Section: New Results
Participants : Le Cui, Eric Marchand.
Le Cui's Ph.D.  ended with a contribution related to visual tracking and estimation of the 3D pose of a micro/nano-object. It is indeed a key issue in the development of automated manipulation tasks using visual feedback. The 3D pose of the micro object can be estimated based on a template matching algorithm. Nevertheless, a key challenge for visual tracking in a scanning electron microscope (SEM) was the difficulty to observe the motion along the depth direction. We then proposed a template-based hybrid visual tracking scheme that uses luminance information to estimate the object displacement on - plane and uses defocus information to estimate object depth .
3D Localization for Space Debris Removal
Participants : Aurélien Yol, Eric Marchand, François Chaumette.
This study is realized in the scope of the FP7 Removedebris project (see Section 18.104.22.168) . We compared two vision-based navigation methods for tracking space debris in a low Earth orbit environment. The proposed approaches rely on a frame to frame model-based tracking in order to obtain the complete 3D pose of the camera with respect to the target . The proposed algorithms robustly combine points of interest and edge features, as well as color-based features if needed. Experimental results have been obtained demonstrating the robustness of the approaches on synthetic image sequences simulating a CubeSat satellite orbiting the Earth .
3D Localization for Airplane Landing
Participants : Noël Mériaux, François Chaumette, Patrick Rives, Eric Marchand.
This study is realized in the scope of the ANR VisioLand project (see Section 9.2.2). In a first step, we have considered and adapted our model-based tracker  to localize the aircraft with respect to the airport surroundings. Satisfactory results have been obtained from real image sequences provided by Airbus. In a second step, we are now considering to perform this localization from a set of keyframe images corresponding to the landing trajectory.
Scene Registration based on Planar Patches
Participants : Renato José Martins, Eduardo Fernandez Moral, Patrick Rives.
Image registration has been a major problem in computer vision over the past decades. It implies searching an image in a database of previously acquired images to find one (or several) that fulfill some degree of similarity, e.g. an image of the same scene from a similar viewpoint. This problem is interesting in mobile robotics for topological mapping, re-localization, loop closure and object identification. Scene registration can be seen as a generalization of the above problem where the representation to match is not necessarily defined by a single image (i.e. the information may come from different images and/or sensors), attempting to exploit all information available to pursue higher performance and flexibility. This problem is ubiquitous in robot localization and navigation. We propose a probabilistic framework to improve the accuracy and efficiency of a previous solution for structure registration based on planar representation. Our solution consists of matching graphs where the nodes represent planar patches and the edges describe geometric relationships. The maximum likelihood estimation of the registration is estimated by computing the graph similarity from a series of geometric properties (areas, angles, proximity, etc.) to maximize the global consistency of the graph. Our technique has been validated on different RGB-D sequences, both perspective and spherical .
Direct RGB-D Registration
Participants : Renato José Martins, Eduardo Fernandez Moral, Patrick Rives.
Dense direct RGB-D registration methods are widely used in tasks ranging from localisation and tracking to 3D scene reconstruction . This work addresses a peculiar aspect which drastically limits the applicability of direct registration, namely the weakness of the convergence domain. In general, registration is performed only between close frames (small displacements), since dense registration tasks are particularly sensible to the local convexity of the cost error function. The main contribution of this work is an adaptive RGB-D error cost function that has a larger convergence domain and a faster convergence in both simulated and real data , . This formulation employs the relative condition number metric to update the weighting of the RGB and depth costs. This approach is performed within a multi-resolution framework, where an efficient pixel selection for both RGB and ICP costs reduces the computational cost whilst preserving the precision. The formulation results in a larger region of attraction and faster convergence than classical RGB, ICP and RGB-D costs. Experiments was conducted using real sequences of indoor and outdoor images using perspective and spherical RGB-D sensors. Significant improvements was denoted in terms of the convergence stability and the speed of convergence in comparison with state-of-the-art methods.
Online localization and mapping for UAVs
Participants : Muhammad Usman, Paolo Robuffo Giordano, Eric Marchand.
Localization and mapping in unknown environments is still an open problem, in particular for what concerns UAVs because of the typical limited memory and processing power available onboard. In order to provide our quadrotor UAVs with high autonomy, we started studying how to exploit onboard cameras for an accurate (but fast) localization and mapping in unknown indoor environments. We chose to base both processes on the newly available Semi-Direct Visual Odometry (SVO) library (http://rpg.ifi.uzh.ch/software) which has gained considerable attention over the last years in the robotics community. The idea is to exploit dense images (i.e., with little image pre-processing) for obtaining an incremental update of the camera pose which, when integrated over time, can provide the camera localization (pose) w.r.t. the initial frame. In order to reduce drifts during motion, a concurrent mapping thread is also used for comparing the current view with a set of keyframes (taken at regular steps during motion) which constitute a “map” of the environment. We have started porting the SVO library to our UAVs and the preliminary results showed good performance of the localization accuracy against the Vicon ground truth. We are now planning to close the loop and base the UAV flight on the reconstructed pose from the SVO algorithm.
Reflectance and Illumination Estimation for Realistic Augmented Reality
Participants : Salma Jiddi, Eric Marchand.
The acquisition of surface material properties and lighting conditions is a fundamental step for photo-realistic Augmented Reality. Human visual cues remain sensitive to the global coherence within a computer-generated image. Absence or bad rendered virtual shadows, unconsidered specular reflections and/or occlusions, confused color perception such as an exuberantly bright virtual object are all elements which may not help an AR user interact and commit to a target application. In this work, we studied a new method for the estimation of the diffuse and specular reflectance properties of an indoor real static scene. Using an RGB-D sensor, we further estimate the 3D position of light sources responsible for specular phenomena and propose a novel photometry-based classification for all the 3D points. The resulting algorithm allows convincing AR results such as realistic virtual shadows as well as proper illumination and specularity occlusions .
Optimal Active Sensing Control
Participants : Paolo Salaris, Riccardo Spica, Paolo Robuffo Giordano.
This study concerns the problem of active sensing control. The objective is to improve the estimation accuracy of an observer by determining the inputs of the system that maximize the amount of information gathered by the outputs. In  this problem has been solved within the Structure from Motion (SfM) framework for 3D structure estimation problems, i.e. a point, a sphere and a cylinder, in the particular case where the observability property is instantaneously guaranteed. The optimal estimation strategy is hence given in terms of the instantaneous velocity direction of the camera velocities.
Recently, we have extended the optimal active sensing control to the case where the observability property is not instantaneously guaranteed. To simplify the analysis, we considered nonlinear differentially flat systems. Moreover, to quantify the richness of the acquired information the Observability Gramian (OG) has been used. We have hence defined a trajectory for the flat outputs of the system by using B-Spline curves and then, we have exploited an online gradient descent strategy to move the control points of such B-Spline in order to actively maximise the smallest eigenvalue of the OG over the whole fixed planning time horizon. While the system travels along its planned (optimized) trajectory, an Extended Kalman Filter (EKF) is used to estimate the system state. In order to keep memory of the past acquired sensory data for online re-planning, the OG is also computed on the past estimated state trajectories. This is then used for an online replanning of the optimal trajectory during the robot motion which is continuously refined by exploiting the estimated system state by the EKF. In order to show the effectiveness of our method we have considered a simple but significant case of a planar robot with a single range measurement. The simulation results show that, along the optimal path, the EKF converges faster and provides a more accurate estimate than along any other possible (non-optimal) paths. These results have been submitted to ICRA'2017.