Section: New Results

Visual tracking and state estimation

3D model-based tracking

Participant : Eric Marchand.

This study focused on the issue of estimating the complete 3D pose of the camera with respect to a potentially textureless object, through model-based tracking. We proposed to robustly combine complementary geometrical and color edge-based features in the minimization process, and to integrate a multiple-hypotheses framework in the geometrical edge-based registration phase [45] . This method will be tested in the scope of the FP7 RemoveDebris project [36] .

Pose estimation through plane tracking

Participants : Aurélien Yol, Eric Marchand.

We proposed a method for localizing an Unmanned Aerial Vehicle (UAV) using georeferenced aerial images. Here we provide a multiple usage localization algorithm based on vision only. To ensure robustness, we choose to use the Mutual Information (MI) within a dense tracking process. MI proved to be very robust toward local and global scene variations. However, dense approaches are often related to drift disadvantages. We solve this problem by using georeferenced images. The localization algorithm has been demonstrated through the localization of a hexarotor UAV fitted with a downward looking camera during real flight tests [53] .

3D tracking of deformable objects

Participants : Bertrand Delabarre, Eric Marchand.

We consider the problem of dense non-rigid visual tracking robust towards global illumination perturbations of the observed scene. The similarity function is based on the sum of conditional variance (SCV). With respect to most approaches that minimize the sum of squared differences, which is poorly robust towards illumination variations in the scene, the choice of SCV as our registration function allows the approach to be naturally robust towards global perturbations. Moreover, a thin-plate spline warping function is considered in order to take into account deformations of the observed template [28] .

Structure from motion

Participants : Riccardo Spica, Paolo Robuffo Giordano, François Chaumette.

Structure from motion (SfM) is a classical and well-studied problem in computer and robot vision, and many solutions have been proposed to treat it as a recursive filtering/estimation task. However, the issue of actively optimizing the transient response of the SfM estimation error has not received a comparable attention. In the work [18] , we showed how to design an online active SfM scheme characterized by an error transient response equivalent to that of a reference linear second-order system with desired poles. Indeed, in a nonlinear context, the observability properties of the states under consideration are not (in general) time-invariant but may depend on the current state and on the current inputs applied to the system. It is then possible to simultaneously act on the estimation gains and system inputs (i.e., the camera velocity for SfM) in order to optimize the observation process and impose a desired transient response to the estimation error. The theory has a general validity and can be applied to many different contexts such as when dealing with point features [18] , solid objects like spheres or cylinders [51] , or planar regions [47] . Furthermore, the active SfM scheme can also be embedded within a classical visual servoing law exploiting the redundancy of the camera motion w.r.t. the considered visual task [52] .

Robust visual odometry

Participants : Tawsif Gokhool, Patrick Rives, Renato José Martins.

Our aim is concentrated around building ego-centric topometric maps represented as a graph of salient keyframe nodes [14] . Additionally, visual odometry from frame to keyframe alignment helps significantly in drift reduction. On the other hand, the sparsity in this kind of graphical representation leads to reduced overlapping between keyframes which can degrade localisation robustness. Our chosen spherical 3600 field of view (FOV) configuration alleviates the overlapping issue by providing an enriched model of the environment with photometric and geometric information content. Following a multitude of advantages with information fusion, merging of frames in a single representation deals with the problem of data redundancy and sensor noise suppression.

Therefore, the second fold of this work consisted in improving the identified conceptual loopholes above by first proposing a generic uncertainty propagation model as applied to our spherical RGB-D database. Secondly, a probabilistic framework was derived which led to a Mahalanobis inconsistency test incorporating both geometric and photometric uncertainty models [32] . Our framework was further improved by adding up a probabilistic model to filter out dynamic points temporally. Finally, the entire probabilistic framework was applied in order to track the most stable points over time.