Analyse en simulation de projection immersive par l'avant

National audienceDepuis quelques années des systèmes caméra-projecteur sont utilisés, dans le cadre de la réalité mixte, pour éclairer des environnements non dédiés comme les murs d'une pièce d'habitation. L'objectif de ce travail est de caractériser le potentiel d'un tel système. Pour cela, nous proposons une étude en simulation des échanges énergétiques, depuis le projecteur jusqu'à la caméra. Nous considérons également l'efficacité de la compensation. Enfin, nous analysons, d'après les résultats obtenus, l'influence de différents facteurs

Compensation radiométrique d'un système de projection immersive grand-public

National audienceCatopsys est un système de projection immersive grand-public destiné à des applications de réalité mixte. Il est composé d'un vidéo-projecteur, d'un miroir convexe et d'une caméra, et peut être utilisé dans des salles qui n'ont pas été conçues spécifiquement pour la projection. Le système permet d'afficher un environnement immersif en projetant une image dans toute la salle. Cependant, l'environnement ainsi affiché est perturbé par la réponse du projecteur, les matériaux composant la salle et les réflexions multiples inhérentes à toute pièce partiellement close. La compensation radiométrique de la projection a pour but de réduire l'influence de ces perturbations. Dans ce papier, nous présentons notre système de projection et le processus radiométrique correspondant. Nous proposons ensuite un modèle radiométrique prenant en compte, en autres, la réponse du projecteur, les matériaux de la salle et les réflexions multiples particulièrement importantes avec notre système. Nous proposons également des méthodes de calibrage et de compensation basées sur ce modèle. Notre méthode de compensation peut être intégrée dans une application de réalité virtuelle ou mixte comme un post-traitement corrigeant les perturbations radiométriques de la projection. A notre connaissance, nos méthodes sont les premières à prendre en compte à la fois les réflexions multiples et la non-linéarité du projecteur

Immersive front-projection analysis using a radiosity-based simulation method

International audienceVideo projectors are designed to project onto flat white diffuse screens. Over the last few years, projector-based systems have been used, in virtual reality applications, to light non-specific environments such as the walls of a room. However, in these situations, the images seen by the user are affected by several radiometric disturbances, such as interreflection. Radiometric compensa tion methods have been proposed to reduce the disturbance caused by interreflection, but nothing has been proposed for evaluating the phenomenon itself and the effectiveness of compensation methods. In this paper, we propose a radiosity-based method to simulate light transfer in immersive environments, from a projector to a camera (the camera gives the image a user would see in a real room). This enables us to evaluate the disturbances resulting from interreflection. We also consider the effectiveness of interreflection compensation and study the influence of several parameters (projected image, projection onto a small or large part of the room, reflectivity of the walls). Our results show that radiometric compensation can reduce the influence of interreflection but is severely limited if we project onto a large part of the walls around the user, or if all the walls are bright

Practical photoquantity measurement using a camera

International audienceAn image output by a camera is generally not a faithful representation of the real scene, because it undergoes a series of radiometric disturbances during the imaging process. This paper proposes a method for obtaining a more accurate measure of the light seen by a camera. Our method requires no specific calibration apparatus and only minimal supervision. Nevertheless, it is quite comprehensive, since it accounts for response function, exposure, vignetting, spatial non-uniformity of the sensor and colour balancing. Our method works in two steps. First, the camera is calibrated off-line, in a photoquantity sense. Then, the photoquantity of any scene can be estimated in-line. Our method is therefore geared to a wide range of computer vision applications where a camera is expected to give a measurement of the visible light. The paper starts by presenting a photoquantity model of the camera imaging process. It then describes the key steps of calibration and correction method. Finally, results are given and analyzed to evaluate the relevance of our approach

Radiometric compensation for a low-cost immersive projection system

International audienceCatopsys is a low-cost projection system aiming at making mixed reality (virtual, augmented or diminished reality) affordable. It combines a videoprojector, a camera and a convex mirror and works in a non-specific room. This system displays an immersive environment by projecting an image onto the different parts of the room. However, the presence of an uncalibrated projector, heterogeneous materials and light inter-reflections influence the colors of the environment displayed in the room. Radiometric compensation of the projection process enables the system to reduce this problem. In this paper, we present our low-cost immersive projection system and propose a radiometric model and a compensation method which handle the projector response, surface materials and inter-reflections between surfaces. Our method works in two stages. First, the radiometric response of the projection process is evaluated. Then, this radiometric response is used to compensate the projection process in the desired environments

Comparison of different segmentation approaches without using gold standard. Application to the estimation of the left ventricle ejection fraction from cardiac cine MRI sequences.

International audienceA statistical method is proposed to compare several estimates of a relevant clinical parameter when no gold standard is available. The method is illustrated by considering the left ventricle ejection fraction derived from cardiac magnetic resonance images and computed using seven approaches with different degrees of automation. The proposed method did not use any a priori regarding with the reliability of each method and its degree of automation. The results showed that the most accurate estimates of the ejection fraction were obtained using manual segmentations, followed by the semiautomatic methods, while the methods with the least user input yielded the least accurate ejection fraction estimates. These results were consistent with the expected performance of the estimation methods, suggesting that the proposed statistical approach might be helpful to assess the performance of estimation methods on clinical data for which no gold standard is available

Improved estimation of the left ventricular ejection fraction using a combination of independent automated segmentation results in cardiovascular magnetic resonance imaging

—This work aimed at combining different segmenta-tion approaches to produce a robust and accurate segmentation result. Three to five segmentation results of the left ventricle were combined using the STAPLE algorithm and the reliability of the resulting segmentation was evaluated in comparison with the result of each individual segmentation method. This comparison was performed using a supervised approach based on a reference method. Then, we used an unsupervised statistical evaluation, the extended Regression Without Truth (eRWT) that ranks different methods according to their accuracy in estimating a specific biomarker in a population. The segmentation accuracy was evaluated by focusing on the left ventricular ejection fraction (LVEF) estimate resulting from the LV contour delineation using a public cardiac cine MRI database. Eight different segmentation methods, including three expert delineations, were studied, and sixteen combinations of the five automated methods were investigated. The supervised and unsupervised evaluations demonstrated that in most cases, STAPLE results provided better estimates of the LVEF than individual automated segmentation methods. In addition, LVEF obtained with STAPLE were within inter-expert variability. Overall, combining different automated segmentation methods improved the reliability of the segmenta-tion result compared to that obtained using an individual metho

A mutual reference shape based on information theory

International audienceIn this paper, we propose to consider the estimation of a refer-ence shape from a set of different segmentation results using both active contours and information theory. The reference shape is defined as the minimum of a criterion that benefits from both the mutual information and the joint entropy of the input segmentations and called a mutual shape. This energy criterion is here justified using similarities between informa-tion theory quantities and area measures, and presented in a continuous variational framework. This framework brings out some interesting evaluation measures such as the speci-ficity and sensitivity. In order to solve this shape optimization problem, shape derivatives are computed for each term of the criterion and interpreted as an evolution equation of an active contour. Some synthetical examples allow us to cast the light on the difference between our mutual shape and an average shape. Our framework has been considered for the estimation of a mutual shape for the evaluation of cardiac segmentation methods in MRI

Interventional planning and assistance for ascending aorta dissections

International audienceIn this paper, we present our global image processing framework of interventional planning and assistance for ascending aorta dissections. The preoperative stage of that framework performs the extraction of aortic dissection features in Computed Tomography Angiography (CTA) images. It mainly consists of a customized fast marching segmentation. The intraoperative stage of that framework realizes medical images registration and proposes data visualization enhancement; standard X-ray fluoroscopic images are used as the reference modality. We use our recently introduced registration method based on image transformation descriptors (ITDs) and usual 3D/2D techniques (based on digitally reconstructed radiographs). The first stage provides aortic dissection features and is to help clinicians for the planning. The second stage provides an augmented reality visualization and would be used for assistance during the intervention. As far as we know, this is the first complete image processing framework which focuses on the ascending aorta dissection (minimally invasive) endovascular treatment