Composed Scattering Model for Direct Volume Rendering

Based on the equation of transfer in transport theory of optical physics,a new volume rendering model,called composed scattering model(CSM),is presented.In calculating the scattering term of the equation,it is decomposed into volume scattering intensity and surface scattering intensity,and they are composed with the boundary detection operator as the weight function.This proposed model differs from the most current volume rendering models in the aspect that in CSM segmentation and illumination intensity calculation are taken as two coherent parts while in existing models they are regarded as two separate ones.This model has been applied to the direct volume rendering of 3D data sets obtained by CT and MRI.The resultant images show not only rich details but also clear boundary surfaces.CSM is demonstrated to be an accurate volume rendering model suitable for CT and MRI data sets.     

Visualization of large data sets with the Active Data Repository

We implement ray-casting-based volume rendering and isosurface rendering methods using the Active Data Repository (ADR) for visualizing out-of-core data sets. We have developed the ADR object-oriented framework to provide support for applications that employ range queries with user-defined mapping and aggregation operations on large-scale multidimensional data. ADR targets distributed-memory parallel machines with one or more disks attached to each node. It is designed as a set of modular services implemented in C++, which can be customized for application-specific processing. The ADR runtime system supports common operations such as memory management, data retrieval, and scheduling of processing across a parallel machine     

Visualization by proxy: a novel framework for deferred interaction with volume data

Interactivity is key to exploration of volume data. Interactivity may be hindered due to many factors, e.g. large data size,high resolution or complexity of a data set, or an expensive rendering algorithm. We present a novel framework for visualizing volume data that enables interactive exploration using proxy images, without accessing the original 3D data. Data exploration using direct volume rendering requires multiple (often redundant) accesses to possibly large amounts of data. The notion of visualization by proxy relies on the ability to defer operations traditionally used for exploring 3D data to a more suitable intermediate representation for interaction--proxy images. Such operations include view changes, transfer function exploration, and relighting. While previous work has addressed specific interaction needs, we provide a complete solution that enables real-time interaction with large data sets and has low hardware and storage requirements.     

基于区域竞争分割结果的体绘制研究

在开发医学图像处理系统时,采用区域竞争模型分割产生的图像只反映图像的轮廓及位置信息,不包含内部的图像信息,所以只能直接进行面绘制.据此,提出了基于区域竞争模型分割结果的体绘制算法.算法根据分割结果中的轮廓信息,对原始医学图像进行数值转换,使目标图像中既包含内部l冬I像信息又包含轮廓边界信息,最后利用转换后的数据对肝脏进行体绘制.实验结果表明本算法有效可行,而且有很高的实用性.     

Interactive Volume Rendering with Dynamic Ambient Occlusion and Color Bleeding

We propose a method for rendering volumetric data sets at interactive frame rates while supporting dynamic ambient occlusion as well as an approximation to color bleeding. In contrast to ambient occlusion approaches for polygonal data, techniques for volumetric data sets have to face additional challenges, since by changing rendering parameters, such as the transfer function or the thresholding, the structure of the data set and thus the light interactions may vary drastically. Therefore, during a preprocessing step which is independent of the rendering parameters we capture light interactions for all combinations of structures extractable from a volumetric data set. In order to compute the light interactions between the different structures, we combine this preprocessed information during rendering based on the rendering parameters defined interactively by the user. Thus our method supports interactive exploration of a volumetric data set but still gives the user control over the most important rendering parameters. For instance, if the user alters the transfer function to extract different structures from a volumetric data set the light interactions between the extracted structures are captured in the rendering while still allowing interactive frame rates. Compared to known local illumination models for volume rendering our method does not introduce any substantial rendering overhead and can be integrated easily into existing volume rendering applications. In this paper we will explain our approach, discuss the implications for interactive volume rendering and present the achieved results.     

海量医学数据处理框架及快速体绘制算法

设计并实现了一套针对海量数据的处理和分析算法框架,并将其融入实验室早先开发完成的医学影像算法研发平台MITK(medical imaging toolkit)中,真正建立起一个海量医学影像数据的处理平台,并在此基础上研究了针对海量数据的基于光线投射和三维纹理的快速体绘制算法,提出了一种半自适应分块的方法对原始数据进行分块,在不对分块速度产生太大影响的基础上得到了更好的分块结果,同时使用图形硬件来进一步加速整个算法的绘制流程.实验结果表明了该平台和算法对于海量医学数据处理和可视化的有效性.     

应用于传递函数设定的交互式体绘制工具

传递函数是体绘制过程中用以定出体数据与光学特征的对应关系,因此,传递函数的设定对成像质量有着直接的影响,文章提出一应用于传递函数设定、简单且有效的交互式体绘制工具,由于二维纹理硬件在通用的个人计算机上被普遍使用,因而该工具采用基于二维纹理硬件的体绘制方法,利用本工具,用户能根据体数据的直方图来交互地分别设定R、G、B和A四种传递函数,以定出体数据与光学特征的对应关系,并获得实时的反馈视觉信息(绘制结果),该工具亦提供一虚拟轨迹球让用户交互地改变观察体数据的视点,用户不但可以交互地控制放大或缩小比率来绘制体数据,还可以选择采用光照或由多重纹理实现的三线性插值来获得不同的绘制效果,该文描述开发此工具的各种技术,并给出利用此工具得到的一些绘制结果。     

Dimensionality Reduction on Multi-Dimensional Transfer Functions for Multi-Channel Volume Data Sets

The design of transfer functions for volume rendering is a non-trivial task. This is particularly true for multi-channel data sets, where multiple data values exist for each voxel, which requires multi-dimensional transfer functions. In this paper, we propose a new method for multi-dimensional transfer function design. Our new method provides a framework to combine multiple computational approaches and pushes the boundary of gradient-based multi-dimensional transfer functions to multiple channels, while keeping the dimensionality of transfer functions at a manageable level, i.e., a maximum of three dimensions, which can be displayed visually in a straightforward way. Our approach utilizes channel intensity, gradient, curvature and texture properties of each voxel. Applying recently developed nonlinear dimensionality reduction algorithms reduces the high-dimensional data of the domain. In this paper, we use Isomap and Locally Linear Embedding as well as a traditional algorithm, Principle Component Analysis. Our results show that these dimensionality reduction algorithms significantly improve the transfer function design process without compromising visualization accuracy. We demonstrate the effectiveness of our new dimensionality reduction algorithms with two volumetric confocal microscopy data sets.     

An architecture for Java-based real-time distributed visualization

In this paper, we present a Java-based software architecture for real-time visualization that utilizes a cluster of conventional PCs to generate high-quality interactive graphics. Normally, a large multiprocessor computer would be needed for interactive visualization tasks requiring more processing power than a single PC can provide. By using clusters of PCs, enormous cost savings can be realized, and proprietary "high-end" hardware is no longer necessary for these tasks. Our architecture minimizes the amount of synchronization needed between PCs, resulting in excellent scalability. It provides a modular framework that can accommodate a wide variety of rendering algorithms and data formats, provided that the rendering algorithms can generate pixels individually and the data is duplicated on each PC. Demonstration modules that implement ray tracing, fractal rendering, and volume rendering algorithms were developed to evaluate the architecture. Results are encouraging-using 15 PCs connected to a standard 100 Megabit/s Ethernet network, the system can interactively render simple to moderately complex data sets at modest resolution. Excellent scalability is achieved; however, our tests were limited to a cluster of 15 PCs. Results also demonstrate that Java is a viable platform for real-time distributed visualization.     

Texture-based transfer functions for direct volume rendering

Visualization of volumetric data faces the difficult task of finding effective parameters for the transfer functions. Those parameters can determine the effectiveness and accuracy of the visualization. Frequently, volumetric data includes multiple structures and features that need to be differentiated. However, if those features have the same intensity and gradient values, existing transfer functions are limited at effectively illustrating those similar features with different rendering properties. We introduce texture-based transfer functions for direct volume rendering. In our approach, the voxel's resulting opacity and color are based on local textural properties rather than individual intensity values. For example, if the intensity values of the vessels are similar to those on the boundary of the lungs, our texture-based transfer function will analyze the textural properties in those regions and color them differently even though they have the same intensity values in the volume. The use of texture-based transfer functions has several benefits. First, structures and features with the same intensity and gradient values can be automatically visualized with different rendering properties. Second, segmentation or prior knowledge of the specific features within the volume is not required for classifying these features differently. Third, textural metrics can be combined and/or maximized to capture and better differentiate similar structures. We demonstrate our texture-based transfer function for direct volume rendering with synthetic and real-world medical data to show the strength of our technique.     

Volume illustration: nonphotorealistic rendering of volume models

Accurately and automatically conveying the structure of a volume model is a problem which has not been fully solved by existing volume rendering approaches. Physics-based volume rendering approaches create images which may match the appearance of translucent materials in nature but may not embody important structural details. Transfer function approaches allow flexible design of the volume appearance but generally require substantial hand-tuning for each new data set in order to be effective. We introduce the volume illustration approach, combining the familiarity of a physics-based illumination model with the ability to enhance important features using non-photorealistic rendering techniques. Since the features to be enhanced are defined on the basis of local volume characteristics rather than volume sample values, the application of volume illustration techniques requires less manual tuning than the design of a good transfer function. Volume illustration provides a flexible unified framework for enhancing the structural perception of volume models through the amplification of features and the addition of illumination effects     

Visibility histograms and visibility-driven transfer functions

Direct volume rendering is an important tool for visualizing complex data sets. However, in the process of generating 2D images from 3D data, information is lost in the form of attenuation and occlusion. The lack of a feedback mechanism to quantify the loss of information in the rendering process makes the design of good transfer functions a difficult and time consuming task. In this paper, we present the general notion of visibility histograms, which are multidimensional graphical representations of the distribution of visibility in a volume-rendered image. In this paper, we explore the 1D and 2D transfer functions that result from intensity values and gradient magnitude. With the help of these histograms, users can manage a complex set of transfer function parameters that maximize the visibility of the intervals of interest and provide high quality images of volume data. We present a semiautomated method for generating transfer functions, which progressively explores the transfer function space toward the goal of maximizing visibility of important structures. Our methodology can be easily deployed in most visualization systems and can be used together with traditional 1D and 2D opacity transfer functions based on scalar values, as well as with other more sophisticated rendering algorithms.     

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.     

三维数据场实时体绘制研究进展

在分析了三维数据场实时体绘制研究现状的基础上，重点探讨了三维数据场实时体绘制的五种方法：降低采样维数法、空间相关性法、跳过空体元法、基于硬件的方法及并行处理的方法，并比较了各种绘制算法的特点，从而指明了三维数据场实时体绘制进一步研究的方向。     

High-level user interfaces for transfer function design with semantics

Many sophisticated techniques for the visualization of volumetric data such as medical data have been published. While existing techniques are mature from a technical point of view, managing the complexity of visual parameters is still difficult for non-expert users. To this end, this paper presents new ideas to facilitate the specification of optical properties for direct volume rendering. We introduce an additional level of abstraction for parametric models of transfer functions. The proposed framework allows visualization experts to design high-level transfer function models which can intuitively be used by non-expert users. The results are user interfaces which provide semantic information for specialized visualization problems. The proposed method is based on principal component analysis as well as on concepts borrowed from computer animation.     

Symmetry in scalar field topology

Study of symmetric or repeating patterns in scalar fields is important in scientific data analysis because it gives deep insights into the properties of the underlying phenomenon. Though geometric symmetry has been well studied within areas like shape processing, identifying symmetry in scalar fields has remained largely unexplored due to the high computational cost of the associated algorithms. We propose a computationally efficient algorithm for detecting symmetric patterns in a scalar field distribution by analysing the topology of level sets of the scalar field. Our algorithm computes the contour tree of a given scalar field and identifies subtrees that are similar. We define a robust similarity measure for comparing subtrees of the contour tree and use it to group similar subtrees together. Regions of the domain corresponding to subtrees that belong to a common group are extracted and reported to be symmetric. Identifying symmetry in scalar fields finds applications in visualization, data exploration, and feature detection. We describe two applications in detail: symmetry-aware transfer function design and symmetry-aware isosurface extraction.     

An Exploratory Technique for Coherent Visualization of Time‐varying Volume Data

The selection of an appropriate global transfer function is essential for visualizing time‐varying simulation data. This is especially challenging when the global data range is not known in advance, as is often the case in remote and in‐situ visualization settings. Since the data range may vary dramatically as the simulation progresses, volume rendering using local transfer functions may not be coherent for all time steps. We present an exploratory technique that enables coherent classification of time‐varying volume data. Unlike previous approaches, which require pre‐processing of all time steps, our approach lets the user explore the transfer function space without accessing the original 3D data. This is useful for interactive visualization, and absolutely essential for in‐situ visualization, where the entire simulation data range is not known in advance. Our approach generates a compact representation of each time step at rendering time in the form of ray attenuation functions, which are used for subsequent operations on the opacity and color mappings. The presented approach offers interactive exploration of time‐varying simulation data that alleviates the cost associated with reloading and caching large data sets.     

Parallel implementing OpenGL on PVM

A 3D polygon rendering system conforming to the specification of OpenGL is implemented on a PVM (parallel virtual machine). The system is targeted to a low speed network of low cost serial computers. In our parallel rendering algorithm, image space is decomposed to exactly the number of regions equal to the number of processors which reduces the volume of communication and the polygons needed to be rendered more than once on separate renderers. We also propose a transmission scheme that distributes the passing of data throughout the whole process of rendering. Taking advantage of frame-to-frame coherence, load balancing is achieved by decomposing the image space unequally so that the workloads in each space region are proportional to the processing power of the corresponding renders. Distribution of workload and processing power that is volatile during rendering are obtained with linear prediction from statistics of previous frames. The system is practical and scalable for moderate number of processors.     

Illustrative interactive stipple rendering

Simulating hand-drawn illustration can succinctly express information in a manner that is communicative and informative. We present a framework for an interactive direct stipple rendering of volume and surface-based objects. By combining the principles of artistic and scientific illustration, we explore several feature enhancement techniques to create effective, interactive visualizations of scientific and medical data sets. We also introduce a rendering mechanism that generates appropriate point lists at all resolutions during an automatic preprocess and modifies rendering styles through different combinations of these feature enhancements. The new system is an effective way to interactively preview large, complex volume and surface data sets in a concise, meaningful, and illustrative manner. Stippling is effective for many applications and provides a quick and efficient method to investigate both volume and surface models.     

Interactive texture-based volume rendering for large data sets

To employ direct volume rendering, TRex uses parallel graphics hardware, software-based compositing, and high-performance I/O to provide near-interactive display rates for time-varying, terabyte-sized data sets. We present a scalable, pipelined approach for rendering data sets too large for a single graphics card. To do so, we take advantage of multiple hardware rendering units and parallel software compositing. The goals of TRex, our system for interactive volume rendering of large data sets, are to provide near-interactive display rates for time-varying, terabyte-sized uniformly sampled data sets and provide a low-latency platform for volume visualization in immersive environments. We consider 5 frames per second (fps) to be near-interactive rates for normal viewing environments and immersive environments to have a lower bound frame rate of l0 fps. Using TRex for virtual reality environments requires low latency - around 50 ms per frame or 100 ms per view update or stereo pair. To achieve lower latency renderings, we either render smaller portions of the volume on more graphics pipes or subsample the volume to render fewer samples per frame by each graphics pipe. Unstructured data sets must be resampled to appropriately leverage the 3D texture volume rendering method