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3D object data acquisition and processing for virtual reality ...

3D object data acquisition and processing for virtual reality ...

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3D object data acquisition and processing for virtual reality applications teleimmersion [7], to insert humans or other 3D objects with real appearance into the virtual environment. This concept of true 3D video reported above is based on capturing 2D images from multiple directions which in consequence creates a limited 3D effect. Much better solution is to capture 3D data representing the shape and texture of real 3D object and transferring them into virtual reality environment. The general concept of methodology and system realizing this task is presented in Section 2 of this paper. It describes also the roles of the main modules of the system including measurement, processing, and virtual environment modules. The method of gathering data about static and variable in time 3D object are presented in Sections 3 and 4. The initial experimental-numerical results of 3D data capture and transfer realized in the proposed set-up are presented in Section 5. Finally, the remarks (Section 6) on the further works required for creating fully interactive true 3D video combined with virtual reality environment are given. 2. General concept The basic novelty of the proposed true 3D video relies on the usage of an optical 3D object measurement system based on active fringe projection method [5–7] supported by photogrametry [8]. Measurement system provides the sets of data including: • cloud of co-ordinate points representing 3D static object seen from multiple directions (x,y,z), • texture of 3D object (R,G,B), • trajectory(ies) of the rigid body motion of the object or its parts (also trajectory of an arbitrary object point), • 3D shape modifications (morphing) of an object or its actual shape in the form of cloud of co-ordinate points (x,y,z). The general scheme of the true 3D video system is shown in Fig. 1. It has modular structure which includes: • measurement module which consists of N sets with digital light projector (DLP) and two cameras. Each of sets captures original stereo-pair images of an object and a set of object images with fringes and Gray code projected by DLP as seen from single direction. N measurement sets cover full hemisphere field of view and enable us to capture the data from multiple directions, • processing module which: – for static objects, converts the intensity images (original and with projected fringes) into the cloud of points with texture (x,y,z,R,G,B). The clouds of points gathered by sets from N directions are merged in order to provide true 3D data. Then, they are converted into triangle mesh with or without texture [9,10]. This true 3D object representation is loaded into virtual environment in which it can be visualized. – for objects, variable in time, the module calculates actual position of markers within the measured volume (from the stereo-pair images). These values form a movement trajectory of any marker located at the object. If the marker is situated at nonmorphing part of an object, the rigid body motion is determined. Combining these data with spatio- -temporal phase analysis concept enables us to monitor and calculate the actual shape and its variation of any morphing object [8,11]. – virtual environment module which provides the VR scene and includes the virtual camera due to which the proper interactive visualization of 3D data is realized [12]. In order to provide interactions between the user and 3D objects placed in VR environment, the system includes the feedback loop between measurement and VR modules. It returns information about the position and offers parameters of virtual camera which are chosen by the interactive user of the system. Modular structure of the system enables us the division of the data gathering, processing and visualization process into three main parts and provides an effective use of three computers stations connected by fast links. The variable streaming between processing module and VR module enables us visualization at different levels of quality (amount of processing data) depending on the computational resources of the outside (remote) user’s computer and the type of object and its movement/morphing (rigid body motion only, morphing of whole or parts of an object). Fig. 1. General scheme of 3D data acquisition and processing system for virtual reality applications. True 3D object measurement system provides enormous amount of data about object shape and texture. However, the information required for visualization in virtual environment at any moment t i is limited to these data points which are seen from the chosen viewpoint. A virtual camera (VC), placed at this point is the essential element of the system presented. Data taken for real-time processing are limited to the points which are included in the solid angle determined by the virtual camera. Due to the key-role of the measure- 182 Opto-Electron. Rev., 11, no. 3, 2003 © 2003 COSiW SEP, Warsaw

Contributed paper Fig. 2. Scheme of 3D object shape, morphing, and localization measurement process. ment module, the special attention is paid to the methodology of 3D object analysis, its changes both object localization and its orientation. The general scheme of a measurement process, which includes the most possible measurement paths, is presented in Fig. 2. The below descriptions of the main modules, which rely on variety of measurements paths and the rules of their operation are presented together with the experimental-numerical results of 3D data capture, data processing and transfer realized in the proposed system. 3. True 3D shape measurement of static object The true 3D measurement system is built with commercially available instrumentation. To realize full hemisphere measurement, N directional modules are situated around the measurement volume [9]. Each of them consists of digital light projector (DLP), two CCD cameras, and a fixing frame (see Fig. 3). Additionally, they are equipped with calibration and processing modules. The measurements are performed according to the main assumptions: • phase measurement is based on fringe/Gray code projection method, • (x,y,z,R,G,B) co-ordinates calculation is based on the pre-calibrated measurement volume, • common co-ordinate system for all directional modules is determined experimentally, • clouds of (x,y,z,R,G,B) points are converted into triangle mesh, • the final triangle mesh of an 3D object (with or without texture) is fully transferred into VR environment and serves as the model for further modifications and visualization. 3.1. Directional 3D-measurement module The DLP provides active projection of 2D patterns on the measured object. CCD camera is a common RGB camera with digital or analogue interface. The fixing ramp is constructed on the base of aluminium profiles. The actual geometrical configuration makes possible to work with measurement volumes up to 2´2´2 m 3 (here for experiment the 1´1´1 m 3 volume was calibrated). The knowledge about the parameters of the objectives in the DLP and CCD is not important and does not influence on the measurement accuracy. Calibration module consists of a calibration model and its manipulator. Processing unit is an ordinary PC based computer with a set of several special cards, frame-grabber card (acquisition of CCD camera image), DLP pattern generator, and digital IO card (managing the manipulator of calibration model). The system works alternatively in the calibration and measurement modes. After each change in the geometrical camera-projector set-up, the system should be recalibrated. The system calculates the values of Cartesian co-ordinates (x,y,z) on the basis of calibration matrix which is generated in the calibration process. Opto-Electron. Rev., 11, no. 3, 2003 M. Kujawiñska 183

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