# CS 534: Computer Vision 3D Model-based recognition

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1 CS 534: Computer Vision 3D Model-based recognition Ahmed Elgammal Dept of Computer Science CS 534 3D Model-based Vision - 1 High Level Vision Object Recognition: What it means? Two main recognition tasks:! Categorization: what is the class of the object: face, car, motorbike, airplane, etc.! Identification: John s face, my car Which of the two tasks is easier and which comes first? The answers from neuroscience and computer vision are different:! Computer vision: identification is relatively easy. Categorization very hard.! Psychologists and neuroscientists: in biological visual systems, categorization seems to be simpler and immediate stage in the recognition process. Two main schools:! Object centered approaches! View-based (image based) approaches. CS 534 3D Model-based Vision - 2 1

2 Outlines Geometric Model-Based Object Recognition Choosing features Approaches for model-based vision! Obtaining Hypotheses by Pose Consistency! Obtaining Hypotheses by Pose Clustering! Obtaining Hypotheses by Using Invariants Geometric Hashing Affine invariant geometric hashing CS 534 3D Model-based Vision - 3 Geometric Model-Based Object Recognition Object 1 Model Object 2 Model Object N Model Image Modelbase Geometric Models Problem definitions: Given geometric object models (modelbase) and an image, find out which object(s) we see in the image. CS 534 3D Model-based Vision - 4 2

3 Object Model? Geometric Transformation Pose Recovery problem Features Find Correspondences Features Solve for Transformation Pose Recovery CS 534 3D Model-based Vision - 5 Framework Object Model? Geometric Transformation Object position and orientation Pose Recovery problem Features Hypothesize Correspondences Features Solve for Transformation Pose Recovery Verification: Compare the rendering to the image. If the two are sufficiently similar, accept the hypothesis Back projection (rendering) Verification CS 534 3D Model-based Vision - 6 3

4 General idea! Hypothesize object identity and pose! Recover camera! Render object in camera (widely known as backprojection)! Compare to image (verification) Issues! Where do the hypotheses come from?! How do we compare to image (verification)? CS 534 3D Model-based Vision - 7 Choosing Features Given a 3-D object, how do we decide which points from its surface to choose as its model?! choose points that will give rise to detectable features in images! for polyhedra, the images of its vertices will be points in the images where two or more long lines meet these can be detected by edge detection methods! points on the interiors of regions, or along straight lines are not easily identified in images. CS 534 3D Model-based Vision - 8 4

5 Verification We do not know anything about scene illumination. If we know scene illumination we can render close images So, Verification should be robust to changes in illumination. Use object silhouettes rather than texture or intensities. Edge score! are there image edges near predicted object edges?! very unreliable; in texture, answer is usually yes Oriented edge score! are there image edges near predicted object edges with the right orientation?! better, but still hard to do well (see next slide) No-one s used texture! e.g. does the spanner have the same texture as the wood? model selection problem! more on these later; no-ones seen verification this way, though CS 534 3D Model-based Vision - 9 Example images CS 534 3D Model-based Vision

6 Choosing Features Example: why not choose the midpoints of the edges of a polyhedra as features! midpoints of projections of line segments are not the projections of the midpoints of line segments! if the entire line segment in the image is not identified, then we introduce error in locating midpoint CS 534 3D Model-based Vision - 11 Obtaining Hypothesis M geometric features in the image N geometric features on each object L objects Brute Force Algorithm: Enumerate all possible correspondences O(LM N )!! Instead: utilize geometric constraints small number of correspondences are needed to obtain the projection parameters, once these parameters are known the positions of the rest of all other features are known too. CS 534 3D Model-based Vision

7 Obtaining Hypotheses Three basic approaches: Obtaining Hypotheses by Pose Consistency Obtaining Hypotheses by Pose Clustering Obtaining Hypotheses by Using Invariants CS 534 3D Model-based Vision - 13 Pose Consistency Object Model Geometric Transformation Object position and orientation Pose Recovery problem? Features Hypothesize Correspondences Using a small number of features (frame group) Features Solve for Transformation Pose Recovery Back projection (rendering) Verification The rest of the features have to be consistent with the recovered pose CS 534 3D Model-based Vision

8 Pose consistency Also called Alignment: object is being aligned to the image Correspondences between image features and model features are not independent. Geometric constraints A small number of correspondences yields missing camera parameters --- the others must be consistent with this. Typically intrinsic camera parameters are assumed to be known Strategy:! Generate hypotheses using small numbers of correspondences (e.g. triples of points for a calibrated perspective camera, etc., etc.)! Backproject and verify frame groups : a group that can be used to yield a camera hypothesis. Example- for Perspective Camera:! three points;! three directions (trihedral vertex) and a point (necessary to establish scale)! Two directions emanating from a shared origin (dihedral vertex) and a point CS 534 3D Model-based Vision - 15 CS 534 3D Model-based Vision

9 Figure from Object recognition using alignment, D.P. Huttenlocher and S. Ullman, Proc. Int. Conf. Computer Vision, 1986, copyright IEEE, 1986! CS 534 3D Model-based Vision - 17 RANSAC RANdom SAmple Consensus Searching for a random sample that leads to a fit on which many of the data points agree Extremely useful concept Can fit models even if up to 50% of the points are outliers. Repeat! Choose a subset of points randomly! Fit the model to this subset! See how many points agree on this model (how many points fit that model)! Use only points which agree to re-fit a better model Finally choose the best fit CS 534 Model Fitting

10 RANSAC RANdom SAmple Consensus Four parameters n : the smallest # of points required k : the # of iterations required t : the threshold used to identify a point that fits well d : the # of nearby points required Until k iterations have occurred Pick n sample points uniformly at random Fit to that set of n points For each data point outside the sample Test distance; if the distance < t, it is close If there are d or more points close, this is a good fit. Refit the line using all these points End use the best fit CS 534 Model Fitting - 19 Pose Clustering Most objects have many frame groups There should be many correspondences between object and image frame groups that verify satisfactorily. Each of these correspondences yield approximately the same estimate for the position and orientation of the object w.r.t. the camera Wrong correspondences will yield uncorrelated estimates!! Cluster hypothesis before verification. Vote for the pose: use an accumulator array that represents pose space for each object CS 534 3D Model-based Vision

11 CS 534 3D Model-based Vision - 21 Figure from The evolution and testing of a model-based object recognition system, J.L. Mundy and A. Heller, Proc. Int. Conf. Computer Vision, 1990 copyright 1990 IEEE! CS 534 3D Model-based Vision

12 Figure from The evolution and testing of a model-based object recognition system, J.L. Mundy and A. Heller, Proc. Int. Conf. Computer Vision, 1990 copyright 1990 IEEE! CS 534 3D Model-based Vision - 23 Figure from The evolution and testing of a model-based object recognition system, J.L. Mundy and A. Heller, Proc. Int. Conf. Computer Vision, 1990 copyright 1990 IEEE! CS 534 3D Model-based Vision

13 Figure from The evolution and testing of a model-based object recognition system, J.L. Mundy and A. Heller, Proc. Int. Conf. Computer Vision, 1990 copyright 1990 IEEE! CS 534 3D Model-based Vision - 25 Figure from The evolution and testing of a model-based object recognition system, J.L. Mundy and A. Heller, Proc. Int. Conf. Computer Vision, 1990 copyright 1990 IEEE! CS 534 3D Model-based Vision

14 Problems Correspondences should not be allowed to vote for poses from which they would be invisible In images with noise or texture generates many spurious frame groups, the number of votes in the pose array corresponding to real objects may be smaller than the number of spurious votes Choosing the size of the bucket in the pose array is difficult:! too small mean no much accumulation,! too large mean too many buckets will have enough votes CS 534 3D Model-based Vision - 27 Invariants There are geometric properties that are invariant to camera transformations CS 534 3D Model-based Vision

15 Geometric Hashing A technique originally developed in computer vision for matching geometric features against a database of such features Widely used for pattern-matching, CAD/CAM, medical imaging Also used in other domains: molecular biology and medicinal chemistry! Matching is possible under transformations Matching is possible from patrial information efficient CS 534 3D Model-based Vision - 29 Geometric hashing basic idea Consider the following simple 2-D recognition problem.! We are given a set of object models, M i! each model is represented by a set of points in the plane M i = {P i,1,..., P i,ni }! We want to recognize instances of these point patterns in images from which point features (junctions, etc.) have been identified So, our input image, B, is a binary image where the 1 s are the feature points We only allow the position of the instances of the Mi in B to vary - orientation is fixed. Only translation We want our approach to work even if some points from the model are not detected in B. We will search for all possible models simultaneously Model Translation Image CS 534 3D Model-based Vision

16 Geometric hashing Consider two models! M 1 = {(0,0), (10,0), (10,10), (0,10)}! M 2 = {(0,0), (4,4), (4,-4), (-4,-4), (-4,4)} We will build a table containing, for each model, all of the relative coordinates of these points given that one of the model points is chosen as the origin of its coordinate system! this point is called a basis, because choosing it completely determines the coordinates of the remaining points.! examples for M 1 choose (0,0) as basis obtain (10,0), (10,10), (0,10) choose (10,0) as basis obtain (-10,0), (0,10), -10,10)) CS 534 3D Model-based Vision - 31 Hashing table M 1, M 1,1 M 2,2 M 2,2 M 2,2 5 M 2,1 M 2,1 2 M 1,2 1 M 1 0 M 2,2 M 1,1 1 2 M 2,1 M 2,1 M 2 M 1,2 M 1,2 CS 534 3D Model-based Vision

17 Hash table creation How many entries do we need to make in the hash table.! Mode M i has n i point each has to be chosen as the basis point coordinates of remaining n i -1 points computed with respect to basis point entry (M i, basis) entered into hash table for each of those coordinates! And this has to be done for each of the m models.! So complexity is mn 2 to build the table But the table is built only once, and then used many times. CS 534 3D Model-based Vision - 33 Using the table during recognition Pick a feature point from the image as the basis.! the algorithm may have to consider all possible points from the image as the basis Compute the coordinates of all of the remaining image feature points with respect to that basis. Use each of these coordinates as an index into the hash table! at that location of the hash table we will find a list of (M i, p j ) pairs - model basis pairs that result in some point from M i getting these coordinates when the j th point from M i is chosen as the basis! keep track of the score for each (M i, p i ) encountered! models that obtain high scores for some bases are recorded as possible detections CS 534 3D Model-based Vision

18 Some observations If the image contains n points from some model, M i, then we will detect it n times! each of the n points can serve as a basis! for each choice, the remaining n-1 points will result in table indices that contain (M i, basis) If the image contains s feature points, then what is the complexity of the recognition component of the geometric hashing algorithm?! for each of the s points we compute the new coordinates of the remaining s-1 points! and we keep track of the (model, basis) pairs retrieved from the table based on those coordinates! so, the algorithm has complexity O(s 2 ), and is independent of the number of models in the database CS 534 3D Model-based Vision - 35 Geometric Hashing Consider a more general case: translation, scaling and rotation one point is not a sufficient basis. But two points are sufficient. Model Translation, Rotation, Scaling Image Figure from Haim J. Wolfson Geometric Hashing an Overview IEEE computational Science and Engineering, Oct CS 534 3D Model-based Vision

19 Figure from Haim J. Wolfson Geometric Hashing an Overview IEEE computational Science and Engineering, Oct CS 534 3D Model-based Vision - 37 Revised geometric hashing Table construction! need to consider all pairs of points from each model for each pair, construct the coordinates of the remaining n-2 points using those two as a basis add an entry for (model, basis-pair) in the hash table complexity is now mn 3 Recognition! pick a pair of points from the image (cycling through all pairs)! compute coordinates of remaining point using this pair as a basis! look up (model, basis-pair) in table and tally votes CS 534 3D Model-based Vision

20 Recall: Affine projection models: Weak perspective projection Also called Scaled Orthographic Projection SOP is the magnification (scaling factor). When the scene depth is small compared its distance from the Camera, we can assume every thing is on one plane, m can be taken constant: weak perspective projection also called scaled orthography (every thing is a scaled version of the scene) CS 534 3D Model-based Vision - 39 Affine combinations of points t(p 2 -P 1 ) Let P 1 and P 2 be points Consider the expression P = P 1 + t(p 2 - P 1 )! P represents a point on the line joining P 1 and P 2.! if 0 <= t <= 1 then P lies between P 1 and P 2.! We can rewrite the expression as P = (1-t)P 1 + tp 2 Define an affine combination of two points to be! a 1 P 1 + a 2 P 2! where a 1 + a 2 = 1! P = (1-t)P 1 + tp 2 is an affine combination with a 2 = t. P 1 P 2 P 1 + t(p 2 -P 1 ) CS 534 3D Model-based Vision

21 Affine combinations P 1 P 3 P Generally,! if P 1,...P n is a set of points, and a 1 + +a n = 1, then! a 1 P 1 + a n P n is the point P 1 + a 2 (P 2 -P 1 ) + a n (P n -P 1 ) Let s look at affine combinations of three points. These are points! P = a 1 P 1 + a 2 P 2 + a 3 P 3 = P 1 + a 2 (P 2 -P 1 ) + a 3 (P 3 -P 1 )! where a 1 + a 2 + a 3 = 1! if 0 <= a 1, a 2, a 3, <=1 then P falls in the triangle, otherwise outside! (a 2, a 3 ) are the affine coordinates of P homogeneous representation is (1,a 2,a 3 )! P 1, P 2,, P 3 is called the affine basis P 2 CS 534 3D Model-based Vision - 41 Affine combinations Given any two points, P = (a 1, a 2 ) and Q = (a 1, a 2 )! Q-P is a vector! its affine coordinates are (a 1 -a 1, a 2 -a 2 )! Note that the affine coordinates of a point sum to 1, while the affine coordinates of a vector sum to 0. CS 534 3D Model-based Vision

22 Affine transformations Rigid transformations are of the form where a 2 + b 2 =1 An affine transformation is of the form for arbitrary a,b,c,d and is determined by the transformation of three points CS 534 3D Model-based Vision - 43 Representation in homogeneous coordinates In homogeneous coordinates, this transformation is represented as: CS 534 3D Model-based Vision

23 Affine coordinates and affine transformations The affine coordinates of a point are unchanged if the point and the affine basis are subjected to the same affine transformation Based on simple properties of affine transformations Let T be an affine transformation! T(P 1 - P 2 ) = TP 1 - TP 2! T(aP) = atp, for any scalar a. CS 534 3D Model-based Vision - 45 Proof Let P 1 = (x,y,1) and P 2 = (u,v,1)! Note that P 1 - P 2 is a vector TP 1 = (ax + by + t x, cx + dy + t y, 1) TP 2 = (au + bv + t x, cu + dv + t y,1)! TP 1 - TP 2 = (a(x-u) + b(y-v), c(x-u) + d(y-v), 0) P 1 - P 2 = (x-u, y-v,0) T(P 1 -P 2 ) = (a(x-u) + b(y-v), c(x-u)+d(y-v), 0) CS 534 3D Model-based Vision

24 Invariance for Geometric hashing Affine Coordinates represent an invariant under Affine Transformation. So can be used for Geometric hashing Let P 1, P 2, P 3 be an ordered affine basis triplet in the plane. Then the affine coordinates (",#) of a point P are:! P = "(P 2 - P 1 ) + # (P 3 - P 1 ) + P 1 Applying any affine transformation T will transform it to! TP = "(TP 2 - TP 1 ) + # (TP 3 - T P 1 ) + TP 1 So, TP has the same coordinates (",#) in the basis triplet as it did originally. CS 534 3D Model-based Vision - 47 What do affine transformations have to do with 3-D recognition Suppose our point pattern is a planar pattern - i.e., all of the points lie on the same plane.! we construct out hash table using these coordinates, choosing three at a time as a basis We position and orient this planar point pattern in space far from the camera, (So SOP is a good model) and take its image.! the transformation of the model to the image is an affine transformation! so, the affine coordinates of the points in any given basis are the same in the original 3-D planar model as they are in the image. CS 534 3D Model-based Vision

25 Preprocessing Suppose we have a model containing m feature points For each ordered noncollinear triplet of model points! compute the affine coordinates of the remaining m-3 points using that triple as a basis! each such coordinate is used as an entry into a hash table where we record the (base triplet,model) at which this coordinate was obtained. Complexity is m 4 per model. p1, p2, p3,m p1, p2, p3,m p1, p2, p3,m p1, p2, p3,m CS 534 3D Model-based Vision - 49 Recognition Scene with n interest points Choose an ordered triplet from the scene! compute the affine coordinates of remaining n-3 points in this basis! for each coordinate, check the hash table and for each entry found, tally a vote for the (basis triplet, model)! if the triplet scores high enough, we verify the match If the image does not contain an instance of any model, then we will only discover this after looking at all n 3 triples. p1, p2, p3,m p1, p2, p3,m p1, p2, p3,m p1, p2, p3,m CS 534 3D Model-based Vision

26 CS 534 3D Model-based Vision - 51 Pose Recovery Applications In many applications recovering pose is far more important than recognition. We know what we are looking at but we need to get accurate measurements of pose Examples: Medical applications. CS 534 3D Model-based Vision

27 Sources Forsyth and Ponce, Computer Vision a Modern approach: chapter 18. Slides by D. UC Berkeley Slides by L.S. UMD Haim J. Wolfson Geometric Hashing an Overview IEEE computational Science and Engineering, Oct (posted on class web page) CS 534 3D Model-based Vision

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