for correspondences that require less than 8 pairs as a minimal case. Essential matrix E (N = 5) P3P (N=3) 31 March 2017 TSBB15, lecture Visual appearance and RANSAC BREAK Remove the low-probability correspondences before RANSAC Use the RANSAC algorithm for finding corresponding points based on the tentative correspondences Use only high-probability inliers (D 0 ) in the initial selection of n points: w > 0.5 ) fewer iterations are needed Use medium and high-probability correspondences (D 1 ) to form the consensus step ) increases the probability of including correct correspondences in the consensus set 31 March 2017 TSBB15, lecture March 31, 2017 TSBB15, Lecture

7 Project 2 Example: multiple views of a dinosaur Reconstruction of 3D object / scene from multiple views March 31, 2017 TSBB15, Lecture March 31, 2017 TSBB15, Lecture Initial assumptions A single camera is moving around in 3D space, taking pictures at multiple distinct positions of one and the same object/scene. These positions are not known with sufficient accuracy The camera has known internal calibration parameters that are constant Lens distortion effects are neglected Or has been compensated for The pin-hole camera model valid Initial assumptions (II) The images are ordered, for example, over a temporal parameter Two consecutive images in the sequence have a smaller baseline than images that are far apart in the sequence Adjacent images in the sequence can be expected to have a significant overlap. This means that many points are visible in both images. The camera path may or may not be closed March 31, 2017 TSBB15, Lecture March 31, 2017 TSBB15, Lecture

8 Project goal Based only on these images and the camera calibration: Example: initial data Generate a 3D representation of the object/scene The 3D representation can then be rendered from any viewpoint, even one not included in the data set Two examples of images from the dinosaur sequence, with corresponding interest points March 31, 2017 TSBB15, Lecture March 31, 2017 TSBB15, Lecture The 3D representation The object(s) in the scene is represented in terms of a set of 3D points Initially unordered (a point cloud) Using spatial relations in 3D space and in the 2D images the 3D points can be connected into one or more 2D surfaces in 3D spaces These surfaces can be texture mapped using the 2D images Example: result Results from 2011 project by Bertil Grelsson and Freddie Åström March 31, 2017 TSBB15, Lecture March 31, 2017 TSBB15, Lecture

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