I Introduction
Odometry plays an essential role in many applications such as robot navigation or autonomous driving where robust and precise estimation of the trajectory is a main challenge. Nowadays, this has become more interesting as a variety of sensors that could include the spatial information as well have emerged [1]. A LiDAR (Light Detection And Ranging) is such an active optical sensor that can provide highly accurate range measurements where errors are usually constant irrespective of the distance.
Odometry is often estimated by incrementally integrating the relative transformations resulting from consecutive point cloud registrations. Traditional point cloud registration approaches such as Iterative Closest Point [2, 3] alternate between the translation and rotation estimation in an iterative fashion. This has the overhead of estimating the three degrees of translational freedom and three degrees of rotational freedom simultaneously. In this paper, we propose the new DeLiO algorithm that estimates the transformation by decoupling the rotation from translation and thus making the six DoF odometry estimation boil down to two times three DOF. To the best of our knowledge, we are the first to propose this decoupling for LiDAR odometry.
The decoupling of rotation estimation in DeLiO is driven by the following main observation in two consecutive LiDAR point clouds: The rotational difference between surface normals corresponds to the rotation difference between their corresponding LiDAR scans. This property is independent of any translation, since translation does not affect the surface normals. Thus, this can be exploited to estimate the rotation independent of translation. The main challenge in DeLiO is to determine the rotation difference in the normals of two LiDAR scans. In Section III we propose a new approach to track these surface normals over time. Later, point clouds are unrotated using the estimated rotation so that the underlying transformation is a pure translation. This translation is estimated in Section IV using a stateoftheart line cloud odometry inspired from the work of Velas et al. [4]. In Section VI we verify the stateoftheart performance of DeLiO on the KITTI benchmark. An overview of our new approach is given in Figure 2.
Ii Related Work
In the literature plenty of odometry algorithms exist for several visual sensor types [5]. Several algorithms – such as LIMO [6], DEMO [7], etc. – estimate the odometry out of a monocular or stereo camera and use the LiDAR points for support only. However, for our approach we want to rely on pure LiDAR information.
Since LiDAR information is a largescale point cloud, point cloud registration is usually applied in the stateoftheart for LiDARonly odometry. For point cloud registration often variations of the traditional Iterative Closest Point (ICP) [3]
are utilized. ICP and its variations estimate the transformation involved between two scans by iteratively estimating rotation and translation in an alternating fashion. But, ICP is prone to local minima when the point clouds are considerably far from each other. This is because the point cloud correspondences can be accurately estimated only when initial alignment is already very good. Also ICP is highly sensitive to noise and outliers. The exact point to point matching in standard ICP does not take into account the differential sampling of the two point clouds and thus may lead to pairs without good equivalence. A few ICP variants were proposed to overcome this issue by taking information from surface normals into consideration. Two prominent variants are
PointToPlane approach that considers the surface normals from the target point cloud only and PlaneToPlane variant that uses the normals from both source and target point cloud.A recent stateoftheart approach in odometry using LiDAR sensors is LiDAR Odometry and Mapping in Realtime (LOAM) [8] that extracts features that are on sharp edges and planar surface patches using the scan sweep information. Also, LOAM integrates data from IMU units as well to improve the accuracy of estimation. VisualLOAM (VLOAM) [9], is a variation of LOAM that aims at motion estimation and mapping using a monocular camera with a 3D LiDAR instead of IMU. A recent variation of ICP is Implicit Moving Least SquaresSLAM (IMLS) [10], that relies on a scantomodel matching framework. Initially the LiDAR scans are sampled with a sampling strategy and then a model is defined from the previous localized LiDAR sweeps and use an Implicit Moving Least Squares (IMLS) surface representation. Unlike LOAM, IMLS does not use any data from other sensors like IMU or cameras but removes dynamic objects from the scene as a preprocessing step. The DLO [11] approach estimates the odometry directly, by projection the LiDAR scan to a ground plane and registering the resulting grid maps.
Unlike the before mentioned approaches, our method DeLiO decouples for the first time the rotation completely from translation in LiDAR odometry. Thus, rotation can be estimated in one step. The point clouds are later unrotated with the estimated rotation so that the underlying transformation is pure translation, which is estimated using a modified line cloud odometry. A further refinement of the rotation is no longer necessary in comparison to the stateoftheart algorithms. Also DeLiO does not use any data from other sensors like IMU, cameras or GPS.
Iii Rotation Estimation
DeLiO determines odometry by decoupling rotation from translation estimation. Thus, a robust rotation estimation scheme is necessary, which can be estimated independently of the translation. Utilizing the 3D coordinates captured by the LiDAR scanner directly is not feasible, since their alignment involves rotation as well as translation.
Therefore, our main idea is to use the surface normals of the captured LiDAR scans for rotation estimation. Surface normals can be extracted directly from the captured scans and convey rich geometric information. The normal vector distributions typically contain a characteristic structure depending on the captured environment as visible in Figure
3. These structural regularities lead to robust patterns in the normal vector density distribution. The rotational difference between two subsequent patterns corresponds to the rotation difference between their corresponding LiDAR scans. Thus, we track the normal distribution over time in order to determine rotation. This can be done completely independent of the translation difference.
In the following, our new rotation approach for DeLiO is described in more detail. We start with the normal extraction out of the captured LiDAR scans. In order to distinguish characteristic patterns in the normal distribution from random noise, we apply a statistical outlier filtering. If the retained subset is not enough for a pattern registration, a mean shift seeking is performed to extract more patterns from the surface normals. Later rotation is estimated out of the displacement of these patterns using a Singular Value Decomposition (SVD).
Iiia Surface Normal Extraction
Surface normals are extracted, since they are translation independent geometric properties. A leastsquare plane fitting estimation problem [12]
that estimates the normal of a point on a surface by approximating the normal of a plane tangent to the surface is used for surface normal estimation. The analysis of eigenvectors and eigenvalues (estimated using Principal Component Analysis (PCA)
[13]) of a covariance matrix formulated from the knearest neighbors [14] of each query point helps in estimating the surface normals corresponding to the query points. The covariance matrix is formulated as(1) 
where is the number of nearest neighbors of point , is the centroid of the nearest neighbors of , and is the th eigenvector of the covariance matrix, and is the th eigenvalue. In order to be more robust against outliers, bilateral filtering on the surface normals is applied [15].
Prominent aspects from the surface normals are now identified by analyzing the pattern formed by these normals on an unit sphere. The motivation behind this idea is the fact that the resulting characteristic pattern on unit sphere is moving over the unit sphere over time as depicted in Figure 3. Tracking this pattern over time yields rotation independent of translation.
IiiB Statistical Outlier Removal
The presence of large number of scattered points on the unit sphere affects adversely the tracking of the pattern. Hence a robust outlier removal scheme has to be considered to select a subset of the points on the unit sphere that effectively represent the underlying pattern.
Therefore, we utilize a statistical outlier removal scheme [16]
that performs a statistical analysis in the neighborhood of each point, removing those points violating a certain criteria. For every point the mean distance to all its neighbors are calculated. Under the assumption that the resulting distribution is Gaussian with a mean and standard deviation, all points whose mean distances are outside a defined interval are considered as outliers and removed. Considering various number of neighboring points, this is done iteratively to retain a small but stable set of points. The resulting surface normals on the unit sphere are shown in Figure
4.If the scene lacks significant characteristic features, the resulting pattern on the unit sphere may be sparse and does not convey meaningful information to estimate the rotation. If the number of points retained after statistical outlier removal is lower than a threshold value, the may mean that the only remaining points represent ground because of the large number of ground points resulting in very dense areas over the unit sphere. A mean shift seeking [17] explained in detail in the following section is used if the retained number of points fall below a certain threshold value. This would facilitate finding more structures or dense points corresponding to new structures other than ground. Once the dense positions are discovered, a ball query discovers neighbors in a certain radius around each of the dense points. Since these points are prone to outliers or noise, statistical outlier removal is performed until a good number of points remain. The obtained points are downsampled and filtered using uniform sampling. Here a 3D voxel grid is assembled over the input points and in each voxel, all the points are assumed with their centroid. This approach reduces the number of points significantly, retaining only the best points and no outliers.
IiiC Mean Shift Seeking
As explained before, in case too few normals are retained after the Statistical Outlier Removal, a mean shift seeking is applied in order to preserve also fine structures for normal tracking. The mean shift seeking procedure applies local Kernel Density Estimation (KDE), if an initial approximate location of the mode is known and takes steps iteratively in the direction of increasing density. To ensure that the whole surface of the sphere is covered, the sphere surface is divided into a grid of uniform size which gives a set of coordinates that can be used to initialize the mode seeking. A nearest neighbor approach was used to clean the identified set of modes since mean shift iterations may converge to same modes if the initialization points are close to each other. The idea is to perform a single mean shift iteration for each mode given a subset of normal vectors on unit sphere. The subset is selected by considering all normal vectors that are in the neighborhood of considered center
. The range of the neighborhood is a conical window centered at the center of the sphere with an apex angle of . The required subset of normal vectors lie within this conical window and satisfies the condition(2) 
To compute the mean shift, these normal vectors are then projected into the tangential plane at . A Riemann exponential map [18] is applied so that the projected vectors represent proper angular deviations in the tangential plane. Then the mean shift in the tangential plane is computed as
(3) 
where are the tangential plane coordinates. is a design parameter that defines the width of the kernel in the tangential plane and can be derived from . The mean shift after computation is transformed onto the unit sphere again using Riemann logarithmic map.
IiiD Pattern Registration
Finally, rotation is estimated using a Singular Value Decomposition (SVD) [19]. For each target point, the corresponding source point is obtained using a knearest neighbor method. A KDTree [14] is constructed using the source point set and thus reducing the search space significantly. Once the correspondences are obtained, SVD can be used to estimate rotation. This is performed iteratively till estimated rotation converges. Using our new described approach makes it possible to estimate rotation completely independent from translation. Also an alternation between rotation and translation estimation in not necessary, since the final rotation is estimated after executing the procedure explained in this Section.
Iv Translation Estimation
After the rotation is estimated, this result can be used to unrotate the two LiDAR scans. The remaining transformation between them is then a pure translation. A major problem involved in LiDAR point cloud registration is the sparsity of the point cloud. Also the ring structure of the point cloud leads to incorrect registration [4]. Since classical ICP approaches [2, 3] try to minimize the distance of closest points, the resulting transformation could be the one that fits the ring structures that represent the floor because of the larger number of floor ring points. These issues associated with point clouds can be effectively handled by a line cloud proposed by Velas et al. [4]. In contrast to their algorithm where rotation and translation are estimated simultaneously, we use this approach for translation estimation only.
The line cloud approach has two main parts. First, the points in the point cloud are transformed into polar coordinate system and line cloud is generated. Second, the generated lines are registered and translation is obtained by aligning the centroids. Conversion to line cloud addresses the sparsity issue of the point cloud by generating lines such that the lines represent the underlying scene geometry efficiently. The original point cloud is iteratively resampled so that the new points for registration do not come from the point cloud, but the closest points between two matching pair of lines.
Iva Line Cloud Generation
As an initial step, each 3D point is converted from Cartesian coordinate system to a polar coordinate system for the line cloud generation. A polar coordinate system is a two dimensional coordinate system where each point is determined by a distance, r from the origin (radial coordinate) and an angle, from the reference direction (angular coordinate). The Cartesian coordinates [x,y,z] of the point cloud can be converted to polar coordinates r and by
(4) 
where is the angle within ground plane xy and r represents the distance from origin. Vertical axis z is not considered for the conversion because of the horizontal ring layout of the point cloud.
The LiDAR scanner produces rays such that each ray captures a ring of points as depicted by the red arrows in Figure 5. Points in the point cloud are shown as blue dots. Line cloud is generated by connecting some of the promising points in the point cloud in such a way that the underlying local surface properties are captured. The scanner position is considered as origin and the space around is divided into polar bins. Each point in the point cloud is represented by the ring and bin. One sample bin is shown by blue color in Figure 5. To ensure points of similar angle are selected, random lines are generated in each bin connecting points from consequent rings. Only few promising points from the line cloud are selected using a filtering criteria. Preserving only shorter lines (green lines) ensures that real planes are connected and points connecting different planes (red lines) are discarded.
IvB Line Cloud Registration
Once the point cloud is sampled into line cloud, the translation is estimated using a variation of ICP. Since the point clouds are unrotated with absolute rotation, translation is calculated iteratively by finding closest matches from the line cloud. Instead of finding pointtopoint correspondences, the algorithm finds correspondences between source and target line segments. This is done by finding the line segment in the target cloud whose center is close to the center of source line segment using a KDTree implementation. A distance threshold is included to eliminate incorrect matches in such a way that only the source and target line center pairs whose distance is less than the mean distance of all match pairs are kept.
Once the matching pair of line segments are found out, the line segments are aligned into a 3D plane that gives the translation involved. Consider a matching pair of lines and that are taken from the source cloud and target cloud respectively. The parametric representation of the lines given 3D points and and vectors and are
(5) 
(6) 
where and are vectors representing the direction of the line segments.
In any dimensional space, the two lines and are closest at points and when is the unique minimum of [20]. These closest points and are the correspondences between the source and target line clouds. The closest points between these lines are given by
(7) 
(8) 
where,
(9) 
(10) 
(11) 
The final transformation is estimated from the centroids of both sets of points under the assumption that the underlying transformation is pure translation. When the line clouds are registered, the distance between the preserved lines become very small or close to zero and the lines are coplanar.
V Optimization
Odometry integrates small relative transformations incrementally over time. The incremental nature of this estimation is subject to drift that accumulates to large errors [21] This drift can be reduced to a certain extend using a variety of optimization techniques. Estimated transformations can be improved using multiframe processing [22] as well as prediction based on constant velocity motion model.
Since DeLiO relies on a tracking of the surface normals for rotation estimation, a good initialization is beneficial. The closer the prediction is to the actual rotation value, the better is the final rotation result. Thus, we use a linear prediction model. The prediction for pairs of frames at time and can be estimated from previous frames using the linear prediction as
(12) 
Even though prediction improves results considerably, there is increased accumulation of drift as the length of trajectory increases. A recent trend is to use graphical models [23] to solve the accumulated drift through optimization. One way of formulating a posegraph for optimization is to construct a graph whose nodes correspond to the poses of the sensor at different points in time and whose edges represent constraints between the poses. The constructed posegraph can be further optimized by minimizing a nonlinear error function that corresponds to the graph.
Graph based optimization approaches build a pose graph incrementally taking the sensor pose at each point of time. The first node is constructed using the initial pose of the sensor. As soon as the sensor moves to a new position, the transformation between these positions is computed using the DeLiO algorithm of Sections III and IV. When this transformation is applied to position , a new measurement at position is acquired. The edge between positions and correspond to this estimated transformation. When the car moves further, a new transformation is available which when applied to position gives measurement at position . The transformation between position and can be estimated if a sufficient overlap exists between these two positions. A third edge may be added between position and in the posegraph. Due to imperfections in the incremental registration and noise from the environment, the two estimations at position may differ a bit. This error can be minimized by fine tuning the graph using optimization algorithms that update the pose with the optimized values.
Before optimization, an error function needs to be defined. A non linear least squares optimization can be described by
(13) 
(14) 
where is a vector of parameters. and represent the mean and information matrix relating the parameters in . defines how well the parameters fits the constraints in and . The information matrix contains value indicating how good the current measurement is. This is the inverse of covariance matrix. The LevenbergMarquardt algorithm [24] implemented in [25] is used to carry out the optimizations.
Vi Evaluation
For evaluation of our new DeLiO approach, we use the KITTI [26]
odometry benchmark. This dataset consists of 22 sequences out of which 11 sequences have ground truth trajectories available for training. The benchmark includes monochromic and color image sequences, GPSIMU data and laser scanner data collected in both urban and rural areas of City of Karlsruhe, Germany. For our work we utilize the laser scanner data only; all other data is neglected. The laser scanner data is represented as a point cloud acquired by a Velodyne HDL64E LiDAR. The evaluation metrics suggested by KITTI compute the rotation and translation errors separately by evaluating errors as a function of the trajectory length and velocity .
Final transformation is computed by combining the rotation (see Section III) and translation (see Section IV) as well as an optimization of this transformation (see Section V). When concatenating the relative transformations between scans, a long trajectory of the full sequence can be created as exemplary visible in Figure 7 and the supplementary video ^{1}^{1}1https://youtu.be/HzwwxQfpKNo. The estimated transformation can also be applied over time to the point clouds resulting in an environment map. Figure 6 shows such an environmental map formulated for sequence #07 validating the functionality of DeLiO.
Figure 7 shows two trajectories corresponding to sequences #07 and #05 before and after optimization (cp. Section V). Unoptimized trajectory errors are resulting from few erroneous registration caused due to overturning vehicles, pedestrians and other dynamic objects in the scene. Lack of characteristic features in the scene can also introduce errors. Since odometry accumulates small transformations over time, few erroneous frames may result in significant drift towards the end of the trajectory. This is reduced to a certain extend using a linear motion model and pose graph optimization.
In Figure 8 the stateoftheart algorithms are compared against DeLiO in a ground truth comparison of their trajectories. While BCC and BLO [27] show a strong drift, DeLiO demonstrates similar accuracy to DEMO [7], LIMO [6] and SuMa [28]. This verifies the stateoftheart performance of our new decoupling approach.
Table I shows a quantitative comparison of DeLiO with the stateoftheart approaches. It is important to know that the better performing algorithms utilize additional sensor information or special preprocessing. For example, LOAM [8] integrates IMU data for rotation estimation, whereas IMLS [10] preprocesses the LiDAR scans to remove dynamic objects from the scene. LIMO [6] and DEMO [7] utilize additional color information. Our proposed approach DeLiO uses only LiDAR data without removing the dynamic objects and without using any loop detection. The stated error in Table I results from very few erroneous registrations leading to the increased average value. Erroneous registration is caused due to overturning vehicles, pedestrians and other dynamic objects in the scene. A vehicle moving close to the sensor can result in clusters over the unit sphere. Since the algorithm retains only the major clusters/dense regions and rejects everything else from the scene as outliers, the registration may introduce large error and can affect the whole odometry. Those frames with no characteristic patterns can also cause substantial errors because of the lack of clusters or reasonable patterns on the unit sphere. This is reduced using the optimization techniques. Other challenges include relatively high car speed, where the sequences at recorded at 10 fps.
Method  Avg trans err  Avg rot err 

LOAM [8]  0.59%  0.0014[d/m] 
IMLS [10]  0.64%  0.0016[d/m] 
DEMO [7]  1.14%  0.0049[d/m] 
SuMA [28]  1.39%  0.0034[d/m] 
BCC [27]  4.59%  0.0175[d/m] 
DeLiO (our)  5.32%  0.0213[d/m] 
NCICP  7.17%  0.0050[d/m] 
BLO [27]  9.21%  0.0163[d/m] 
Vii Conclusion
This paper presents a new algorithm called DeLiO for odometry using LiDAR point clouds by decoupling rotation from translation. The rotation estimation approach based on the distribution pattern of surface normals on unit sphere is independent of translation and thus stands out from the stateoftheart approaches. To the best of our knowledge, we are the first presenting such an approach for LiDAR odometry. In the evaluation we showed the effectiveness of DeLiO on the wellknown KITTI benchmark. In order to achieve even better results on the benchmark further engineering is necessary.
Consequentially, as future work it will be interesting to remove the dynamic objects from the scenes as proposed in [10]. These dynamic objects are contributing a lot to rotational error. For the detection of dynamic objects, optical flow [29] or scene flow [30] algorithms could be utilized. Also objects that are observed in one frame and not in another may also be removed to improved the performance. Another potential improvement would be to consider a new place recognition algorithm for loop closure like SegMatch [31]. Detected loop closures could be easily integrated into the already used pose graph and considered for optimization.
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