1 Introduction
An accurate, fast and robust estimation of 6D object pose is of great importance to many application fields such as robotic manipulation, augmented reality, scene interpretation, and autonomous driving. In robotics, for example, the 6D object pose facilitates spatial reasoning and allows an endeffector to act upon the object. In an augmented reality scenario, object pose can be used to enhance one’s perception of reality by augmenting objects with extra information such as hints for assembly.
The introduction of consumer and industrial grade RGBD sensors have allowed for substantial improvement in 6D object pose estimation as these sensors concurrently capture both the appearance and geometry of the scene. However, there still remain challenges to be addressed, including robustness against occlusion and clutter, scalability to multiple objects, and fast and reliable object modelling, including capturing of reflectance properties. Extending contemporary methods to work reliably and with sufficient execution speed in an industrial setting is still an open problem. Many recent methods focus on specific rigid objects, but pose estimation of deformable or articulated objects and of object categories is also an important research direction.
The field of 6D object pose estimation has gained more attention last years. A big achievement for the field is the best paper award of ECCV 2018 given to Martin Sundermeyer, Zoltan Marton, Maximilian Durner, Manuel Brucker, and Rudolph Triebel for their work titled Implicit 3D Orientation Learning for 6D Object Detection from RGB Images.
The 4th edition of the International Workshop on Recovering 6D Object Pose [4]^{1}^{1}1The previous editions were held at ICCV 2015 [1], ECCV 2016 [2], and ICCV 2017 [3]. was organized in conjunction with ECCV 2018 and was attended by 100+ people from both academia and industry who shared uptodate advances and discussed open problems. Four invited speakers talked about their current work (Section 2), the BOP benchmark for 6D object pose estimation was introduced (Section 3), and the accepted workshop papers were presented.
The workshop covered the following topics: (a) 6D object pose estimation (a.k.a. 3D object detection) and tracking, (b) 3D object modeling and reconstruction, (c) surface representation and registration, (d) robustness to occlusion and background clutter, (e) multiple object instance detection, (f) pose estimation of nonrigid objects and object categories, (g) robotic grasping and grasp affordances, and (h) object manipulation and interaction.
Many methods for 6D pose estimation of specific rigid objects (Fig. 1) have been published recently, but were usually compared with only a few competitors on a small subset of datasets. It had been therefore unclear which methods perform well and in which scenarios. To capture the status quo of the field, we organized the SIXD Challenge [5] at the 3rd workshop edition held at ICCV 2017 [3]. The results submitted to the challenge were published in the BOP benchmark paper [25] and presented at the 4th workshop edition.
Papers submitted to the workshop were peerreviewed. Out of 13 submissions, the program committee accepted 10 papers which were introduced at the workshop through oral and poster presentations. The best paper award was given to
ImagetoVoxel Model Translation with Conditional Adversarial Networks
by Vladimir Knyaz, Vladimir V. Kniaz, and Fabio Remondino.2 Invited talks
The invited talks were given by Federico Tombari from Technical University of Munich, Kostas Bekris from Rutgers University, Bertram Drost from MVTec, and Thibault Groueix from Ecole Nationale des Ponts et Chaussées. The talks are summarized below and the slides are available on the workshop website [4].
2.1 From 3D Descriptors to Monocular 6D Pose: What Have We Learned? – Federico Tombari
While 6D rigid pose estimation has been an important research task for more than twenty years, recently the design of new algorithms is more and more focused on overcoming the limitations provided by real world applications, so to bridge the gap between lab research and products. This translates to the necessity to move on from the simplified scenario of estimating the pose of a single object on a clutterless planar surface, towards scenarios with high clutter and occlusion [42]
. At the same time, algorithms need to process input data at a very high frame rate, so to reduce the latency of the output and avoid lagging, provide accurate pose estimation (e.g. to reduce jitter during tracking) and cope with resourcelimited hardware architectures such as mobile phones or embedded computers. This is particularly motivated by applications that already have a strong market interest such as augmented reality, personal/industrial robotics and autonomous driving. A disruptive technology that strongly influenced the field of 6D rigid pose estimation in the past 5 years is deep learning. Hence, the talk presented an overview of the current trends and results regarding the use of deep learning for this task, in view of overcoming the aforementioned limitations. In particular, it highlighted two main research directions where deep learning has been leveraged to improve the state of the art: i) the definition of 3D descriptors for 3D data; ii) 6D object pose estimation from RGB (or monocular) data.
As for the first aspect, we briefly went over the development of 3D descriptors for unorganized 3D representations, starting from the handcrafted ones [46, 41] until the more recent ”learned” ones. An important aspect regarding the influence of deep learning on this field is that certain 3D representations such as point clouds and 3D meshes, frequently utilized for 6D rigid pose estimation tasks, are not well suited to convolutions due to their intrinsic unorganized nature. Hence, an important step in the direction of learning 3D features was the introduction of methods such as PointNet [35] and [50], that, conversely to approaches such as 3D Match [52] and [29]
, can directly operate on point clouds or meshes without the need to either voxelize or histogram the data. The recent global and fullyconvolutional architecture for point cloud processing and scene understanding proposed in
[40] was also introduced.As for the second research direction, the intuition of estimating the 6D pose of an object based on monocular information relies on the fact that humans can often have a rough idea of the pose of the objects in the surrounding 3D space simply from monocular cues, provided that they are familiar with the shape of the objects. Several works have recently explored this direction, which were briefly introduced and compared in terms of characteristics. One distinctive trait that differentiates such approaches is the type of output that the network is trained to infer: from the 8 corners of the projected bounding box [36, 45] to the regression of the 6D pose [49, 15] or the classification of the viewpoint and inplane rotations [27]. The method in [30] was also introduced, that proposes to learn the 6D pose refinement from pairs of RGB patches using a CNN. Finally, the extension of monocular 6D pose estimation to the autonomous driving domain was also discussed, by referencing recent directions such as [12, 13].
In conclusion, deep learning appears as a powerful tool for 6D rigid pose estimation, although seems still strongly limited by open issues such as generalizability, learning of geometric invariance and computational efficiency (especially in a field where most applications need to deal with resourcelimited hardware). Monocular pose estimation can be promisingly carried out via deep learning, although not yet as accurately as with a depth sensor, as also showcased by a qualitative comparison on a real sequence between the two tracking approaches in [30] (monocular) and [42] (RGBD). Finally, deploying monocular 6D pose estimation jointly with monocular semantic SLAM such as CNNSLAM [43], which also leverages deep learning to obtain dense semantic reconstruction, appears as an interesting direction to explore towards full (i.e. semantic+geometry) scene understanding from monocular data.
2.2 Towards Robust 6D Pose Estimation: Physicsbased Reasoning and Data Efficiency – Kostas Bekris
Towards the objective of robust 6 DoF pose estimation at an accuracy and speed level that allows robots to manipulate objects in their surroundings, this talk focused on warehouses tasks, such as picking from bins, packing and sorting. Warehouses can be seen as a stepping stone between the success story of robotics in manufacturing and the vision of deploying robots in everyday human environments. Warehouses involve a large variety of objects, which can appear in general, unpredictable configurations, as well as cluttered scenes and tight spaces, such as shelving units, which limit the observability of onboard sensors and introduce occlusions. They allow, however, access to known object models, which frequently correspond to standard geometric shapes, due to packaging.
In these setups, it is critical for a robot to both utilize physicsbased reasoning to achieve 3D scenelevel understanding as well as minimize the dependence of solutions to excessive human labeling, which negatively impacts scalability. With these priorities in mind, the talk highlighted a pipeline for robust 6D pose estimation, which includes the following four steps: 1) semantic object segmentation given physically realistic synthetic data, 2) pose hypothesis generation through robust point registration, 3) pose improvement via consideration of physical constraints at the scene level, and 4) lifelong selflearning through active physical interaction with objects.
Recently, deep learning methods, such as those employing Convolutional Neural Networks (CNN’s), have become popular for object detection
[38, 37] and pose estimation [27, 48], outperforming alternatives in most benchmarks. These results are typically obtained by training CNN’s using datasets with a large number of labeled images. Such datasets, however, need to be collected in a way that captures the intricacies of the environment the robot is deployed in, such as lighting conditions, occlusion and clutter. This motivates the development of synthetic dataset that can capture the known parameters of the environment and generate data accordingly, while avoid overfitting to the unknown parameters.The first component of the proposed pipeline is to use a physics engine in the synthetic dataset generation pipeline [33]. The physics engine defines environmental constraints on object placement, which naturally capture in the training set, the distribution of object poses that can realistically appear during testing. Furthermore, a physics engine is a convenient tool to parameterize the unknown scene features, such as illumination. A randomization over such parameters is very effective in avoiding overfitting to synthetic textures of objects.
Given semantic object segmentation, the problem of estimating the 6D object poses involves geometric reasoning regarding the position and orientation of the detected objects. Solutions that became popular in the context of the Amazon Picking Challenge (APC) [14], use a Convolutional Neural Network (CNN) for object segmentation [51, 21] followed by a 3D model alignment step using point cloud registration techniques [31, 6]. The quality of the pose estimate, however, can still suffer due to overreliance on the learned models.
The second insight of the talk
is that CNN output can be seen as a probability for an object to be visible at each pixel. These segmentation probabilities can then be used during the registration process to achieve robust and fast pose estimation. This requires sampling a base of points on a point cloud segment, such that all points on the base belong to the target object with high probability. The resulting approach, denoted as “Stochastic Congruent Sets” (StoCS)
[32], builds a probabilistic graphical model given the obtained soft segmentation and information from the preprocessed geometric object models. The preprocessing corresponds to building a global model descriptor that expresses oriented point pair features [18]. This geometric modeling, not only biases the base samples to lie within the object bound, but is also used to constrain the search for finding the congruent sets, which provides a substantial computational benefit.The third key observation of the talk is to treat individualobject predictions with some level of uncertainty and perform a global, scenelevel optimization process that takes object interactions into account [34]. This information arises from physical properties, such as respecting gravity and friction as well as the requirement that objects do not penetrate one another. In particular, a Monte Carlo Tree Search (MCTS) process utilizes local detections to achieve scenelevel optimization. It generates multiple candidate poses per object and then searches over the cartesian product of these individual object pose candidates to find the optimal scene hypothesis. The scene is evaluated according to a score defined in terms of similarity of the rendered hypothesized scenes against input data. The search performs constrained local optimization via physics correction and ICP [6]. Through this physical reasoning, the resulting pose estimates for the objects are of improved accuracy and by default consistent.
Once the system has access to an object detector and a pose estimation process, it can already be deployed for the desired task. Nevertheless, as the system performs its task, it also gets access to data in the operation domain, which it did not have access to during training. This data could be very useful in further improving the performance of the system but they are not labelled.
The fourth aspect of the talk is a solution for automatically labeling real images acquired from multiple viewpoints using a robotic manipulator [33]. A robotic manipulator autonomously collects multiview images of real scenes and labels them automatically using the object detector trained with the above physicsbased simulation tool. The key insight is the fact that the robot can often find a good viewing angle that allows the detector to accurately label the object and estimate its pose. The object’s predicted pose is then used to label images of the same scene taken from more difficult and occluded views. The transformations between different views are known because they are obtained by moving the robotic manipulator. Overall, the data can be added to the existing synthetic dataset to retrain the model for better performance as part of a lifelong, selflearning procedure.
2.3 Detecting Geometric Primitives in 3D Data – Bertram Drost
Even though methods based on (deep) learning lead to major advances in many areas of computer vision over the last years, the topperforming methods for 6D object detection are still handcrafted, classic methods. This is evident from the recent results on the BOP benchmark (Section
3).Methods that currently perform best on BOP are based on a voting scheme that can be interpreted as a meetinthemiddle between RANSAC and a generalized Hough transform. The base method [18]
uses a local parametrization for the object pose, where an oriented scene point (reference point) is fixed and assumed to be on the target object. The remaining local parameters are then the point on the model surface that corresponds to the reference point and the rotation around the normal vector, which combined represent three degrees of freedom. The optimal local parameters are recovered using a voting scheme. For this, the reference point is paired with neighboring scene points, and similar point pairs on the model are searched using an efficient hashing scheme. Each such match then casts a vote. While the base method shows good results for 3D shapes with a distintive geometry, it has weaknesses for shapes that are symmetric of strongly selfsimilar. This is mostly because point pairs on such shapes are no longer very discriminative. The voting thus finds all symmetric poses simultaneously, which is both slower and less robust.
The talk presented a way of adapting the base method for geometric primitive shapes  spheres, planes, cylinders, and to some extend cones. First, the local parameter space was reduced by removing duplicate entries due to symmetries. For example, since a sphere is identical no matter the corresponding point on its surface or the rotation around the normal, the local parameter space becomes zerodimensional, i.e. a single counter. Additionally, the local parameters can be extended by shape parameters such as the radius of a sphere or cylinder, or the opening angle of a cone. Second, instead of using a hashing scheme for matching point pairs between scene and model, an explicit point pair matching can be used thanks to the explicit nature of the geometric primitives. Those changes make the method both faster and more robust for such geometric primitives, while also allowing the recovery of shape parameters.
2.4 Parameteric Estimation of 3D Surfaces and Correspondences – Thibault Groueix
Pose of a rigid object has 6 degrees of freedom and is well defined. A broader class of objects are nonrigid shapes such as articulated robots or humans. Articulated robots have an additional degree of freedom for each joint. Those additional intrinsic parameters also have to be estimated. In this case, it is easy to manually design a parametrization with N degrees of freedom, where each parameter naturally encodes the angle of a joint. Humans are much harder to parameterize and doing it manually is hard. We propose to learn the parameterization [19].
To that end, we want to map each shape of category
, represented as a point cloud, to a parameter space. A relevant prior on deformable shapes is the ability to find a neutral template, already encoding lots of general information on humans. To learn a parameterization, we use autoencoder neural networks. The encoder is a PointNet
[35] and the decoder is a Shape Deformation Network [19] that deforms the template by iteratively transforming each point on the template to a point in 3D. Given correspondence annotation between a training example and the template, the L2 distance between the generated and the input point clouds is penalized. Annotation of correspondences is expensive and can be avoided through an unsupervised reconstruction loss, the chamfer distance, and a proper regularization. The parameterization is a mapping from the parameter space to the point cloud space, and is given by the learned decoder.At test time, the parameters associated with this parametrization can be estimated using the encoder. We propose to use this estimate as an initialization for a local exploration of the parameter space through gradient descent. The learned parametrization is evaluated on the task of 3D correspondences on the FAUST dataset [7] and achieves stateoftheartresults. It also displays strong robustness to holes and noise in the data.
The method is not limited to humans, but applies to any category of shape for which a template can be found. For broader category without natural templates such as chairs, or general furniture, the template can be replaced by a set a square patches [20]. The assumption is that any object can be reconstruct through the deformation of a set of patches. The proposed parametrization, Atlasnet, is learned on Shapenet and applied for the task of Single View Reconstruction, leading to stateoftheart results. This confirms that learning a parametrization can be the key to extending 6D pose estimation beyond rigid objects.
3 BOP: Benchmark for 6D Object Pose Estimation
The BOP benchmark considers the task of 6D pose estimation of a rigid object from a single RGBD input image, when the training data consists of a texturemapped 3D object model or images of the object in known 6D poses. The benchmark comprises of: (i) eight datasets [22, 8, 44, 16, 23, 39] in a unified format that cover different practical scenarios, including two new datasets focusing on varying lighting conditions, (ii) an evaluation methodology with a poseerror function that deals with pose ambiguities, (iii) an evaluation of 15 diverse recent methods that captures the state of the art, and (iv) an online evaluation system at bop.felk.cvut.cz that is open for continuous submission of new results.
The evaluation shows that methods based on pointpair features [47, 18], introduced in 2010, currently perform best. They outperform template matching methods [26], learningbased methods [8, 9, 28, 44] and methods based on 3D local features [10, 11]. Occlusion is a big challenge for current methods, as shown by scores dropping swiftly already at low levels of occlusion. As another important future research directions, our analysis identified robustness against object symmetries and similarities, varying lighting conditions, and noisy depth images.
3.1 Future Plans
Various possible extensions of the BOP benchmark regarding datasets, problem statement and evaluation metrics were discussed at the workshop and are summarized in the following paragraphs.
The benchmark was started with the simple task of 6D localization of a single instance of a single object. This task allowed to evaluate most of the recent methods out of the box and is a common denominator of the other 6D localization variants – a single instance of multiple objects, multiple instances of a single object, and multiple instances of multiple objects. The plan is to move step by step and add the more complicated variants to the online evaluation system in the near future, as well as the 6D detection task, where no prior information about the object presence is provided [24]. Note that while it is easy to extend the evaluation to other tasks, it is nontrivial to extend the methods.
The plan is also to keep adding new datasets, e.g. [17], and for every new dataset to reserve a set of test images for which the groundtruth annotation will be private. The datasets that are currently included in BOP are fully public.
Another topic of discussion were the poseerror functions. In the BOP benchmark, 6D pose estimates are evaluated using the Visible Surface Discrepancy (VSD) which measures misalignment of the visible part of the object surface. One limitation of the current version of VSD is that it does not consider color. The same holds for the other poseerror functions commonly used in the literature [24]. While the alignment of color texture is less relevant for a number of robotic applications, it may be important for augmentedreality applications. The plan is to fix this limitation in a new version of VSD with an extended pixelwise test checking not only the distance of the object surface but also its color. We are open to other feedback and discussion about the evaluation methodology.
4 Conclusions
This document summarized the 4th International Workshop on Recovering 6D Object Pose which was organized at ECCV 2018 and featured four invited talks, oral and poster presentations of accepted workshop papers, and an introduction of the BOP benchmark for 6D object pose estimation.
An accurate, fast and robust method for 6D object pose estimation is still in need, as is evident from the current scores on the BOP benchmark [25]. We would like to encourage authors of relevant methods to keep submitting results to the online evaluation system at bop.felk.cvut.cz. We will present snapshots of the leaderboards at the next workshop editions, which are planned for the upcoming major conferences.
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