# Jiří Matas. Hough Transform

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1 Hough Transform Jiří Matas Center for Machine Perception Department of Cybernetics, Faculty of Electrical Engineering Czech Technical University, Prague Many slides thanks to Kristen Grauman and Bastian Leibe

2 Why HT and not Recognition with Local Features? Strengths: applicable to many objects (e.g. in image stitching) is real-time scales well to very large problems (retrieval of millions of images) handles occlusion well insensitive to a broad class of image transformations Weaknesses: applicable to recognition of specific objects (no categorization) applicable only to objects with distinguished local features Slide credit: David Lowe 2

3 Why HT and not the Scanning Window (Viola-Jones) Method? Strengths: applicable to many classes of objects not restricted to specific objects often real-time Weaknesses: extension to a large number of classes not straightforward (standard implementation: linear complexity in the number of classes) occlusion handling not easy full 3D recognition requires too many windows to be checked training time is potentially very long 3/25

4 Hough Transform A method for detecting geometric primitives based on evaluation of an objective function: is the parameter space, are tokens (image points of interest) Origin: Detection of straight lines Examples of for different geometric primitives: Straight line: Circle: Parameters evaluated on a grid Discretization of : 4

5 Comparison: Template Matching and HT Template Matching: for all! 2 J(!) = 0 for all x = (x; y) 2 Image = for all x i» tokens if satisfies J(!) = J(!) + p(x) else /* nothing */ Complexity: O(j j jpj) HT: (basic idea: each token votes for all primitives it is consistent with) for all find x i (x i ) J(!) = J(!) + p(x i ) Complexity: O(j (x i )j jpj); j (x i )j j j 5

6 Hough Transform for Straight Lines 1. Define the minimal parametrisation (p,q) of the space of lines: Most common: angle distance from origin (½, µ) Other options: tangent of angle intercept (a,b), nearest point to center, Quantize the Hough space: Identify the maximum and minimum values of a and b, and the number of cells, 3. Create an accumulator array A(p,q); set all values to zero 4. (if gradient available) : For all edge points (x i,y i ) in the image Use gradient direction Compute a from the equation Increment A(p,q) by one (if gradient not available): For all edge points (x i,y i ) in the image Increment A(p,q) by one for all lines incident on x,y 5. For all cells in A(p,q) Search for the maximum value of A(p,q) Calculate the equation of the line 6. To reduce the effect of noise more than one element (elements in a neighborhood) in the accumulator array are increased 6

7 HT for Straight Lines: Variations Representation of a line Usual form y = a x + b has a singularity around 90º. Can be overcome by considering two cases, y = a x + b and x = a y + b Common parametrization parameterization: x cos( ) + y sin( ) = y Using gradient orientation Uses not only point but also orientation consistent with the edge orientation Variables: In HT: for x P; ; Á : P! h0; ¼) (x i ; Á(x i )) Can be used by weighting the strength of the vote by: Ã line orientation, Á gradient orientation y x ρ já Ãj θ K. Grauman, B. Leibe 7

8 Examples Hough transform for a square (left) and a circle (right) K. Grauman, B. Leibe 8

9 Hough Transform: Noisy Line ρ Tokens Votes θ Problem: Finding the true maximum K. Grauman, B. Leibe Slide credit: David Lowe 9

10 Hough Transform: Noisy Input ρ Tokens Votes θ Problem: Lots of spurious maxima K. Grauman, B. Leibe Slide credit: David Lowe 10

11 HT for different primitive: circles, circles with fixed radius circles squares with a known orientation and size rectangles 11

12 HT for multiple instances 1. p 1 = HT(P; ) : strongest result of HT 2. Set 3. Unvote 4. P 1 = P n p 1 p 1 p 2 = HT(P 1 ; ) 5. Cont. to get as many instances as required Greedy Sequential 12

13 Hough Transform Problems 1. Search space (accumulator size) gets prohibitively large easily Line segments: Circular arc: r; c x ; c y ; t 1 ; t 2 2. Cost function must be additive. 3. Greedy assignment rule of a token to primitive 4. No global objective function for multiple primitives (global optimization for one primitive only) 13

14 When is the Hough transform useful? Textbooks often imply that it is useful mostly for finding lines In fact, it can be very effective for recognizing arbitrary shapes or objects (Generalized HT) The key to efficiency is to have each feature (token) determine as many parameters as possible For example, lines can be detected much more efficiently from small edge elements (or points with local gradients) than from just points For object recognition, each token should predict location, scale, and orientation (4D array) Bottom line: The Hough transform can extract feature groupings from clutter in linear time! K. Grauman, B. Leibe Slide credit: David Lowe 14

15 Generalized Hough Transform [Ballard81] Generalization for an arbitrary contour or shape Choose reference point for the contour (e.g. center) For each point on the contour remember where it is located w.r.t. to the reference point Remember radius r and angle relative to the contour tangent Recognition: whenever you find a contour point, calculate the tangent angle and vote for all possible reference points Instead of reference point, can also vote for transformation The same idea can be used with local features! K. Grauman, B. Leibe Slide credit: Bernt Schiele 15

16 Gen. Hough Transform with Local Features For every feature, store possible occurrences For new image, let the matched features vote for possible object positions Object identity Pose Relative position K. Grauman, B. Leibe 16

17 Finding Consistent Configurations Global spatial models Generalized Hough Transform [Lowe99] RANSAC [Obdrzalek02, Chum05, Nister06] Basic assumption: object is planar Assumption is often justified in practice Valid for many structures on buildings Sufficient for small viewpoint variations on 3D objects K. Grauman, B. Leibe 17

18 3D Object Recognition Gen. HT for Recognition Typically only 3 feature matches needed for recognition Extra matches provide robustness Affine model can be used for planar objects [Lowe99] K. Grauman, B. Leibe Slide credit: David Lowe 18

19 Comparison Gen. Hough Transform Advantages Very effective for recognizing arbitrary shapes or objects Can handle high percentage of outliers (>95%) Extracts groupings from clutter in linear time Disadvantages Quantization issues Only practical for small number of dimensions (up to 4) Improvements available Probabilistic Extensions Continuous Voting Space [Leibe08] RANSAC Advantages General method suited to large range of problems Easy to implement Independent of number of dimensions Disadvantages Only handles moderate number of outliers (<50%) Many variants available, e.g. PROSAC: Progressive RANSAC [Chum05] Preemptive RANSAC [Nister05] K. Grauman, B. Leibe 19

20 RHT = Randomized Hough Transform [Xu93] In: Out: E = fe i g; m( ; e) = 0 S1 ; S2 ; :::; SN Repeat: I. Hypothesis 1. Select random M feature points e k1 ; :::; e km 2. Compute k : m( k ; e kj ) = 0; j = 1; :::; M II. Pre-Verification 3. Add 1 to accumulator k ( k ) > T 1 4. If (accumulator ) goto III. Else goto I. III. Verification 5. Find support for k 6. If (support ) output 7. Reset accumulator ( k ) > T 2 k Proc v Out Omega-S-N, a v algoritmu Omega-N 20

21 Probabilistic Hough Transform [Kiryati et al. 91] Idea: Evaluate Algorithm: NX i=1 p(x i ; ) using only a fraction f = k MAX N 1. Select points at random 2. Perform standard HT Analysis: k MAX Selection of k MAX is incorrect of N points x i the number L of selected points from L N points of a line in a random subset of k MAX points is governed by hypergeometric, not binomial distance L N L L P (L N ) = N k MAX L N N k MAX ¹ = N ¾ 2 = k MAX L N (N L N ) N 2 µ 1 k MAX 1 N 1 21

22 Idea: 1. PHT = Monte Carlo Evaluation of N X i=1 p(x i ; ) 2. Apply standard MC analysis to find k MAX in PHT to guarantee Pffalse positiveg and Pffalse negativeg < ² Algorithm: 1. Select a random point 2. Vote and return it 3. Finish if k MAX reached 22

23 CHT = Cascaded Hough Transform [Tuytelaars et al. 97] Finds structures at different hierarchical levels by iterating one kind of HT (fixed points, fixed lines, lines of fixed points, pencils of fixed lines) Uses duality of lines and points in image and parameter spaces Algorithm: 1. First HT: detects lines in the image and keeps dominant peaks in the parameter space 2. Second HT: detects lines of collinear peaks in parameter space and keeps vertices where several straight lines in the original image intersect (vanishing points) 3. Third HT: applied to the peaks of thto detect collinear vertices (vanishing lines) 23

24 CHT: Experiments Lines belonging to one of the three detected vanishing points Aerial image of buildings and streets (left), the corresponding edges (right) 24

25 macros.tex sfmath.sty cmpitemize.tex Thank you for your attention. 25

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