Four-dimensional dynamic flow measurement by holographic particle image velocimetry

The ultimate goal of holographic particle image velocimetry (HPIV) is to provide space- and time-resolved measurement of complex flows. Recent new understanding of holographic imaging of small particles, pertaining to intrinsic aberration and noise in particular, has enabled us to elucidate fundamental issues in HPIV and implement a new HPIV system. This system is based on our previously reported off-axis HPIV setup, but the design is optimized by incorporating our new insights of holographic particle imaging characteristics. Furthermore, the new system benefits from advanced data processing algorithms and distributed parallel computing technology. Because of its robustness and efficiency, for the first time to our knowledge, the goal of both temporally and spatially resolved flow measurements becomes tangible. We demonstrate its temporal measurement capability by a series of phase-locked dynamic measurements of instantaneous three-dimensional, three-component velocity fields in a highly three-dimensional vortical flow--the flow past a tab

RGB-D Salient Object Detection: A Survey

Salient object detection (SOD), which simulates the human visual perception system to locate the most attractive object(s) in a scene, has been widely applied to various computer vision tasks. Now, with the advent of depth sensors, depth maps with affluent spatial information that can be beneficial in boosting the performance of SOD, can easily be captured. Although various RGB-D based SOD models with promising performance have been proposed over the past several years, an in-depth understanding of these models and challenges in this topic remains lacking. In this paper, we provide a comprehensive survey of RGB-D based SOD models from various perspectives, and review related benchmark datasets in detail. Further, considering that the light field can also provide depth maps, we review SOD models and popular benchmark datasets from this domain as well. Moreover, to investigate the SOD ability of existing models, we carry out a comprehensive evaluation, as well as attribute-based evaluation of several representative RGB-D based SOD models. Finally, we discuss several challenges and open directions of RGB-D based SOD for future research. All collected models, benchmark datasets, source code links, datasets constructed for attribute-based evaluation, and codes for evaluation will be made publicly available at https://github.com/taozh2017/RGBDSODsurveyComment: 24 pages, 12 figures. Has been accepted by Computational Visual Medi

Investigation of Flow Disturbances and Multi-Directional Wall Shear Stress in the Stenosed Carotid Artery Bifurcation Using Particle Image Velocimetry

Hemodynamics and shear forces are associated with pathological changes in the vascular wall and its function, resulting in the focal development of atherosclerosis. Flow complexities that develop in the presence of established plaques create environments favourable to thrombosis formation and potentially plaque rupture leading to stroke. The carotid artery bifurcation is a common site of atherosclerosis development. Recently, the multi-directional nature of shear stress acting on the endothelial layer has been highlighted as a risk factor for atherogenesis, emphasizing the need for accurate measurements of shear stress magnitude as well direction. In the absence of comprehensive patient specific datasets numerical simulations of hemodynamics are limited by modeling assumptions. The objective of this thesis was to investigate the relative contributions of various factors - including geometry, rheology, pulsatility, and compliance – towards the development of disturbed flow and multi-directional wall shear stress (WSS) parameters related to the development of atherosclerosis An experimental stereoscopic particle image velocimetry (PIV) system was used to measure instantaneous full-field velocity in idealized asymmetrically stenosed carotid artery bifurcation models, enabling the extraction of bulk flow features and turbulence intensity (TI). The velocity data was combined with wall location information segmented from micro computed tomography (CT) to obtain phase-averaged maps of WSS magnitude and direction. A comparison between Newtonian and non-Newtonian blood-analogue fluids demonstrated that the conventional Newtonian viscosity assumption underestimates WSS magnitude while overestimating TI. Studies incorporating varying waveform pulsatility demonstrated that the levels of TI and oscillatory shear index (OSI) depend on the waveform amplitude in addition to the degree of vessel constriction. Local compliance resulted in a dampening of disturbed flow due to volumetric capacity of the upstream vessel, however wall tracking had a negligible effect on WSS prediction. While the degree of stenosis severity was found to have a dominant effect on local hemodynamics, comparable relative differences in metrics of flow and WSS disturbances were found due to viscosity model, waveform pulsatility and local vessel compliance

Towards Robust and Physically Plausible Shaded Stereoscopic Segmentation

International audienceWe address the multi-view shape from shading problem, that is the recovery of 3-D shape, lighting configuration and surface albedo from multiple calibrated views. Previous approaches to this problem relied on physically impossible illumination models (negative light) and only work on constant albedo and resulted in biased estimates of shape and lighting positions. Furthermore, since the solution involves infinite-dimensional optimization, existing approaches were quite slow. We develop a new model that explicitly enforces positivity in the light sources with the assumption that the object is Lambertian and its albedo is piecewise constant and show that the new model significantly improves the accuracy and robustness relative to existing approaches. Furthermore, we show that the most computationally expensive step in the optimization can actually be solved in closed form. This significantly improves speed of convergence over existing schemes. We illustrate the behavior of our algorithm directly on the same data used by previous authors, so direct comparison is possible

Golgi clusters and vesicles mediate mitotic inheritance independently of the endoplasmic reticulum

We have examined the fate of Golgi membranes during mitotic inheritance in animal cells using four-dimensional fluorescence microscopy, serial section reconstruction of electron micrographs, and peroxidase cytochemistry to track the fate of a Golgi enzyme fused to horseradish peroxidase. All three approaches show that partitioning of Golgi membranes is mediated by Golgi clusters that persist throughout mitosis, together with shed vesicles that are often found associated with spindle microtubules. We have been unable to find evidence that Golgi membranes fuse during the later phases of mitosis with the endoplasmic reticulum (ER) as a strategy for Golgi partitioning (Zaal, K.J., C.L. Smith, R.S. Polishchuk, N. Altan, N.B. Cole, J. Ellenberg, K. Hirschberg, J.F. Presley, T.H. Roberts, E. Siggia, et al. 1999. Cell. 99:589–601) and suggest that these results, in part, are the consequence of slow or abortive folding of GFP–Golgi chimeras in the ER. Furthermore, we show that accurate partitioning is accomplished early in mitosis, by a process of cytoplasmic redistribution of Golgi fragments and vesicles yielding a balance of Golgi membranes on either side of the metaphase plate before cell division

Shape and Reflectance Recovery using Multiple Images with Known Illumination Conditions

We develop a variational method to recover both the shape and the reflectance of a scene surface(s) using multiple images, assuming that illumination conditions are fixed and known in advance. Scene and image formation are modeled with known information about cameras and illuminants, and scene recovery is achieved by minimizing a global cost functional with respect to both shape and reflectance. Unlike most previous methods recovering only the shape of Lambertian surfaces, the proposed method considers general dichromatic surfaces. We verify the method using synthetic data sets containing specular reflection

Structured-light-based surface measuring for application in fluid–structure interaction

International audienceThe study implements a method based on the analysis of a sinusoidal fringe pattern. This method allows the retrieval of a three dimensional surface shape from a two dimensional video recording of the deformation of fringes projected on the surface. A detailed description of the method and the implementation is provided which will facilitate its adaption to and utilization in custom FSI applications (the code is published under open-source license). An application example is presented in the form of an experiment. A hyperflexible hydrofoil is set under forced rotational oscillation in a closed water tunnel to mimic the characteristics of a vertical-axis hydrokinetic cross-flow turbine. The flexible structures encounter large deformations which are linked to corresponding hydrodynamic forces. To capture this interdependence, the deformations were recorded in sync with forces registered with a six-axis load cell. A validation of the method is presented and shows good accuracy in the deformation measurements of a hyperflexible hydrofoil. A benefit compared to other common techniques like Laser-interferometer measurements is the simplicity and modest hardware requirements of the method with the possibility to acquire height fields with good spatial and temporal resolution, which allows a spectral analysis of the surface deformation. The hydrodynamic forces are presented together with the corresponding deformation of the structure, and spectral analyses are performed which demonstrates the possibilities of the proposed method. In summary, the presented method allows the analysis of fluid-structure interactions using a simple assembly consisting of a projector and a high-speed camera. Graphic abstract frequency [Hz] oscillation base frequency flicker fluid-structure interaction imposed motion frequency [Hz] inlet outlet projection high-speed camer

Comprehensive review of vision-based fall detection systems

Vision-based fall detection systems have experienced fast development over the last years. To determine the course of its evolution and help new researchers, the main audience of this paper, a comprehensive revision of all published articles in the main scientific databases regarding this area during the last five years has been made. After a selection process, detailed in the Materials and Methods Section, eighty-one systems were thoroughly reviewed. Their characterization and classification techniques were analyzed and categorized. Their performance data were also studied, and comparisons were made to determine which classifying methods best work in this field. The evolution of artificial vision technology, very positively influenced by the incorporation of artificial neural networks, has allowed fall characterization to become more resistant to noise resultant from illumination phenomena or occlusion. The classification has also taken advantage of these networks, and the field starts using robots to make these systems mobile. However, datasets used to train them lack real-world data, raising doubts about their performances facing real elderly falls. In addition, there is no evidence of strong connections between the elderly and the communities of researchers