信息熵在体绘制视图选取中的研究与应用

在三维体数据可视化处理过程中,选取一个好的视图可以提高对体数据理解的速度和效率.光投射算法是体绘制中一种非常重要的算法.由于该算法成熟,成像质量高,它已在许多医学可视化处理系统中得到了广泛的应用.为该算法提供了一种视图选择方法,它将信息熵公式与光投射算法结合在一起对所选视图质量的优劣做出客观的评价.该方法充分考虑了传递函数,数据分布和各个体素的可见度.方法既可以用于非交互可视化处理时对给定场景找到一幅最具可视化信息的视图;也可用于交互的可视化处理中引导用户确定好的视点,以便对所获取的可视化信息进一步地深入研究和探索.     

State‐of‐the‐Art in Compressed GPU‐Based Direct Volume Rendering

Great advancements in commodity graphics hardware have favoured graphics processing unit (GPU)‐based volume rendering as the main adopted solution for interactive exploration of rectilinear scalar volumes on commodity platforms. Nevertheless, long data transfer times and GPU memory size limitations are often the main limiting factors, especially for massive, time‐varying or multi‐volume visualization, as well as for networked visualization on the emerging mobile devices. To address this issue, a variety of level‐of‐detail (LOD) data representations and compression techniques have been introduced. In order to improve capabilities and performance over the entire storage, distribution and rendering pipeline, the encoding/decoding process is typically highly asymmetric, and systems should ideally compress at data production time and decompress on demand at rendering time. Compression and LOD pre‐computation does not have to adhere to real‐time constraints and can be performed off‐line for high‐quality results. In contrast, adaptive real‐time rendering from compressed representations requires fast, transient and spatially independent decompression. In this report, we review the existing compressed GPU volume rendering approaches, covering sampling grid layouts, compact representation models, compression techniques, GPU rendering architectures and fast decoding techniques.     

Three Architectures for Volume Rendering

Volume rendering is a key technique in scientific visualization that lends itself to significant exploitable parallelism. The high computational demands of real-time volume rendering and continued technological advances in the area of VLSl give impetus to the development of special-purpose volume rendering architectures. This paper presents and characterizes three recently developed volume rendering engines which are based on the ray-casting method. A taxonomy of the algorithmic variants of ray-casting and details of each ray-casting architecture are discussed. The paper then compares the machinefeatures and provides an outlook onfuture developments in the area of volume rendering hardware.     

Mobile collaborative medical display system

Because of recent advances in wireless communication technologies, the world of mobile computing is flourishing with a variety of applications. In this study, we present an integrated architecture for a personal digital assistant (PDA)-based mobile medical display system that supports collaborative work between remote users. We aim to develop a system that enables users in different regions to share a working environment for collaborative visualization with the potential for exploring huge medical datasets. Our system consists of three major components: mobile client, gateway, and parallel rendering server. The mobile client serves as a front end and enables users to choose the visualization and control parameters interactively and cooperatively. The gateway handles requests and responses between mobile clients and the rendering server for efficient communication. Through the gateway, it is possible to share working environments between users, allowing them to work together in computer supported cooperative work (CSCW) mode. Finally, the parallel rendering server is responsible for performing heavy visualization tasks. Our experience indicates that some features currently available to our mobile clients for collaborative scientific visualization are limited due to the poor performance of mobile devices and the low bandwidth of wireless connections. However, as mobile devices and wireless network systems are experiencing considerable elevation in their capabilities, we believe that our methodology will be utilized effectively in building quite responsive, useful mobile collaborative medical systems in the very near future.     

Direct volume rendering of unstructured tetrahedral meshes using CUDA and OpenMP

Direct volume visualization is an important method in many areas, including computational fluid dynamics and medicine. Achieving interactive rates for direct volume rendering of large unstructured volumetric grids is a challenging problem, but parallelizing direct volume rendering algorithms can help achieve this goal. Using Compute Unified Device Architecture (CUDA), we propose a GPU-based volume rendering algorithm that itself is based on a cell projection-based ray-casting algorithm designed for CPU implementations. We also propose a multicore parallelized version of the cell-projection algorithm using OpenMP. In both algorithms, we favor image quality over rendering speed. Our algorithm has a low memory footprint, allowing us to render large datasets. Our algorithm supports progressive rendering. We compared the GPU implementation with the serial and multicore implementations. We observed significant speed-ups that, together with progressive rendering, enables reaching interactive rates for large datasets.     

针对全空子数据体的GPU体绘制

目的 体绘制是3维数据可视化的主要方法之一。用于体绘制的数据体中包含有大量的空体素,导致光线投射算法进行没有意义的重采样计算,必然降低绘制算法效率。针对全空子数据体体绘制低效问题,提出基于GPU体高效绘制方法。方法 利用八叉树数据结构组织数据,有效管理包含许多空体素的子数据体。通过绘制八叉树非全空叶子节点子数据体表面,使光线投射算法中起始和终止重采样位置更接近数据体中的可视部分,同时根据八叉树全空节点子数据体判定纹理查询结果,计算合适的跳跃步长,快速跳过八叉树中全空节点子数据体。结果 当数据体中空体素较多时,确定合适的八叉树深度,有效地跳过数据体中的空体素,减少体绘制运算量,实现对原基于体包围盒表面绘制的GPU光线投射算法的加速。结论 设计不透明度函数,凸显数据体中层位面,并将算法成功应用于地震数据可视化,取得很好应用效果。     

渲染器与Web服务器耦合实现远程体 渲染的交互优化

目的 为使用户随时随地获得需要较大计算和存储资源的交互真实感体渲染服务,设计并实现了一种面向Web的远程真实感体渲染方法。方法 计算量较大的实时体渲染任务由远端渲染服务器中的GPU加速完成并通过WebSocket将渲染图像发送至客户端;客户端浏览器只需负责接收显示渲染图像并监听发送用户交互事件。提出了一种输出系统耦合算法用以连接输出图像速率较大的渲染服务器和发送图像速率较慢的Web服务器。算法能根据Web服务器发送图像的情况动态调整每次图像输出的迭代计算次数,改变渲染服务器输出图像的时间间隔以达到与Web服务器发送速度相平衡,同时保持渲染服务器持续工作。结果 实验比较了在不同网络传输条件下,采用输出系统耦合算法与直接连接渲染器和Web服务器,渲染4个不同数据集所需的完成时间及帧率等性能评价指标。在局域网和广域网环境下,本文方法分别最多只需17 s和14 s即可完成整个渲染过程,而采用直接连接渲染器和Web服务器的方法则分别至少需要31 s和60 s才能完成整个渲染过程。实验结果表明采用输出系统耦合算法在不同网络条件下均可较大地缩短整个渲染过程所需时间,使用户在较短时间内获得高质量渲染图像。结论 本文渲染器与Web服务器耦合实现远程体渲染交互优化的方法可让用户使用与计算能力无关的桌面或移动设备通过网络透明使用高性能渲染系统;系统采用的输出系统耦合算法能够根据网络承载能力自适应调整渲染器输出速度,使用户在不同的网络环境中均可以较快的速度获得高质量渲染图像。     

基于Web的时变体数据的体绘制方法

传统Web体绘制方法主要集中在利用服务器端进行预处理和绘制任务,浏览器端仅用于呈现绘制结果,这样会造成服务器负载过高,同时,当绘制参数发生更改时,必须向服务器请求新的绘制结果,这样也易受网络延迟的影响。为了解决以上问题,实现在浏览器本地进行体绘制和交互,本文提出一种基于WebGL的体绘制方法,以时变体数据为例,在浏览器端实现光线投射体绘制算法。同时,为了提升绘制效率和减少内存占用,本文基于维度压缩方法,优化时变体数据的预处理过程。最后,本文设计了Web体绘制系统,引入暴风时变数据集以验证方法的有效性,结果表明,本文方法能够在浏览器本地对时变体数据进行体绘制,绘制时间在50ms以下,帧速率可达到50 FPS以上,同时支持实时交互,并且当绘制参数发生更改时,系统能够直接在浏览器端进行重新绘制。     

Extinction-based shading and illumination in GPU volume ray-casting

Direct volume rendering has become a popular method for visualizing volumetric datasets. Even though computers are continually getting faster, it remains a challenge to incorporate sophisticated illumination models into direct volume rendering while maintaining interactive frame rates. In this paper, we present a novel approach for advanced illumination in direct volume rendering based on GPU ray-casting. Our approach features directional soft shadows taking scattering into account, ambient occlusion and color bleeding effects while achieving very competitive frame rates. In particular, multiple dynamic lights and interactive transfer function changes are fully supported. Commonly, direct volume rendering is based on a very simplified discrete version of the original volume rendering integral, including the development of the original exponential extinction into a-blending. In contrast to a-blending forming a product when sampling along a ray, the original exponential extinction coefficient is an integral and its discretization a Riemann sum. The fact that it is a sum can cleverly be exploited to implement volume lighting effects, i.e. soft directional shadows, ambient occlusion and color bleeding. We will show how this can be achieved and how it can be implemented on the GPU.     

Scalable Remote Rendering with Depth and Motion‐flow Augmented Streaming

In this paper, we focus on efficient compression and streaming of frames rendered from a dynamic 3D model. Remote rendering and on‐the‐fly streaming become increasingly attractive for interactive applications. Data is kept confidential and only images are sent to the client. Even if the client's hardware resources are modest, the user can interact with state‐of‐the‐art rendering applications executed on the server. Our solution focuses on augmented video information, e.g., by depth, which is key to increase robustness with respect to data loss, image reconstruction, and is an important feature for stereo vision and other client‐side applications. Two major challenges arise in such a setup. First, the server workload has to be controlled to support many clients, second the data transfer needs to be efficient. Consequently, our contributions are twofold. First, we reduce the server‐based computations by making use of sparse sampling and temporal consistency to avoid expensive pixel evaluations. Second, our data‐transfer solution takes limited bandwidths into account, is robust to information loss, and compression and decompression are efficient enough to support real‐time interaction. Our key insight is to tailor our method explicitly for rendered 3D content and shift some computations on client GPUs, to better balance the server/client workload. Our framework is progressive, scalable, and allows us to stream augmented high‐resolution (e.g., HD‐ready) frames with small bandwidth on standard hardware.     

A scalable architecture for 3D map navigation on mobile devices

Mobile devices such as smart phones or tablets are rapidly increasing their graphics and networking capabilities. However, real-time visualization of 3D maps is still a challenging task to accomplish on such limited devices. In this paper, we describe the principles involved in the design and development of a scalable client–server architecture for delivering 3D maps over wireless networks to mobile devices. We have developed a hybrid adaptive streaming and rendering method that distributes the 3D map rendering task between the mobile clients and a remote server. This architecture provides support for efficient delivery of 3D contents to mobile clients according to their capabilities. As a proof of concept, we have implemented a prototype and carried out exhaustive experiments considering different scenarios and hundreds of concurrent connected clients. The analysis of the server workload and the mobile clients performance show that our architecture achieves a great scalability and performance even when using low-end hardware.     

基于CUDA 的体数据可视化工具

GPU的可编程性和并行计算能力的飞速发展为可视化提供了新的解决途径。基于支持CUDA的GPU,利用光线投射,实现了一个可以对体数据进行交互式可视化的工具,包括阻光度融合、等值面绘制、MIP绘制以及X光线投影等多种绘制效果,并加入了Phong光照模型以提高阻光度融合和等值面绘制的图像质量。实验表明,该工具较好的利用了GPU的并行计算能力,能够绘制出较高质量的图像,并具有良好的可交互性和可扩展性。     

GPU-based remote visualization of dynamic molecular data on the web

Visualization of dynamic data from molecular dynamics simulations is crucial for understanding the functioning of molecules. Many existing visualization tools have mainly focused on supporting a single user working on a desktop computer. Technical advancements in browser features increase the potential for the development of web-based collaborative visualization tools. Although web-based molecular viewers already exist, their support for dynamic molecular data at interactive rates is lacking. To address this gap, we present an efficient web application for visualization of dynamic molecular data using WebGL that exploits HTML5 technologies like WebSockets and Web Workers. GPU-based ray casting techniques offer fast rendering times and produce images with higher visual quality. Efficient data encoding techniques are used to minimize the data transferred to the client; therefore saving bandwidth and improving the transfer times. We demonstrate the feasibility of visualizing large dynamic molecular data with more than one million atoms in the browser at interactive frame rates. Our approach allows scientists at arbitrary locations to concurrently visually analyze the same data. The interactive parameterization of the visualization can be shared among multiple clients, providing the basis for collaborative research. Moreover, the application can be employed for remote simulation monitoring on mobile devices.     

基于体元投影的一种非规则数据场体绘制方法

随着科学计算可视化的发展及其在实践中的应用,非规则数据场的可视化已成为当前的研究热点之一。由于非规则场中样点的大小、形状和分布是不一致的,在成像时面临更多的困难。常用的方法有光线投射法,单元投影法,以及单元投影与光线投射相结合等算法。吸取了上述算法的优点,同时利用非规则数据场的一些特性,采用了一些加速措施来加速绘制。通过实验验证,可以达到比较理想的效果。     

An efficient direct volume rendering approach for dichromats

Color vision deficiency (CVD) affects a high percentage of the population worldwide. When seeing a volume visualization result, persons with CVD may be incapable of discriminating the classification information expressed in the image if the color transfer function or the color blending used in the direct volume rendering is not appropriate. Conventional methods used to address this problem adopt advanced image recoloring techniques to enhance the rendering results frame-by-frame; unfortunately, problematic perceptual results may still be generated. This paper proposes an alternative solution that complements the image recoloring scheme by reconfiguring the components of the direct volume rendering (DVR) pipeline. Our approach optimizes the mapped colors of a transfer function to simulate CVD-friendly effect that is generated by applying the image recoloring to the results with the initial transfer function. The optimization process has a low computational complexity, and only needs to be performed once for a given transfer function. To achieve detail-preserving and perceptually natural semi-transparent effects, we introduce a new color composition mode that works in the color space of dichromats. Experimental results and a pilot study demonstrates that our approach can yield dichromats-friendly and consistent volume visualization in real-time.     

Fast high-quality volume ray-casting with virtual samplings

Volume ray-casting with a higher order reconstruction filter and/or a higher sampling rate has been adopted in direct volume rendering frameworks to provide a smooth reconstruction of the volume scalar and/or to reduce artifacts when the combined frequency of the volume and transfer function is high. While it enables high-quality volume rendering, it cannot support interactive rendering due to its high computational cost. In this paper, we propose a fast high-quality volume ray-casting algorithm which effectively increases the sampling rate. While a ray traverses the volume, intensity values are uniformly reconstructed using a high-order convolution filter. Additional samplings, referred to as virtual samplings, are carried out within a ray segment from a cubic spline curve interpolating those uniformly reconstructed intensities. These virtual samplings are performed by evaluating the polynomial function of the cubic spline curve via simple arithmetic operations. The min max blocks are refined accordingly for accurate empty space skipping in the proposed method. Experimental results demonstrate that the proposed algorithm, also exploiting fast cubic texture filtering supported by programmable GPUs, offers renderings as good as a conventional ray-casting algorithm using high-order reconstruction filtering at the same sampling rate, while delivering 2.5x to 3.3x rendering speed-up.     

一种实时体切割绘制的光线分段预处理方法

针对传统图形绘制算法速度慢,质量不高的缺点,提出了一种快速的基于光线投射法的体切割预处理方法。通过外接球面和进行分段的光线投射预处理,在保证绘制质量的同时,实现了实时体切割。实验结果表明,本方法对于医学可视化应用,在使用规则体数据时,有令人满意的结果。     

A point-based rendering approach for real-time interaction on mobile devices

Mobile device is an important interactive platform. Due to the limitation of computation, memory, display area and energy, how to realize the efficient and real-time interaction of 3D models based on mobile devices is an important research topic. Considering features of mobile devices, this paper adopts remote rendering mode and point models, and then, proposes a transmission and rendering approach that could interact in real time. First, improved simplification algorithm based on MLS and display resolution of mobile devices is proposed. Then, a hierarchy selection of point models and a QoS transmission control strategy are given based on interest area of operator, interest degree of object in the virtual environment and rendering error. They can save the energy consumption. Finally, the rendering and interaction of point models are completed on mobile devices. The experiments show that our method is efficient. Supported by the National Natural Science Foundation of China (Grant No. 60873159), the Program for New Century Excellent Talents in University (Grant No. NCET-07-0039), the National High-Tech Research & Development Progrom of China (Grant No. 2006AA01Z333)     

Transform coding for hardware-accelerated volume rendering

Hardware-accelerated volume rendering using the GPU is now the standard approach for real-time volume rendering, although limited graphics memory can present a problem when rendering large volume data sets. Volumetric compression in which the decompression is coupled to rendering has been shown to be an effective solution to this problem; however, most existing techniques were developed in the context of software volume rendering, and all but the simplest approaches are prohibitive in a real-time hardware-accelerated volume rendering context. In this paper we present a novel block-based transform coding scheme designed specifically with real-time volume rendering in mind, such that the decompression is fast without sacrificing compression quality. This is made possible by consolidating the inverse transform with dequantization in such a way as to allow most of the reprojection to be precomputed. Furthermore, we take advantage of the freedom afforded by off-line compression in order to optimize the encoding as much as possible while hiding this complexity from the decoder. In this context we develop a new block classification scheme which allows us to preserve perceptually important features in the compression. The result of this work is an asymmetric transform coding scheme that allows very large volumes to be compressed and then decompressed in real-time while rendering on the GPU.     

移动设备上基于三维模型的线条画绘制

提出一种综合考虑移动设备客户端绘制能力和传输速度的混合型远程绘制方法.该方法采用渐进网格得到不同分辨率的细节层次模型,并利用渐进传输机制逐步传输数据,以缓解单次网络负载.服务器端可以根据网络质量状况、移动客户端的处理能力和用户交互请求来选择适宜的三维细节层次模型,提取特征轮廓线并将生成的线条集合传输给客户端进行本地风格化绘制.将该方法应用于考古数字博物馆展示中取得了良好的效果.