Visualizing dynamic geosciences phenomena using an octree-based view-dependent LOD strategy within virtual globes

Geoscientists build dynamic models to simulate various natural phenomena for a better understanding of our planet. Interactive visualizations of these geoscience models and their outputs through virtual globes on the Internet can help the public understand the dynamic phenomena related to the Earth more intuitively. However, challenges arise when the volume of four-dimensional data (4D), 3D in space plus time, is huge for rendering. Datasets loaded from geographically distributed data servers require synchronization between ingesting and rendering data. Also the visualization capability of display clients varies significantly in such an online visualization environment; some may not have high-end graphic cards. To enhance the efficiency of visualizing dynamic volumetric data in virtual globes, this paper proposes a systematic framework, in which an octree-based multiresolution data structure is implemented to organize time series 3D geospatial data to be used in virtual globe environments. This framework includes a view-dependent continuous level of detail (LOD) strategy formulated as a synchronized part of the virtual globe rendering process. Through the octree-based data retrieval process, the LOD strategy enables the rendering of the 4D simulation at a consistent and acceptable frame rate. To demonstrate the capabilities of this framework, data of a simulated dust storm event are rendered in World Wind, an open source virtual globe. The rendering performances with and without the octree-based LOD strategy are compared. The experimental results show that using the proposed data structure and processing strategy significantly enhances the visualization performance when rendering dynamic geospatial phenomena in virtual globes.     

一种场景内容分布的交互式渲染系统

近年来,三维虚拟场景的规模和复杂程度不断提高,受到硬件的限制,一些应用 中的超大规模场景(如建筑群,城市等)很难在单机上进行渲染或满足可交互的需求。针对该问 题,提出了一种分布式渲染框架,将大规模场景在内容上进行划分,得到单一节点可渲染的子 场景。这些子场景被分布到集群中不同的渲染节点进行处理,其渲染结果根据深度信息进行合 成得到整个场景的最终渲染结果。为了降低交互响应时间,需对子场景的渲染结果进行压缩传 输。实验充分验证了提出的分布式渲染系统能够高效处理超大规模场景的渲染和交互,并且具 有良好的可扩展性,能够满足很多领域中对大规模场景交互式渲染的需求。     

Collaborative Virtual Simulation Environment for Radiotherapy Treatment Planning

The simulation of Radiotherapy Treatment Planning (RTP) is a normal procedure in oncology clinics carried out on a Simulator machine. The Virtual Simulation of RTP replaces the real Simulator machine with a virtual one by using the CT data sets of a patient instead of the real patient. In this paper, we present a collaborative virtual simulation environment of RTP, named EU-VIRTUOSO, which is based on volume rendering and telecommunication techniques. The RTP procedure is visualised on a virtual patient, which is created by using the CT data of the patient. Different volume rendering and volume interaction techniques, such as DRR, MIP, gradient surface, and iso-surface, supply physicians with high quality rendering images to simulate the real working environment of the Simulator machine. In the collaborative environment, physicians distributed at different locations can work together via network to plan the treatment or to validate the treatment plan on-line by a collaborative application sharing approach. Both concepts virtualised planning and collaborative planning improve the efficiency and accuracy of a radiotherapy treatment while reducing the effort for an individual patient.     

A Fast Ambient Occlusion Method for Real-Time Plant Rendering

Global illumination effects are crucial for virtual plant rendering. Whereas real-time global illumination rendering of plants is impractical, ambient occlusion is an efficient alternative approximation. A tree model with millions of triangles is common, and the triangles can be considered as randomly distributed. The existing ambient occlusion methods fail to apply on such a type of object. In this paper, we present a new ambient occlusion method dedicated to real time plant rendering with limited user interaction. This method is a three-step ambient occlusion calculation framework which is suitable for a huge number of geometry objects distributed randomly in space. The complexity of the proposed algorithm is O（n）, compared to the conventional methods with complexities of O（n^2）. Furthermore, parameters in this method can be easily adjusted to achieve flexible ambient occlusion effects. With this ambient occlusion calculation method, we can manipulate plant models with millions of organs, as well as geometry objects with large number of randomly distributed components with affordable time, and with perceptual quality comparable to the previous ambient occlusion methods.     

CyberWalk: a web-based distributed virtual walkthrough environment

A distributed virtual walkthrough environment allows users connected to the geometry server to walk through a specific place of interest, without having to travel physically. This place of interest may be a virtual museum, virtual library or virtual university. There are two basic approaches to distribute the virtual environment from the geometry server to the clients, complete replication and on-demand transmission. Although the on-demand transmission approach saves waiting time and optimizes network usage, many technical issues need to be addressed in order for the system to be interactive. CyberWalk is a web-based distributed virtual walkthrough system developed based on the on-demand transmission approach. It achieves the necessary performance with a multiresolution caching mechanism. First, it reduces the model transmission and rendering times by employing a progressive multiresolution modeling technique. Second, it reduces the Internet response time by providing a caching and prefetching mechanism. Third, it allows a client to continue to operate, at least partially, when the Internet is disconnected. The caching mechanism of CyberWalk tries to maintain at least a minimum resolution of the object models in order to provide at least a coarse view of the objects to the viewer. All these features allow CyberWalk to provide sufficient interactivity to the user for virtual walkthrough over the Internet environment. In this paper, we demonstrate the design and implementation of CyberWalk. We investigate the effectiveness of the multiresolution caching mechanism of CyberWalk in supporting virtual walkthrough applications in the Internet environment through numerous experiments, both on the simulation system and on the prototype system.     

A QoS model for collaboration through Distributed Virtual Environments

Today’s virtual environments are expected to be distributed to allow collaboration for common purposes. However, they must ensure a high level of Quality of Service (QoS) to the user, especially in an open context with unknown, a priori, man-in-the-loop event occurrence. This paper presents our Distributed Virtual Environment Collaboration Model (DVECOM) and its implementation which aims to provide an end-user QoS support for distributed virtual reality applications. This model aims to reconcile openness and real-time requirements for collaborative relationships using virtual environments. Real-time requirements ensure the logical synchronization between the displayed scenes of the same virtual world on a set of distributed machines. The DVECOM model is based partially upon COREMO concepts and further work done in the context of the Amusement European Esprit project.DVECOM integrates QoS provision and management. The major idea is to guarantee the consistency of scene rendering and the synchronization of the display from the user point of view. The other idea is to auto-adapt rendering in accordance with the retained strategy, ensuring best effort and least suffering virtual world rendering. Representation degradation is driven by the users’ choices and is assisted by the system. The receiver-side protocol is based upon end-user preferences, physical level capability information, as well as pertinence of notification to each client (contextual end-user information). The model should be able to guarantee logical consistency and synchronization of virtual world distributed displays, with the least rendering degradation possible. When available, such guarantees would make it possible to use DVE out of a closed, oversized, very restricted context for industrial collaborative applications with an expected QoS.     

分布式并行绘制系统中带宽降低方法的研究

把静态与视点无关网格简化技术和动态与视点相关的网格简化技术结合起来,根据应用领域的要求,采用考虑了离散曲率边折叠算法构造的多分辨率模型,把得到的简化模型提交给与视点相关的网格简化技术处理,在基于联网PC的分布式并行绘制系统中实现模型的渐进传输并进行实时绘制;同时采用几何索引等方式来降低立即模式分布式系统的网络传输数据量,进一步缓解网络带宽压力,增强数据的实时动态交互性。     

分布式虚拟集群渲染系统研究

针对三维动画领域的渲染瓶颈和分布式渲染的资源搜索问题,提出了一种基于双层体系结构的分布式虚拟集群渲染系统,并详细介绍了系统的各个功能模块.通过对渲染资源的统一描述,定义了虚拟集群渲染能力的计算方法,给出了分布式渲染资源的搜索算法,并验证了算法的有效性.     

非规则数据场并行体绘制算法

并行算法是实现体绘制加速的重要途径，然而现有的并行体制绘制算法大部分是针对规则数据场的。     

Hybrid client-server architecture and control techniques for collaborative product development using haptic interfaces

In this paper, a collaborative product development and prototyping framework is proposed by using distributed haptic interfaces along with deformable objects modeling. Collaborative Virtual Environment (CVE) is a promising technique for industrial product development and virtual prototyping. Network control problems such as network traffic and network delay in communication have greatly limited collaborative virtual environment applications. The problems become more difficult when high-update-rate haptic interfaces and computation intensive deformable objects modeling are integrated into CVEs for intuitive manipulation and enhanced realism. A hybrid network architecture is proposed to balance the computational burden of haptic rendering and deformable object simulation. Adaptive artificial time compensation is used to reduce the time discrepancy between the server and the client. Interpolation and extrapolation approaches are used to synchronize graphic and haptic data transmitted over the network. The proposed techniques can be used for collaborative product development, virtual assembly, remote product simulation and other collaborative virtual environments where both haptic interfaces and deformable object models are involved.     

可微绘制技术研究进展

可微绘制技术是当前虚拟现实、计算机图形学与计算机视觉领域研究的热点,其目标是改造计算机图形学中以光栅化或光线跟踪算法为主的真实感绘制流程,支持梯度信息回传以计算由输出图像的变化导致的输入几何、材质属性变化,通过与优化及深度学习技术等相结合支持从数据中学习绘制模型和逆向推理,是可微学习技术在计算机图形学绘制技术中的应用的具体体现,在增强/虚拟现实内容生成、三维重建、表观采集建模和逆向光学设计等领域中有广泛的应用前景。本文对可微绘制当前的发展状况进行调研,重点对该技术在真实感绘制、3维重建和表观采集建模中的研究和应用情况进行综述,并对可微绘制技术发展趋势进行展望,以期推动可微技术在学术界和产业界的进一步发展。     

Evaluation of per-pixel linked lists for distributed rendering and comparative analysis

Rendering scenes with overlapping transparent faces is challenging for several reasons, including sorting geometry, managing multiple per-pixel fragments, and compositing. These questions have been solved for local rendering previously, for example by order-independent transparency approaches like Depth Peeling or A-buffer implementations. We discuss order-independent transparency algorithms in context of distributed rendering for large data sets or simulations which by design split the work load among a number of computation nodes. Furthermore, we evaluate a per-pixel linked list approach for distributed mesh rendering and multi-variant analysis of molecule data sets.     

基于图象的绘制技术

基于图象绘制技术是实现对虚拟场景建模和实时绘制的新的有效方法,可以克服传统的基于几何绘制方式中建立起初模型的困难和实时绘制计算在的缺陷。本文介绍和分析目前图象绘制技术的一些主要方法,并提出改进途径。     

IBRAC技术在分布式虚拟仿真系统中的应用研究

基于图像的方法和基于模型的方法是生成虚拟场景的两种方法,但这两种方法在分布式虚拟场景的生成过程中各有优缺点。将两种方法的优点相结合,在分布式虚拟仿真系统中引进IBRAC技术,有三个优点：绘制速度快、网络带宽限制低、能够进行交互控制。该文结合分布式门式起重机虚拟仿真培训系统对IBRAC技术进行了研究。首先给出了该系统的总体结构,然后分析系统中用到的IBRAC技术重要算法,最后对IBRAC技术在系统中的应用前景进行了展望。     

Forward Light Cuts: A Scalable Approach to Real‐Time Global Illumination

We present Forward Light Cuts, a novel approach to real‐time global illumination using forward rendering techniques. We focus on unshadowed diffuse interactions for the first indirect light bounce in the context of large models such as the complex scenes usually encountered in CAD application scenarios. Our approach efficiently generates and uses a multiscale radiance cache by exploiting the geometry‐specific stages of the graphics pipeline, namely the tessellator unit and the geometry shader To do so, we assimilate virtual point lights to the scene's triangles and design a stochastic decimation process chained with a partitioning strategy that accounts for both close‐by strong light reflections, and distant regions from which numerous virtual point lights collectively contribute strongly to the end pixel. Our probabilistic solution is supported by a mathematical analysis and a number of experiments covering a wide range of application scenarios. As a result, our algorithm requires no precomputation of any kind, is compatible with dynamic view points, lighting condition, geometry and materials, and scales to tens of millions of polygons on current graphics hardware.     

一种基于OGRE图形引擎的实时分布式渲染系统

虚拟现实的实时交互得到了越来越广泛地应用,实时分布式渲染有效地解决了普通PC机渲染的时间瓶颈问题,提高了系统实时渲染性能和输出分辨率。在对分布式渲染原理研究分析的基础上,依据设计模式的思想,设计了一种基于Sort-First结构的实时分布式渲染框架和同步机制,并在实践中应用OGRE图形引擎实现了该系统。     

三维虚拟场景绘制加速技术综述

加速三维图形的绘制对三维虚拟场景的交互式生成有重要作用。该文结合三维图形绘制流水线，概括了提高绘制性能的途径，综述了可见性处理、细节层次方法、基于图象的绘制以及大型几何数据库的操纵等主要的绘制加速技术。     

Parallel rendering of volumetric data set on distributed-memory architectures

A solution is proposed to the problem of interactive visualization and rendering of volume data. Designed for parallel distributed memory MIMD architectures, the volume rendering system is based on the ray tracing (RT) visualization technique, the Sticks representation scheme (a data structure exploiting data coherence for the compression of classified data sets), the use of a slice-partitioning technique for the distribution of the data between the processing nodes and the consequent ray-data-flow parallelizing strategy. The system has been implemented on two different architectures: an inmos Transputer network and a hypercube nCUBE 6400 architecture. The high number of processors of this latter machine has allowed us to exploit a second level of parallelism (parallelism on image space, or parallelism on pixels) in order to arrive at a higher degree of scalability. In both proposals, the similarities between the chosen data-partitioning strategy, the communications pattern of the visualization processes and the topology of the physical system architecture represent the key points and provide improved software design and efficiency. Moreover, the partitioning strategy used and the network interconnection topology reduce the communications overhead and allow for an efficient implementation of a static load-balancing technique based on the prerendering of a low resolution image. Details of the practical issues involved in the parallelization process of volumetric RT, commonly encountered problems (i.e. termination and deadlock prevention) and the sw migration process between different architectures are discussed.     

基于时空连贯性和几何简化技术的复杂场景快速消隐绘制算法

提出了一个结合层次遮挡图像缓存的快速消隐绘制算法,本算法首先利用空间连贯性对场景实行快速保守的消隐,对可能可见的近景、中景使用几何绘制,对可能可见的远景实现了基于图像和几何混合的加速绘制,实验表明,由于充分利用了空间连贯性和图像简化技术,本算法效果良好,可适合各种复杂度场景的快速绘制。     

Interactive fragment tracing

One of the main challenges in real-time rendering is to enable more and more effects that were previously available in offline rendering only. An important effect among these is physically correct reflections of arbitrary objects in curved reflectors like windshields. In this paper we propose fragment tracing on the GPU as a solution to interactively realizing this effect for large scenes as employed in industrial applications. For each rasterized fragment, a ray is traced through an octree representing the original geometry and surface material. By introducing a GPU implementation of an octree traversal, for the first time hierarchical data structures can efficiently be used on the GPU. As a result, the approach allows both handling of large geometries such as those employed in virtual prototyping and accurate rendering. Several examples show the generality and achievable rendering quality of our method.