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Large scenes such as building facades and other architectural constructions often contain repeating elements such as identical windows and brick patterns. In this paper, we present a novel approach that improves the resolution and geometry of 3D meshes of large scenes with such repeating elements. By leveraging structure from motion reconstruction and an off-the-shelf depth sensor, our approach captures a small sample of the scene in high resolution and automatically extends that information to similar regions of the scene. Using RGB and SfM depth information as a guide and simple geometric primitives as canvas, our approach extends the high-resolution mesh by exploiting powerful, image-based texture synthesis approaches. The final results improve on standard SfM reconstruction with higher detail. Our approach benefits from reduced manual labor as opposed to full RGBD reconstruction, and can be done much more cheaply than with LiDAR-based solutions. [ABSTRACT FROM AUTHOR]

The accuracy of 3D reconstruction for metrology applications using active stereo pairs depends on the quality of the calibration of the system. Active stereo pairs are generally composed of cameras mounted on tilt/pan mechanisms separated by a constant or variable baseline. This paper presents a calibration approach based on speckle metrology that allows the separation of translation and rotation in the estimation of extrinsic parameters. To achieve speckle-based calibration, a device called an Almost Punctual Speckle Source (APSS) is introduced. Using the APSS, a thorough method for the calibration of extrinsic parameters of stereo pairs is described. Experimental results obtained with a stereo system called the Agile Stereo Pair (ASP) demonstrate that speckle-based calibration achieves better reconstruction performance than methods using standard calibration procedures. Although the experiments were performed with a specific stereo pair, such as the ASP, which is described in the paper, the speckle-based calibration approach using the APSS can be transposed to other stereo setups. [ABSTRACT FROM AUTHOR]

This paper presents a new stereo sensor for active vision. Its cameras are mounted on two independent 2-DOF manipulators, which are themselves mounted on two translation stages. The system is designed for fast and accurate dynamical adjustments of gaze, vergence, and baseline. A complete description of its software and hardware components is given, including a detailed discussion of its calibration procedure. The performance of the sensor with respect to dynamical properties and measurement accuracy is also demonstrated through both simulations and experiments. [ABSTRACT FROM AUTHOR]

Most man-made objects are composed of a few basic geometric primitives (GPs) such as spheres, cylinders, planes, ellipsoids, or cones. Thus, the object recognition problem can be considered as one of geometric primitives extraction. Among the different geometric primitives, cylinders are the most frequently used GPs in real-world scenes. Therefore, cylinder detection and extraction are of great importance in 3D computer vision. Despite the rapid progress of cylinder detection algorithms, there are still two open problems in this area. First, a robust strategy is needed for the initial sample selection component of the cylinder extraction module. Second, detecting multiple cylinders simultaneously has not yet been investigated in depth. In this paper, a robust solution is provided to address these problems. The proposed solution is divided into three sub-modules. The first sub-module is a fast and accurate normal vector estimation algorithm from raw depth images. With the estimation method, a closed-form solution is provided for computing the normal vector at each point. The second sub-module benefits from the maximally stable extremal regions (MSER) feature detector to simultaneously detect cylinders present in the scene. Finally, the detected cylinders are extracted using the proposed cylinder extraction algorithm. Quantitative and qualitative results show that the proposed algorithm outperforms the baseline algorithms in each of the following areas: normal estimation, cylinder detection, and cylinder extraction. [ABSTRACT FROM AUTHOR]

Background: Electromagnetic tracking (EMT) has been researched for brachytherapy applications, showing a great potential for automating implant reconstruction, and overcoming image‐based limitations such as contrast and spatial resolution. One of the challenges of this technology is that it does not intrinsically share the same reference frame as the patient's medical imaging. Purpose: To present a novel phantom that can be used for a comprehensive quality assurance (QA) program of brachytherapy EMT systems and use this phantom to validate a novel applicator‐based registration method of EMT and image reference frames for gynecological (GYN) interstitial brachytherapy. Materials and Methods: Eleven 6F‐catheters (20 cm long), one 6F round tip catheter (29.4 cm long) and a tandem and ring gynecological applicator (Elekta, CT/MR 60°, 40 mm long tandem, 30 mm diameter ring) were placed in a rigid custom‐made phantom (Elekta Brachytherapy, Veenendaal, The Netherlands) to reconstruct their geometry using a five‐degree of freedom EMT sensor attached to an afterloader's check cable. All EMT reconstructions were done in three different environments: disturbance free (no metal nearby), computed tomography (CT)‐on‐rails brachytherapy suite and magnetic resonance imaging (MRI) brachytherapy suite. Implants were placed parallel to a magnetic field generatorand were reconstructed using two different acquisition methods: step‐and‐record and continuous motion. In all cases, the acquisition is performed at a rate of approximately 40 Hz. A CT scan of the phantom inside a water cube was obtained. In the treatment planning system (TPS), all catheters in the CT images were manually reconstructed and the applicator reconstruction was achieved by manually placing its solid 3D model, found in the applicator library of the TPS. The Iterative Closest Point and the Coherent Point Drift algorithms were used, initialized with four known points, to register both EMT and CT scan reference frames using corresponding points from the EMT and CT based reconstructions of the phantom, following three approaches: one gynecological applicator, four interstitial catheters inside four calibration plates having an S‐shaped path, and four 5 mm diameter ceramic marbles found in each of the four calibration plates. Once registered, the registration error (perpendicular distance) was computed. Results: The absolute median deviation from the expected value for EMT measurements in the disturbance free environment, CT‐on‐rails brachytherapy suite, and MRI‐brachytherapy suite are 0.41, 0.23, and 0.31 mm, respectively, while for the CT scan it is 0.18 mm. These values significantly lie below the sensor's expected accuracy of 0.70 mm (p < 0.001), suggesting that the environment did not have a significant impact on the measurements, given that care is taken in the immediate surroundings. In all three environments, the two acquisitions and three registration approaches have mean and median registration errors that lie at or below 1 mm, which is lower than the clinical acceptable threshold of 2 mm. Conclusions: The novel phantom allowed to successfully evaluate the accuracy of EMT‐based reconstructions of catheters and a GYN tandem and ring applicator in different clinical environments. A registration method based only on the applicator geometry, reconstructed withan EMT sensor and the TPS solid applicator library, was validated and shows clinically acceptable accuracy, comparable to CT‐based reconstruction but within a few minutes. Since the applicator is also visible in MRI, this method could potentially be used in clinics in an EMT‐MR interstitial GYN brachytherapy workflow. [ABSTRACT FROM AUTHOR]

Spheres are popular geometric primitives found in many manufactured objects. However, sphere fitting and extraction have not been investigated in depth. In this paper, a robust method is proposed to extract multiple spheres accurately and simultaneously from unorganized point clouds. Moreover, a novel validation step is presented to assess the quality of the detected spheres, which help remove the confusion between perfect spheres and sphere-like shapes such as ellipsoids and paraboloids. A novel sampling strategy is introduced to reduce computational burden for sphere extraction. Experiments on both synthetic and scanned point clouds with different levels of noise and outliers are conducted and the results compared to state-of-the-art methods. These experiments demonstrate the efficiency and robustness of the proposed sphere extraction method. [ABSTRACT FROM AUTHOR]

With their large field of view, anamorphosis, and areas of enhanced magnification, panomorph lenses are an interesting choice for navigation systems for mobile robotics in which knowledge of the surroundings is mandatory. However, panomorph lenses special characteristics can be challenging during the calibration process. This study focuses on the calibration of two panomorph stereoscopic systems with a model and technique developed for narrow-angle lenses, the “Camera Calibration Toolbox for MATLAB.” In order to assess the performance of the systems, the mean reprojection error (MRE) related to the calibration and the reconstruction error of control points of an object of interest at various locations in the field of view are used. The calibrations were successful and exhibit MREs of less than one pixel in all cases. However, some poorly reconstructed control points illustrate that an acceptable MRE guarantees neither the quality of 3-D reconstruction nor its uniformity in the field of view. In addition, the nonuniformity in the 3-D reconstruction quality indicates that panomorph lenses require a more accurate estimation of the principal point (center of distortion) coordinates to improve the calibration and therefore the 3-D reconstruction. [ABSTRACT FROM AUTHOR]

In this work we present a pairwise 3D vision-based inspection method for comparing locally deformable 3D models which refers to models whose parts have undergone some local deformation or change while the rest of the parts remained rigid and intact. In general, local deformations result from two types of transformations: non-volume conservative transformations caused by either the inflation or the deflation of models' parts and volume conservative transformations caused by bending, twisting, rotation or displacement of either internal or external parts of the model. The internal parts are the middle parts of an object, while the external parts are the parts on the extremity or the end of a part. We have experimented with different cases of deformations on man-made objects and mechanical parts for industrial inspection of possible defects, and for quality control purposes as well. We have also applied our pairwise comparison method on parts of the human body. Examples relevant to medical applications are the detection and inspection of a lesion which are also tested in this work. Our algorithm is not only successful for detecting local change but it also achieves high-accuracy results comparable to other well-known registration and industrial inspection techniques. [ABSTRACT FROM AUTHOR]

In this paper, a novel algorithm is proposed for extracting cylinders and estimating their parameters from 3D point cloud data. First, normal vectors and curvature information are computed for each data point as a preprocessing step. Then potential points that could belong to cylindrical surfaces are extracted by using curvature information. For each potential cylinder point, its neighborhood is considered as inlier points and a robust cylinder fitting algorithm is applied on these inlier points. Inlier points are updated and the fitting process is applied iteratively to propagate to all remaining points belonging to a cylinder. A validation method is proposed to assess whether the detected cylinder is reliable or not. Finally, by applying mean shift clustering, final descriptive parameters of cylinders are estimated accurately. To demonstrate its robustness, the method is tested on both synthetic and complex point clouds with different levels of noise and outliers. [ABSTRACT FROM AUTHOR]

• 3D shape descriptors are computed to estimate the morphology of gold grains from Quaternary sediments. • New 3D shape descriptors for ellipticity and sphericity of gold particles are introduced. • The morphology of gold grains collected along a transect of the Amaruq gold deposit shows changes according to glacial processes. Native gold is malleable and deforms during glacial, fluvial and eolian sedimentary processes. Quantification of its morphology is used in mineral exploration to estimate a distance of transport from its primary source. However, this quantification depends on textural aspects, or on partial 2D measurements of gold grain shapes. In the glacial domain, the classification of the shape remains qualitative and subjective. Microtomography and photogrammetry using scanning electron microscope images are two methods used to reconstruct the gold grain shape in 3D and to quantify accurately grain shape parameters. Reconstructions are applied on gold grains ranging in size from 35 μm to 415 μm for the long axis and 2716 μm3 to 760,713 μm3 for the volume. Shape parameters such as particle length, volume, and surface area are estimated in order to compute 3D shape descriptors of roundness, sphericity, and convexity. This study integrates two new shape descriptors (the ellipticity shape descriptor and inscribed sphericity) in order to estimate the ellipticity and sphericity of gold particles. These descriptors can be used to evaluate a morphological change with the distance of transport in glacial sediments such as in the Amaruq gold deposit area. [ABSTRACT FROM AUTHOR]

Purpose: In permanent seed implants, 60 to more than 100 small metal capsules are inserted in the prostate, creating artifacts in x-ray computed tomography (CT) imaging. The goal of this work is to develop an automatic method for metal artifact reduction (MAR) from small objects such as brachytherapy seeds for clinical applications. Methods: The approach for MAR is based on the interpolation of missing projections by directly using raw helical CT data (sinogram). First, an initial image is reconstructed from the raw CT data. Then, the metal objects segmented from the reconstructed image are reprojected back into the sinogram space to produce a metal-only sinogram. The Steger method is used to determine precisely the position and edges of the seed traces in the raw CT data. By combining the use of Steger detection and reprojections, the missing projections are detected and replaced by interpolation of non-missing neighboring projections. Results: In both phantom experiments and patient studies, the missing projections have been detected successfully and the artifacts caused by metallic objects have been substantially reduced. The performance of the algorithm has been quantified by comparing the uniformity between the uncorrected and the corrected phantom images. The results of the artifact reduction algorithm are indistinguishable from the true background value. Conclusions: An efficient algorithm for MAR in seed brachytherapy was developed. The test results obtained using raw helical CT data for both phantom and clinical cases have demonstrated that the proposed MAR method is capable of accurately detecting and correcting artifacts caused by a large number of very small metal objects (seeds) in sinogram space. This should enable a more accurate use of advanced brachytherapy dose calculations, such as Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

Traditional approaches for surface reconstruction from range data require that the input data be either range images or unorganized sets of points. With these methods, range data acquired along curvilinear patterns cannot be used for surface reconstruction unless constraints are imposed on the shape of the patterns or on sensor displacement. This paper presents a novel approach for reconstructing a surface from a set of arbitrary, unorganized and intersecting curves. A strategy for updating the reconstructed surface during data acquisition is described as well. Curves are accumulated in a volumetric structure in which a vector field is built and updated. The information that is needed for efficient curve registration is also directly available in the vector field. The proposed modeling approach combines surface reconstruction and curve registration into a unified procedure. The algorithm implementing the approach is of linear complexity with respect to the number of input curves and makes it suitable for interactive modeling. Simulated data based on a set of six curvilinear patterns as well as data acquired with a range sensor are used to illustrate the various steps of the algorithm. [Copyright &y& Elsevier]

This paper presents a newly developed method to estimate, in magnetic resonance (MR) images, the temperatures reached within the volume of an iceball produced by a cryogenic probe. Building on the direct measurements of the MR signal intensity and its correlation with independent temperature variations at the phase transition from liquid to solid, the thermal information embedded in the images was accessed. The volume and diameter of the growing iceball were estimated from a time series of MR images. Using regressions over the volume in the time and thermal domains, this method predicted the cryogenic temperatures beyond the range of sensitivity of the MR signal itself. We present a validation of this method in samples of gelatin and ex vivo pig liver. Temperature predictions are shown to agree with independent thermosensor readings over a range extending from 20 °C down to -- 65 °C, with an average error of less than 6 °C. [ABSTRACT FROM AUTHOR]

This work addresses the problem of automatic head pose estimation and its application in 3D gaze estimation using low quality RGB-D sensors without any subject cooperation or manual intervention. The previous works on 3D head pose estimation using RGB-D sensors require either an offline step for supervised learning or 3D head model construction, which may require manual intervention or subject cooperation for complete head model reconstruction. In this paper, we propose a 3D pose estimator based on low quality depth data, which is not limited by any of the aforementioned steps. Instead, the proposed technique relies on modeling the subject's face in 3D rather than the complete head, which, in turn, relaxes all of the constraints in the previous works. The proposed method is robust, highly accurate and fully automatic. Moreover, it does not need any offline step. Unlike some of the previous works, the method only uses depth data for pose estimation. The experimental results on the Biwi head pose database confirm the efficiency of our algorithm in handling large pose variations and partial occlusion. We also evaluated the performance of our algorithm on IDIAP database for 3D head pose and eye gaze estimation. [ABSTRACT FROM AUTHOR]

Electromagnetic tracking (EMT) can be used to reconstruct the 3D geometry of implants used in interstitial gynecological (GYN) brachytherapy, but it does not share the same reference frame (RF) as the patient's medical image. This work validates the use of a tandem and ring (T&R) GYN applicator and its 3D model counterpart from a treatment planning system (TPS), to register the RF of an EMT-enabled brachytherapy afterloader with the RF of a CT scan image-set of a rigid phantom. Eleven 6F, 20 cm long interstitial catheters and a T&R applicator (Elekta, CT/MR 60°, 30 mm diameter ring and 40 mm long tandem) were placed in a rigid custom-made phantom (allowing different parallel and curved catheter paths) to reconstruct their geometry using a 5 Degree of Freedom EMT sensor attached to the check-cable of an EMT-enabled afterloader prototype. A magnetic field generator (NDI Aurora V3) was placed parallel to the catheters and were reconstructed using two different acquisition step-and-record (SR) and continuous motion (CM). In SR, the sensor moved forward every 2 mm and stopped for 3 s inside all five straight catheters and the tandem channel, and every 1 mm inside the six bent catheters and ring channel. In CM, after the sensor reached the channel tip, the sensor was retracted at a constant speed of 1 cm/s and the resulting point cloud, representing the reconstruction of the channel, was smoothed using a Savitzky-Golay filter. In all cases, the acquisition was performed at a rate of 40 Hz. A CT scan of the phantom inside a water cube was captured (0.5 mm slice thickness, 120 kV & 420 mAs). A rigid registration of CT and EMT RFs was performed using coherent point drift (CPD) based on three different subgroups of corresponding point clouds: the T&R applicator, four s-shape calibration templates, and four 5 mm diameter ceramic marbles mounted on each of the four templates. The CT-based reconstruction of the applicator was obtained by manually aligning the TPS applicator 3D model to the CT scan. The centroid of the marbles in the EMT RF is estimated by aligning the CAD model of the four calibration templates to their EMT counterpart using CPD. The absolute mean deviation from the expected value for EMT measurements is 0.33 ± 0.27 mm, while for the CT scan is 0.71 ± 0.30 mm, when compared to the 3D model of the phantom. The total mean registration errors between EMT and CT scan RF using the applicator-based registration are 0.85 ± 0.54 mm and 1.35 ± 0.88 mm, for SR and CM EMT acquisition methods respectively. For the S-shape template-based registration, the registration errors are 0.63 ± 0.32 mm (SR) and 0.62 ± 0.36 mm (CM), while for the marble & CAD-based registration the errors are 0.64 ± 0.30 mm (SR) and 0.65 ± 0.36 mm (CM). The calibration template of the proposed rigid phantom constitutes an efficient approach for accurate, submillimeter registration between EMT and imaging data, allowing for the design of efficient quality control tests in the future. While further evaluation is still needed, this work shows that the use of the TPS applicator library model placement within the image-set might be accurate enough (recommended accuracy ≤2 mm) for registration of implants obtained from EMT data to the CT image RF. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented which addresses the authors' claim that automobile drivers with amnestic mild cognitive impairment (aMCI) can benefit from participating in a five-week driving simulator automated training program, and it mentions procedural learning and at-risk drivers.

The intrinsic interaction of a robotic system that includes two 6-degree-of-freedom cable-driven platforms sharing a common workspace might result in cable interferences for random trajectories. This paper presents and analyzes computational methods for geometrically determining and managing these interferences for any trajectory constrained with variable loads. The algorithms considered determine which cable can be released from an active actuation state while allowing control in a minimal tension state, thereby ensuring that both platforms stay in a controllable workspace. The process of managing cable interferences constitutes a challenge as one must take into account the inherent limitations of the workspace, which not only include the possibility of interference itself, but also the geometry of the cable-driven locomotion interface (CDLI), its dynamics, the nonideal behavior of real cables, and the requirement that both platforms must be completely constrained at any time. As releasing a cable from an active actuation state might generate tension discontinuities in the other cables, this paper also proposes collision prediction schemes that are only applied to redundant actuators in order to reduce or completely eliminate such discontinuities. Finally, a simulation of a CDLI embedded as a peripheral in a virtual environment, in which the load applied on each platform comes from the wrench measured under the foot for a natural gait walking, is thoroughly analyzed. [ABSTRACT FROM AUTHOR]

Abstract: In this work, we introduce an extension of the linear elastic tensor–mass method allowing fast computation of non-linear and visco-elastic mechanical forces and deformations for the simulation of biological soft tissue. We aim at developing a simulation tool for the planning of cryogenic surgical treatment of liver cancer. Percutaneous surgery simulation requires accurate modelling of the mechanical behaviour of soft tissue, and previous experimental characterizations have shown that linear elasticity is only a coarse approximation of the real properties of biological tissues. We first show that our model can simulate different types of non-linear and visco-elastic mechanical behaviours at speeds which are compatible with real-time applications. Then an experimental setup is presented which was used to characterize the mechanical properties of deer liver tissue under perforation by a biopsy needle. Experimental results demonstrate that a linear model is not suitable for simulating this application, while the proposed model succeeds in accurately modelling the axial load measured on the needle. [Copyright &y& Elsevier]