On the Two-View Geometry of Unsynchronized CamerasOn the Two-View Geometry of Unsynchronized Cameras Cenek Albl1 Zuzana Kukelova1 Andrew Fitzgibbon2 Jan Heller 3 Matej Smid1 Tomas

On the Two-View Geometry of Unsynchronized Cameras

Cenek Albl1 Zuzana Kukelova1 Andrew Fitzgibbon2 Jan Heller 3 Matej Smid1 Tomas Pajdla1

1Czech Technical University in PraguePrague, Czechia

{alblcene,kukelova}@[email protected],[email protected]

2MicrosoftCambridge, UK

[email protected]

3Magik Eye Inc.New York, US

[email protected]

Abstract

We present new methods for simultaneously estimating cam-era geometry and time shift from video sequences from mul-tiple unsynchronized cameras. Algorithms for simultane-ous computation of a fundamental matrix or a homographywith unknown time shift between images are developed. Ourmethods use minimal correspondence sets (eight for fun-damental matrix and four and a half for homography) andtherefore are suitable for robust estimation using RANSAC.Furthermore, we present an iterative algorithm that extendsthe applicability on sequences which are significantly un-synchronized, finding the correct time shift up to severalseconds. We evaluated the methods on synthetic and widerange of real world datasets and the results show a broadapplicability to the problem of camera synchronization.

1. Introduction

Many computer vision applications, e.g., human body mod-elling [30, 5], person tracking [8, 36], pose estimation [11],robot navigation [1, 12], and 3D object scanning [26],benefit from using multiple-camera systems. In tightly-controlled laboratory setups, it is possible to have all cam-eras temporally synchronized. However, applicability ofmulti-camera systems could be greatly enlarged when cam-eras might run without synchronization [15]. Synchroniza-tion is sometimes not possible, e.g. in automotive indus-try, but even if it was possible, using asynchronous cam-eras may produce other benefits, e.g., reducing bandwidthrequirements and improving temporal resolution of eventdetection and motion recovery [6].

In this paper, we (1) introduce practical solvers thatsimultaneously compute either a fundamental matrix ora homography and time shift between image sequences,and (2) we propose a fast iterative algorithm that usesRANSAC [10] with our solvers in the inner loop to syn-chronize large time offsets. Our approach can accurately

X(t)

x(t) x′(t)

C C′

X(ti+1)

X(ti+2)

si+1

si+2s′i+1

s′i+2

F

X(t′i+1)

X(t′i+2)

X(ti)

X(t′i)

s′(β+ρi)si

s′i

Figure 1. Two cameras capture a moving point at different times,so the projection rays of the two cameras meet nowhere.

calibrate large time shifts, which was not possible before.

1.1. Related work

Many video and/or image sequence synchronization meth-ods are based on image content analysis [23, 2, 35, 4, 3, 7,22, 24, 32], or on synchronizing video by audio tracks [29]and therefore their applicability is limited. Other ap-proaches employed compressed video bitrate profiles [25]and still camera flashes [28].The methods differ in temporaltransformation models. Often, time shift [23, 32, 35, 3], ortime shift combined with variable frame rate [7, 22, 2], areused. The majority of previous work requires rigid sets ofcameras. Notable examples of synchronization methods forindependently moving cameras are [35, 3].

Many methods share a similar basis. A set of trajectoriesis detected in every video sequence using an interest pointdetector and an association rule or a 2D tracker. The tra-jectories are matched across sequences. A RANSAC basedalgorithm is often used to estimate jointly or in an itera-tive manner the parameters of temporal and spatial transfor-

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mations [7, 22, 2]. In [7], RANSAC is used to search formatching trajectory pairs in filtered set of all combinationsof trajectories in a sequence pair. The epipolar geometry hasto be provided. The method [22] enables joint synchroniza-tion of N sequences by fitting a single N -dimensional linecalled timeline in a RANSAC framework. The algorithm[2] estimates temporal and spatial transformation based ontentative trajectory matches.

Methods using exhaustive search to find the homogra-phy [32] and either fundamental matrix or homography [33]along with the time offset were presented. These are search-ing over the entire space of possible time shifts.

The two most closely related works to ours are [21, 20]that jointly estimate two-view geometry together with timeshift from approximated image point trajectories. In [21] es-timated epipolar geometry or homography along with timeshift using non-linear least squares, approximating the im-age trajectory by a straight line. The algorithm is initializedby the 7pt algorithm [14] and a zero time shift. Work [20]extended this approach by estimating difference in framerate and using splines instead of lines. Both the above worksachieve good results only when given a good initialization,e.g., on sequences less than 0.5 seconds time shift and withno gross matching errors.

1.2. Contribution

In this paper we present two new contributions.First, we present a new method for simultaneous com-

putation of two-view camera geometry and temporal off-set parameters from minimal sets of point correspondences.We solve for fundamental matrix or homography togetherwith temporal offset of image sequences. Our methods needonly moving image point trajectories, which are easy totrack. Unlike in [21, 20], we use a small (minimal) numbersof correspondences and we therefore are robust to outlierswhen combined with RANSAC robust estimation.

Secondly, we present an iterative scheme using the min-imal solvers to efficiently estimate large time offsets. Ourapproach is based on RANSAC loop running our minimalsolvers. This approach efficiently searches in the space ofpossible time offsets, which is much more efficient than ex-haustive search methods [32, 33] developed before.

We evaluated our approach on a wide range of scenesand demonstrated its capability of synchronizing variouskinds of real camera setups, such as driving cars, surveil-lance cameras, or sports match recordings with no other in-formation than image data.

We demonstrate that our solvers are able to synchro-nize small time shifts of fractions of a second as well aslarge time shifts of tens of seconds. Our iterative algorithmis capable of synchronizing medium time shifts (i.e. tensof frames) with less than 5 RANSAC iterations and largetime offsets (i.e. tens to hundreds of frames) using tens of

RANSAC iterations. Overall, our approach is much moreefficient than other methods utilizing RANSAC [22].

By solving two-camera synchronization problem, wealso solve the multi-camera synchronization problem sincetemporal offsets of multiple cameras can be determinedpairwise to serve as the initialization point for a global it-erative solutions based on bundle adjustment [34].

2. Problem formulationLet us consider two unsynchronized cameras with a fixedrelative pose [14] producing a stereo video sequence by ob-serving a dynamic scene. Motions of objects in the video se-quence are indistinguishable from camera rig motions, andtherefore, we will present the problem for static camerasand moving objects.

2.1. Geometry of two unsynchronized cameras

The coordinates of a 3D point moving along a smooth tra-jectory in space can be described by function

X(t) = [X1(t), X2(t), X3(t), 1]>, (1)

where t denotes time, see Figure 1. Projecting X(t) into theimage planes of the two distinct cameras produces two 2Dtrajectories x(t) and x′(t). Now, let’s assume that the firstcamera captures frames with frequency f (period p = 1/f)starting at time t0. This leads to a sequence of samples

si = [ui, vi, 1]> = x(ti) = π(X(ti)), i = 1, . . . , n. (2)

of the trajectory x(t) at times ti = t0 + ip.Analogously, assuming a sampling frequency f ′ (period

p′ = 1/f ′), at times t′j = t′0 + jp′, the second camera pro-duces a sequence of samples

s′j = [u′j , v′j , 1]> = x′(t′j) = π′(X(t′j)), j = 1, . . . , n′. (3)

In general, there is no correspondence between the siand s′j samples, i.e., for i = j, si and s′j do not representthe projections of the same 3D point. There are two mainsources of desynchronization in video streams. The first oneis the different recording starts or camera shutters triggeringindependently leading to a constant time shift. The secondsource are different frame rates or imprecise clocks leadingto different time scales. Assuming these two sources, wecan map the time t to t′ for frame i using (i) : N→ R as

(i) =ti−t′0p′

=t0+ip−t′0

p′=

t0−t′0p′

+p

p′i = β + ρi, (4)

where β ∈ R is captures the time shift and ρ ∈ R the timescaling. Note that (i) is an integer-to-real linear mappingwith an analogous inverse mapping ı(j). Given the modelin (4) and a sequence of image samples s′j , j = 1, . . . , n′,we can interpolate a continuous curve s′(), for example

using a spline, so that the 2D point corresponding to si isapproximately given as

si ←→ s′(β + ρi). (5)

Notice that the interpolated image curve s′(·) is not equiva-lent to the true image trajectory x′(·), but may be expectedto be a good approximation under certain conditions. Eventhough it might appear reasonable to assume time shift to beknown within a fraction of a second, it is often the case inpractice that the timestamps are based on CPU clocks whichtogether with startup delays can lead to time shift β beingin the order of seconds. On the other hand, the time scalingρ is more often known or can be calculated accurately.

2.2. Epipolar geometry

At any given (real-valued) time t, the epipolar constraint ofthe two cameras is determined by the following equation:

x′(t)>Fx(t) = 0. (6)

For a sample si in the first camera, we can rewrite (6) usingthe corresponding point x′(ti) in the second camera as

x′(ti)>Fsi = 0, (7)

Using the approximation of the trajectory x′ by s′, we canexpress the approximate epipolar constraint as

s′(β + ρi)>Fsi = 0. (8)

In principle, we can solve for the unknowns β, ρ, and F

given 9 correspondences si, s′j . However, such a solutionwould be necessarily iterative and too slow to be used as aRANSAC kernel. In the following, a further approximationis used to expresses the problem as a system of polynomi-als, which can be solved efficiently [18]. In §6 we showan iterative solution built on this kernel, which can recoveroffsets of up to hundreds of frames.

2.3. Linearization of s′ for known ρ

Let us assume that the relative framerate ρ is known. Inpractice, the image curve s′ is a complicated object. Toarrive to our polynomial solution we approximate s′ by thefirst order Taylor polynomial at β0 + ρi

s′(β + ρi) ≈ s′(β0 + ρi) + (β − β0)v = s′′(β + ρi) (9)

where v is the tangent vector s′(β0+ρi), and β0 is an initialtime shift estimate. We denote this approximation as s′′.

Further, we choose v to approximate the tangent overthe next d samples. Let j0 = bβ0 + ρic be the approximatediscrete correspondence, and then

v = s′j0+d − s′j0 . (10)

x′(t)

si

x(t)

x′(t0 + ip)s′′(β + ρi)

s′(β0 + ρi) = s′(i) = ui

vi

x′(t)

si

x(t)

x′(t0 + ip)s′′(β + ρi)

vi

s′(β0 + ρi) = s′(1/2 i)= ui

Figure 2. Illustration of the proposed trajectory linearization.(Left) Situation for ρ = 1, β0 = 0 and d = 1 (Right) Situationfor ρ = 1/2, β0 = 0 and d = 1.

Note, that now v depends on i. For compactness, we writeui = s′(β0 + ρi)− β0vi, and (8) becomes

(ui + βvi)>Fsi = 0 (11)

In the rest of the paper, we will assume that f = f ′ andthe initial estimate β0 = 0. This situation is illustrated inFigure 2 (Left). However the key results hold for generalknown ρ, Figure 2 (Right), and β0 6= 0.

2.4. Homography

Using the same approach, we can write the equation forhomography between two unsynchronized cameras. In thesynchronized case, the homography between two camerascan be expressed as

Hsi = λis′i. (12)

where λi is an unknown scalar. Approximating the imagemotion locally by a straight line gives for two unsynchro-nized cameras

Hsi = λi (ui + βvi) . (13)

3. Solving the equations3.1. Minimal solution to epipolar geometry

The minimal solution to the simultaneous estimation ofthe epipolar geometry and the unknown time shift β startswith the epipolar constraint (11). The fundamental matrixF = [fij ]

3i,j=1 is a 3× 3 singular matrix, i.e. it satisfies

det(F) = 0. (14)

Therefore, the minimal number of samples si and s′i neces-sary to solve this problem is eight.

For eight samples in general position in two cameras, theepipolar constraint (11) can be rewritten as

Mw = 0, (15)

where M is a 8 × 15 coefficient matrix of rank 8 andw is a vector of monomials w = [f11, f12, f13, f21,f22, f23, f31, f32, f33, βf11, βf12, βf13, βf21, βf22, βf23].Since the fundamental matrix is only given up to scale,

the monomial vector w can be parametrized using the7-dimensional nullspace of the matrix M as

w = n0 +∑6i=1 αini, (16)

where αi, i = 1, . . . , 6 are new unknowns and ni, i =0, . . . , 6 are the null space vectors of the coefficient matrixM. The elements of the monomial vector w satisfy

βwj = wk for (j, k) ∈ {(1, 10), ..., (6, 15)}. (17)

The parametrization (16) used in the rank constraint (14)and in the quadratic constraints (17) results in a quite com-plicated system of 7 polynomial equations in 7 unknownsα1, . . . , α6, β. Therefore, we first simplify these equationsby eliminating the unknown time shift β from these equa-tions using the elimination ideal method presented in [19].This results in a system of 18 equations in 6 unknownsα1, . . . , α6. Even though this system contains more equa-tions than the original system, its structure is less compli-cated. We solve this system using the automatic genera-tor of Grobner basis solvers [18]. The final Grobner basissolver performs Gauss-Jordan elimination of a 194 × 210matrix and the eigenvalue computations of a 16 × 16 ma-trix, since the problem has 16 solutions. Note that by sim-ply applying [18] to the original system of 7 equations in7 unknowns a huge and numerically unstable solver of size633× 649 is obtained.

3.2. Generalized eigenvalue solution to epipolar ge-ometry

Using the non-minimal number of nine point correspon-dences, the epipolar constraint (11) can be rewritten as

(M1 + βM2)f = 0, (18)

where M1 and M2 are 9 × 9 coefficient matrices and f is avector containing nine elements of the fundamental matrix F

The formulation (18) is a generalized eigenvalue prob-lem (GEP) for which efficient numerical algorithms arereadily available. The eigenvalues of (18) give us solutionsto β and the eigenvectors to fundamental matrix F.

For this problem the rank of the matrix M2 is only sixand three from nine eigenvalues of (18) are always zero.Therefore, instead of 9× 9 we can solve only 6× 6 GEP.

This generalized eigenvalue solution is more efficientthan the minimal solution presented in section 3, howevernote that the GEP solution uses non-minimal number ofnine point correspondences and the resulting fundamentalmatrix does not necessarily satisfy det(F) = 0.

3.3. Minimal solution to homography estimation

The minimal solution to the simultaneous estimation ofthe homography and the unknown time shift β starts withthe equations of the form (13).

First, the solver eliminates the scalar values λi from (13).This is done by multiplying (13) by the skew symmetricmatrix [ui + βvi]×. This leads to the matrix equation

[ui + βvi]× Hsi = 0. (19)

The matrix equation (19) contains three polynomialequations from which only two are linearly independent, be-cause the skew symmetric matrix has rank two. This meansthat we need at least 4.5 (5) samples in two images to esti-mate the unknown homography H as well as the time shift β.

Now let us use the equations corresponding to thefirst and second row of the matrix equation (19). Inthese equations β multiplies only the 3rd row of theunknown homography matrix. This lead to nine ho-mogeneous equations in 12 monomials w = [h11, h12,h13, h21, h22, h23, h31, h32, h33, β h31, β h32, β h33]> for4.5 samples in two images (i.e. we use only one equationfrom the three equations (19) for the 5th sample).

We can stack these nine equations into a matrix formMw = 0, where M is a 9 × 12 coefficient matrix. Assum-ing that M has full rank equal to nine, i.e., we have non-degenerate samples, the dimension of null(M) is 3. Thismeans that the monomial vector w can in general be rewrit-ten as a linear combination of three null space basis vectorsni of the matrix M as

w =∑3i=1 γi ni, (20)

where γi are new unknowns. Without loss of generality, wecan set γ3 = 1 to fix the scale of the homography and tobring down the number of unknowns. For 5 or more sam-ples, instead of null space vectors ni, we use in (20) threeright singular vectors corresponding to three smallest sin-gular values of M.

The elements of the monomial vector w are not inde-pendent. We can see that w10 = β w7, w11 = β w8, andw12 = β w9, where wi is the ith element of the vector w.These three constraints, together with the parametrizationfrom equation (20) form a system of three quadratic equa-tions in three unknowns γ1, γ2, and β and only 6 monomi-als. This system of three equations has a very simple struc-ture and can be directly solved by performing G-J elimina-tion of the 3 × 6 coefficient matrix M1 representing thesetree polynomials, and then by computing eigenvalues of the3× 3 matrix obtained from this eliminated matrix M1. Thisproblem results in up to three real solutions.

Note, that the problem of estimating homography and βcan also be formulated as a generalized eigenvalue problem,similarly as the problem of estimating epipolar geometry(Section 3.2). However, due to the lack of space and the factthat the presented minimal solution is extremely efficient,we do not describe the GEP homography solution here.

4. Using RANSAC

In this section we would like to emphasize the role ofRANSAC for our solvers. RANSAC is generally used forrobustness since the minimal solvers are sensitive to noiseand outliers. Outliers in the data will usually come from twosources. One are the mismatches and misdetections and theother is the non-linearity of the point trajectory. Even with-out gross outliers due to false detections, there will alwaysbe outliers with respect to the model in places where the tra-jectory is not straight on the interpolating interval. There-fore, it is usually beneficial to use RANSAC even if we aresure the correspondences are precise.

By using RANSAC, we avoid those parts of the trajec-tory and pick the parts that are approximately straight andlinear in velocity. Basically we only need to sample 8(F)or 5(H) parts of the trajectory where this assumption holdsto obtain a good model, even if the rest of the trajectory ishighly non-linear.

5. Performance of the solvers on synthetic data

First, we investigated the performance of estimating thetime shift β using the proposed F and H minimal solvers.We simulated a random movement of a 3D point in front oftwo cameras. The simulated 3D trajectory was then sam-pled at different times in each camera, the difference be-ing the ground truth time shift βgt. Image noise was addedfrom a normal distribution with σ = 0.5 px. We testedthe minimal solvers with various interpolation distances dand compared them also to the standard seven point funda-mental matrix (7pt-F) and four point homography (4pt-H)solvers [14]. Each algorithm was tested on 100 randomlygenerated scenes for each βgt, resulting in tens of thousandsof experiments.

There are multiple observations we can make from theresults. The main one is that both F and H solvers performwell in terms of estimating βgt, even for the minimal in-terpolation distance d = 1. Figure 3 shows that almost allinliers are correctly classified using d = 1, d = 2, d = 4up to shift of 5 frames forward. Furthermore, even thoughthe inlier ratio begins to decrease with larger shifts, timeshift β is still correctly estimated, up till frame shifts of 20.Overall, for a given d, each algorithm was able to estimatecorrect β at least up to d. This is a nice property, suggest-ing that for larger time shifts we should be able to estimatethem simply by increasing d.

For d = 8, d = 16, d = 32, the situation is slightlydifferent with respect to inliers. Notice that there are twopeaks in the number of inliers, one at βgt = 0 and the otherat βgt = d. This is expected, because at βgt = d, the inter-polating vector v passes through the sample s′i+βgt

whichis in temporal correspondence with si. When βgt 6= 0 oursolvers are for any d well above the number of inliers pro-

-50 0 50

Ground truth shift (frames)

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d=1d=2d=4d=8d=16d=327pt-F

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d=1d=2d=4d=8d=16d=32

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d=1d=2d=4d=8d=16d=324pt-H

-50 0 50

Ground truth shift (frames)

-20

0

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40

Estimated shift (frames)

βgt

d=1d=2d=4d=8d=16d=32

Figure 3. Results on randomly generated scene with various timeshift β between cameras and several different interpolation dis-tances d. Temporal distance of one frame equals to approximately8 pixels distance in 1000x1000px image. Top two figures are re-sults for epipolar matrix and bottom two for homography.

vided by standard F and H algorithms.Another thing to notice is the non-symmetricity of the re-

sults. Obviously, when βgt < 0 (backward) and we are in-terpolating with d-th (forward) sample, the peaks in inliersare not present, since we will never hit the sample which isin correspondence. Also, the performance in terms of in-liers is reduced when interpolating in the wrong direction,although still above the algorithms not modelling the timeshift. Estimation of β deteriorates significantly sooner fornegative βgt, at around -10 frames. We will show how toovercome this non-symmetricity by searching over d in bothdirections using an iterative algorithm.

6. Iterative algorithmAs we observed in the synthetic experiments, the perfor-mance of the minimal solvers will depend on the distancefrom the optimum, i.e. the distance between the initial es-timate β0 and the true time shift βgt, and on the distance dof the samples used for interpolation. The results from syn-thetic experiments (Figure 3) provide useful hints on how toconstruct an iterative algorithm to improve the performanceand applicability of the minimal solvers. In particular, thereare three key observations to consider.

First, the number of inliers obtained from RANSACseems to be a reasonable function to optimize. Generallyit will have two strong local maxima, one at ti = t′i andone at (ti − t′i) = d. At ti = t′i the sequences are synchro-nized and at (ti − t′i) = d, Fig. 3, we obtain the correct β.Both situations give us synchronized sequences. Second,the β computed even far from the optimum, although not

precise, provides often a good indicator of the direction to-wards ti = t′i. Finally, it can be observed that increasing dimproves the estimates when we are far from the optimum.Moreover, as seen from the peaks in Fig. 3, selecting largerd yields increasingly better estimates of β, which are loweror equal than the actual (ti− t′i), but never higher. This sug-gests that we could safely increase d until a better estimateis found.

The observations mentioned above lead us to algo-rithm 1. The basic principle of the algorithm is the follow-ing. In the beginning, assume i = j. At each iteration k,estimate β and F. If this model gives more inliers than pre-vious estimate, change j to the nearest integer to j + β andrepeat. If the new estimate gives less inliers than the lastone, extend the search direction by increasing d by powersof 2 until more inliers are found. If dpmax is reached, p isreset to 0, so interpolation distances keep circling betweend0 and dpmax . This is essentially a line search over the pa-rameter d. Algorithm is stopped when the number of inliersdid not increase pmax times. This ensures, that at each t′j ,all interpolation distances are tested at maximum once. Theresulting estimate of β is then j− i+β, which is the differ-ence in frames the algorithm traveled plus the last estimateof time shift at this point (subframe synchronization).

Estimating of β and F is done using RANSAC and in-terpolating from both the next and previous dth sample,searching the space of β in both directions. Whichever di-rection returns more inliers is taken as current estimate. Bychanging the values pmin and pmax we have the option toadjust the range of search. Having an initial guess aboutthe amount of time shift, e.g. not more than 100 frames,but definitely more than 10 frames, we could start the algo-rithm with values pmin = 3 and pmax = 7 so the search ind would start with d = 8 and not go further than d = 128.

The symbol T represents a geometric relation, in our caseeither a fundamental matrix or a homography.

7. Real data experimentsOur real datasets contain two private datasets and three pub-licly available multi-camera datasets. We aimed at collect-ing various types of scenes to cover wide range of appli-cations. The public data were always synchronized andwe manually shifted the frame to frame correspondencesto simulate the ground truth time shift. We experimentedwith shift of -50 to 50 frames on each dataset, which pro-duced time shifts ranging from 2s to 5s based on the cameraframerate.

7.1. Datasets

Dataset Marker was obtained by moving an Arucomarker in front of a two webcams running at 10fps. A digi-tal clock in the scene was processed by OCR in each frameto provide ground truth timestamps. Further, we used three

Algorithm 1 Iterative syncInput: s0, . . . , sn,s′0, . . . , s

′n′ ,kmax,pmax,pmin

Output: β,Tβ0 ← 0,i = j,skipped← 0, d← 2pmin ,inliers0 ← 0,p← pmin

while k = 1 < kmax doT1,β1 and inliers1 ← RANSAC(si, s′j , d)T2,β2 and inliers2 ← RANSAC(si, s′j ,−d)if inliers1 > inliers2 then

inliersk ← inliers1, βk ← β1, Tk ← T1else

inliersk ← inliers2, βk ← β2, Tk ← T2end ifif skipped > pmax then

return Tk−1,β ← j − i+ βk−1

else if inliersk < inliersk−1 thenif p < pmax thenp← p+ 1

elsep← 0

end ifd← 2p

skipped← skipped + 1elsej ← j + dβkcskipped← 0k ← k + 1

end ifend while

public datasets and one private: UvA [17], KITTI [12],Hockey and PETS [8]. The UvA dataset consists of videosequences taken by three static cameras with manual anno-tations of humans. The KITTI dataset contains stereo videosequences taken from a moving car. In our experiments, weused raw unsynchronized data provided by the authors. TheHockey dataset was synchronized by [31] and the trajecto-ries are manually curated tracks of [16]. The PETS datasetis a standard multi-target tracking dataset. Trajectories weredetected by [13, 9, 27] and manually joined.

7.2. Algorithms

We compared seven different approaches to simultane-ously solving two-camera geometry and time shift. De-pending on the data, either fundamental matrix or homog-raphy was estimated. We denote both geometric relationsby T , where T means H or F was estimated using standard4 or 7 point algorithms [14] and Tβ means that H or F wasestimated together with β. The rightmost column of fig-ure 4 shows which model, i.e. homography or fundamentalmatrix, was estimated on a particular data set.

The closest alternatives to our approach are the least-squares based algorithms presented in [21] and [20]. Bothoptimize F or H and β starting from an initial estimate ofβ = 0 and T . Method [21] uses linear interpolation fromthe next sample, whereas method [20] uses spline interpola-tion of the image trajectory and we will refer to these meth-ods as Tβ-lin and Tβ-spl respectively. In our implementa-tion of those methods, we used Matlab’s lsqnonlin function

βgt 0-10 10-20 20-30 30-40 40-50Tβ-new-iter-pmax0 4.7 4.3 3.5 4.1 3.8Tβ-new-iter-pmax6 23 22 21.2 21.6 21.2Tβ-new-iter-pmaxvar 18 19 17.5 16.7 16.5

Table 1. Average number of RANSACs executed before termina-tion. Evaluated on Marker dataset.

with Levenberg-Marquardt algorithm, all stopping criteriaset to epsilon and maximum number of 100 iterations.

We tested the solvers presented in section 3 with d = 1as algorithm Tβ-new-d1. The proposed iterative algorithm 1that uses the solvers was tested using several different set-tings. The user can control the algorithm using parame-ters pmax and pmin, which determine the distances d thatwill be used for interpolation. As we observed in section 5,there is a good chance of computing a correct β if d > βgt.First, we ran the algorithm with pmin = 0 and pmax = 6,which gives maximum d = 64 as algorithm Tβ-new-iter-pmax6. This version of the algorithm is guaranteed to tryd = 1, 2, 4, 8, 16, 32, 64 at each βk before it stops or it findsmore inliers. This covers the time shifts we tested, but canlead to unnecessary iterations for smaller shifts. Therefore,we also tested pmax = 0 as Tβ-new-iter-pmax0 which onlytried d = 1 at each iteration to see the capabilities of themost efficient version of the algorithm.

The last version of our algorithm, Tβ-new-iter-pmaxvar,adapted both pmax and pmin to βgt such that 2pmin ≤βgt < 2pmax . This represents a case when user has a roughestimate about the expected time shift and sets the algorithmaccordingly. We remind that setting pmin only affects theinitial interpolating distance, after reaching d = 2pmax thealgorithm starts again with d = 20.

Finally, algorithm T -lin [21] also takes the next samplesfor interpolation, making it comparable to our Tβ-new-d1.We used T -lin in the same iterative scheme as Tβ-new-iter-pmax6 and tested it as T -new-lin-iter, where instead of us-ing the number of inliers as a criteria for accepting a step,we used the value of the residual.

7.3. Results and discussion

The results on real datasets demonstrate a wide practi-cal usefulness of the proposed methods. For most datasets,Tβ-new-d1 itself performed at least as good as the leastsquares algorithms Tβ-lin and Tβ-spl. A single RANSACwas enough to synchronize time shifts of 2-5 frames acrossall datasets. The iterative algorithm Tβ-new-iter-pmax6built upon our solvers performed the absolute best acrossall datasets, converging successfully from as far as 5s timedifference on Marker and Hockey datasets, 2s difference onUvA dataset and 2.5s on Kitti dataset as seen in the successrate column of figure 4.

On the Kitti dataset, Tβ-new-iter-pmax6 was outper-formed by the Tβ-new-lin-iter, which uses the iterative al-

gorithm proposed by us, but with a solution from [21] in-side. Tβ-new-lin-iter was able to estimate time differenceslarger than 2.5s but only in roughly half of the cases, whereTβ-new-iter-pmax6 was 100% successful up to 2.5s whenit sharply fell off. We account this to the high non-linearityof the 2D velocity of the image points, where as the objectsgot closer to the car, they moved faster. The tracks of length25 frames and more were very sparse here and the longerthey were the more non-linear in the velocity.

On the contrary, the hockey dataset posed a big challengefor the least squares algorithms, which struggled even withthe smallest time offsets. We attribute this to the poor es-timate of F by the seven point algorithm which causes theLM algorithm to get stuck in local minima. We also testedthe homography version of all algorithms on this dataset,since the trajectories are approximately planar, which re-sulted in the least squares algorithms performing slightlybetter whereas the algorithms with minimal solvers per-formed slightly worse.

PETS dataset was probably the most challenging, be-cause of the low framerate (7FPS), coarse detections andabrupt change of motion. Still, our methods managed tosynchronize the sequences in majority of cases.

Table 1 shows the average number of RANSACs usedbefore termination of different variants of iterative algo-rithm 1 for the dataset Marker. We can see that using 8pt-iter-pmax0 greatly reduces the computations needed, stillallowing this method to reliably estimate time shifts of 0.5s-2s depending on the scene, rendering it useful if we are cer-tain that the sequences are off by only a few tens of frames.Knowing the time shift approximately and setting pmax andpmin can also reduce the computations as shown by 8pt-iter-pmaxvar, which provided identical performance to 8pt-iter-pmax6, sometimes even outperforming it.

8. Conclusion

We have presented solvers for simultaneously estimatingepipolar geometry or homography and time shift betweenimage sequences from unsynchronized cameras. These arethe first minimal solutions to these problems, making themsuitable for robust estimation using RANSAC. Our methodsneed only trajectories of moving points in images, whichare easily provided by state-of-the-art methods, e.g. SIFTmatching, human pose detectors, or pedestrian trackers. Wewere able to synchronize wide range of real world datasetsshifted by several frames using a single RANSAC with oursolvers. For larger time shifts, we proposed an iterativealgorithm using these solvers in succession. The iterativealgorithm proved to be reliable enough for synchronizingreal world camera setups ranging from autonomous cars,surveillance videos, and sport game recordings, which werede-synchronized by several seconds.

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βgt (s) βgt (s) βgt (s)Figure 4. Results on real data. In the two leftmost columns, trajectories used for the computations are depicted in coloured lines over asample images from the dataset. Third column shows the rates with which different algorithms succeeded to synchronize the sequence tosingle frame precision for various ground truth time shifts. Fourth column shows a closer look at the individual results for β for smallerground truth time shifts and five runs for each algorithm, each data point corresponds to one run of the algorithm at corresponding βgt.Letters H and F on the right signalize whether homography or fundamental matrix was computed.

9. Appendix

The appendix contains additional experimental results thatprovide details that are out of the scope of the main paper.

9.1. Subframe synchronization

One issue that was not directly elaborated upon in the mainpaper is the ability of the solvers to synchronize sub-frametime shifts, i.e., shifts where βgt is not an integer. In the real

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datasets, images were either hardware synchronized, i.e.,βgt = 0, or we did not have precise enough ground truthinformation about the subframe time shift. Therefore, wetested the subframe synchronization on the synthetic dataonly. The results in Figure 5 show that the subframe syn-chronization is very precise for various levels of noise. Fig-ure 6 shows an example of a randomly generated scene forthe synthetic experiments.

9.2. Iterative algorithm visualization

In Figure 7, we provide a visualization of one run of the it-erative algorithm with pmax = 5. Each iteration is markedby a black square and denoted by the number of the itera-tion k and the distance d used for interpolation in the giveniteration. The algorithm greedily searches for a larger num-ber of inliers (the top figure) and uses the estimated βk tochange the correspondences, which results in change of thecurrent ground truth shift (bottom figure). This particularrun converged in 6 iterations, even though the initial timeshift (50) was larger than the maximum interpolation dis-tance d = 32. Moreover, the algorithm only used interpo-lation distances d = 1, 2. These distances were enough toprovide a good enough estimate of the time shift that leadto an increased number of inliers.

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9.3. Accuracy of the estimated geometry

9.3.1 Synthetic data

On the same data as used in Section 5.1 of the main paper,we evaluated the estimated relative rotations R and transla-tions t. The results in Figure 8 show that we are able to es-timate R and t significantly better than the classical 7 pointalgorithm. The utility of our solver is especially apparentfrom the zoomed in figures with smaller time shifts. The er-ror in R and t is almost zero up to 5 frames shift, for shorterinterpolation distances d = 1, 2, 4, 8. In contrast, such shiftcauses a significant drop in performance of the classical 7-point algorithm, resulting in errors up to 5 degrees in orien-tation and relative error of 5% in the translation vector.

Even for the long interpolation distances d = 16, 32—although not as good as for d = 1, 2, 4, 8—the performanceis still better than that of the classical 7-point algorithm.The performance of d = 16 and d = 32 improves with in-creasing ground truth time shift and peaks, as expected, ontime shifts 16 and 32, respectively. Note that in our iterativealgorithm, we only use the right hand side of the results inthe above graphs, because both d and −d are used at eachiteration.

9.3.2 Real data

The only real world dataset used for the main paper experi-ments for which the ground truth spatial calibration is pro-

vided is the UvA dataset. We extracted the ground truthrelative Rgt and tgt from the dataset camera matrices andcompared them to the values estimated by all algorithms.Figure 9 shows the angular error of R, measured as the rota-tion angle of Rerr = R>Rgt, and the relative translation errormeasured as ||tgt−t||, where both tgt and t are normalizedto unit lengths. Errors are averaged over 100 runs for eachdatapoint. The results follow the pattern of the results inFigure 4 of the paper, where when an algorithm successfullyestimated the time shift, it also provided a good geometryestimate.

Both iterative algorithms Tβ-new-iter-pmax6 and Tβ-new-iter-pmaxvar, which have pmax large enough to coverthe required time shifts, perform well over almost the en-tire range of time shifts. The efficient Tβ-new-iter-pmax0which iteratively uses d = 1 performed well up till the timeshifts of 0.25 s (5 frames). Tβ-new-d1, which is the solverusing d = 1 in RANSAC, was able to estimate the geometryreliably only for a time shift of 1 frame. All the algorithmsbased on the 7-point algorithm, including the 7-point algo-rithm itself in RANSAC, performed poorly on this dataset.

AcknowledgementThis work was partly done during an internship ofC. Albl and a postdoc position of Z. Kukelovaat Microsoft Research Cambridge and was sup-ported by the EU-H2020 project LADIO (number731970), The Czech Science Foundation Project GACRP103/12/G084, Grant Agency of the CTU Prague projectsSGS16/230/OHK3/3T/13, SGS17/185/OHK3/3T/13 andby SCCH GmbH under Project 830/8301544C000/13162.

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