面向体绘制图像的直接体对象抠取

针对三维体对象抠取的相关工作中存在的尚多不足,提出一种体对象抠取算法.首先计算出与用户勾画结果相关的三维数据点,然后基于K-means聚类方法标记出高置信度的属于目标对象和无关对象的三维数据点,并以此作为种子点,借助基于能量优化的图割算法最终得到正确的体对象抠取结果.用户只需直接在体绘制的二维颜色叠加结果上通过简单的勾画指定目标对象和无关对象,即可抠取出感兴趣的体结构.最后通过实验说明了该算法的有效性.     

直接体绘制技术研究

直接体绘制技术是可视化研究领域的一个重要分支,是目前最活跃的可视化技术之一。该文首先介绍了体绘制技术的原理,分析其实现的关键技术及算法。然后介绍了可视化工具VTK,及其对光线投射算法的实现过程。     

直接体绘制技术研究

直接体绘制技术是可视化研究领域的一个重要分支,是目前最活跃的可视化技术之一。该文首先介绍了体绘制技术的原理,分析其实现的关键技术及算法。然后介绍了可视化工具VTK,及其对光线投射算法的实现过程。     

直接体绘制加速算法综述

针对当前体绘制存在的软件加速算法,将其分为空间剖分技术、光线相关、物空间相关和基于图像绘制技术等四类,分别介绍其算法基本内容.最后利用MIN-MAX八叉树加速算法设计一个直接体绘制系统.     

Depth of Field Effects for Interactive Direct Volume Rendering

In this paper, a method for interactive direct volume rendering is proposed for computing depth of field effects, which previously were shown to aid observers in depth and size perception of synthetically generated images. The presented technique extends those benefits to volume rendering visualizations of 3D scalar fields from CT/MRI scanners or numerical simulations. It is based on incremental filtering and as such does not depend on any pre‐computation, thus allowing interactive explorations of volumetric data sets via on‐the‐fly editing of the shading model parameters or (multi‐dimensional) transfer functions.     

彩色三维体数据场的直接体绘制方法

提出基于光线投射和三维纹理映射的彩色体数据成像算法，研究如何从每个体素的R，G，B三元组映射成不透明度值，即不透明度转换函数。首先把原始的RGB色彩空间转换成LUV色彩空间；然后以亮度分量的中心差分来近似估计法向量，并应用Phong光照模型进行着色，根据亮度分量及其梯度等信息计算不透明度值；最后合成、累积颜色。对美国数字人男子照相彩色体数据分别采用两种算法进行实验。结果表明：基于光线投射的彩色体数据算法成像质量较高，可以表现体表毛细血管等细微结构，但速度较慢；基于三维纹理映射的彩色体数据成像算法速度较快，但成像质量适中。     

A Multidirectional Occlusion Shading Model for Direct Volume Rendering

In this paper, we present a novel technique which simulates directional light scattering for more realistic interactive visualization of volume data. Our method extends the recent directional occlusion shading model by enabling light source positioning with practically no performance penalty. Light transport is approximated using a tilted cone‐shaped function which leaves elliptic footprints in the opacity buffer during slice‐based volume rendering. We perform an incremental blurring operation on the opacity buffer for each slice in front‐to‐back order. This buffer is then used to define the degree of occlusion for the subsequent slice. Our method is capable of generating high‐quality soft shadowing effects, allows interactive modification of all illumination and rendering parameters, and requires no pre‐computation.     

A Directional Occlusion Shading Model for Interactive Direct Volume Rendering

Volumetric rendering is widely used to examine 3D scalar fields from CT/MRI scanners and numerical simulation datasets. One key aspect of volumetric rendering is the ability to provide perceptual cues to aid in understanding structure contained in the data. While shading models that reproduce natural lighting conditions have been shown to better convey depth information and spatial relationships, they traditionally require considerable (pre)computation. In this paper, a shading model for interactive direct volume rendering is proposed that provides perceptual cues similar to those of ambient occlusion, for both solid and transparent surface-like features. An image space occlusion factor is derived from the radiative transport equation based on a specialized phase function. The method does not rely on any precomputation and thus allows for interactive explorations of volumetric data sets via on-the-fly editing of the shading model parameters or (multi-dimensional) transfer functions while modifications to the volume via clipping planes are incorporated into the resulting occlusion-based shading.     

面向医学数据的分层剥离体绘制算法

采用直接体绘制方法显示医学数据时,一般通过修改传递函数来显示感兴趣区域,但仅通过调节传递函数很难清晰地显示被遮挡的人体组织,为此提出一种新的分层剥离直接体绘制算法.通过计算相邻的不透明度极小值和极大值之间的斜率,并根据用户指定的斜率阈值和累积不透明度阈值确定分层点,实现对体数据的分层绘制.实验结果证明,该算法可以根据不同医学数据的特征信息更精确地定位分层点,绘制效果清晰,速度较快.     

State of the Art in Transfer Functions for Direct Volume Rendering

A central topic in scientific visualization is the transfer function (TF) for volume rendering. The TF serves a fundamental role in translating scalar and multivariate data into color and opacity to express and reveal the relevant features present in the data studied. Beyond this core functionality, TFs also serve as a tool for encoding and utilizing domain knowledge and as an expression for visual design of material appearances. TFs also enable interactive volumetric exploration of complex data. The purpose of this state‐of‐the‐art report (STAR) is to provide an overview of research into the various aspects of TFs, which lead to interpretation of the underlying data through the use of meaningful visual representations. The STAR classifies TF research into the following aspects: dimensionality, derived attributes, aggregated attributes, rendering aspects, automation, and user interfaces. The STAR concludes with some interesting research challenges that form the basis of an agenda for the development of next generation TF tools and methodologies.     

Hypergraph-Partitioning-Based Remapping Models for Image-Space-Parallel Direct Volume Rendering of Unstructured Grids

In this work, image-space-parallel direct volume rendering (DVR) of unstructured grids is investigated for distributed-memory architectures. A hypergraph-partitioning-based model is proposed for the adaptive screen partitioning problem in this context. The proposed model aims to balance the rendering loads of processors while trying to minimize the amount of data replication. In the parallel DVR framework we adopted, each data primitive is statically owned by its home processor, which is responsible from replicating its primitives on other processors. Two appropriate remapping models are proposed by enhancing the above model for use within this framework. These two remapping models aim to minimize the total volume of communication in data replication while balancing the rendering loads of processors. Based on the proposed models, a parallel DVR algorithm is developed. The experiments conducted on a PC cluster show that the proposed remapping models achieve better speedup values compared to the remapping models previously suggested for image-space-parallel DVR     

基于PC图形硬件的直接体绘制技术综述

对基于PC图形硬件的直接体绘制技术进行了介绍、归类和总结，力求为本领域的的研究提供一个较为详实的背景资料。     

GPU Accelerated Direct Volume Rendering on an Interactive Light Field Display

We present a GPU accelerated volume ray casting system interactively driving a multi‐user light field display. The display, driven by a single programmable GPU, is based on a specially arranged array of projectors and a holographic screen and provides full horizontal parallax. The characteristics of the display are exploited to develop a specialized volume rendering technique able to provide multiple freely moving naked‐eye viewers the illusion of seeing and manipulating virtual volumetric objects floating in the display workspace. In our approach, a GPU ray‐caster follows rays generated by a multiple‐center‐of‐projection technique while sampling pre‐filtered versions of the dataset at resolutions that match the varying spatial accuracy of the display. The method achieves interactive performance and provides rapid visual understanding of complex volumetric data sets even when using depth oblivious compositing techniques.     

Compressive Volume Rendering

Compressive rendering refers to the process of reconstructing a full image from a small subset of the rendered pixels, thereby expediting the rendering task. In this paper, we empirically investigate three image order techniques for compressive rendering that are suitable for direct volume rendering. The first technique is based on the theory of compressed sensing and leverages the sparsity of the image gradient in the Fourier domain. The latter techniques exploit smoothness properties of the rendered image; the second technique recovers the missing pixels via a total variation minimization procedure while the third technique incorporates a smoothness prior in a variational reconstruction framework employing interpolating cubic B‐splines. We compare and contrast the three techniques in terms of quality, efficiency and sensitivity to the distribution of pixels. Our results show that smoothness‐based techniques significantly outperform techniques that are based on compressed sensing and are also robust in the presence of highly incomplete information. We achieve high quality recovery with as little as 20% of the pixels distributed uniformly in screen space.     

加速体绘制技术

根据体绘制成像的各个操作环节,对体绘制的加速技术进行了较全面且系统的介绍,包括色彩合成、光线与数据的求交、插值计算、排序及视见变换、对体绘制的基于硬件及系统方面的加速技术（如并行体绘制和漫游体绘制）也进行了一些讨论,在实际应用中,只有将各种加速技术进行有机地结合才能充分发挥体绘制的可视化作用。     

Parallel Irradiance Caching for Interactive Monte‐Carlo Direct Volume Rendering

We propose a technique to build the irradiance cache for isotropic scattering simultaneously with Monte Carlo progressive direct volume rendering on a single GPU, which allows us to achieve up to four times increased convergence rate for complex scenes with arbitrary sources of light. We use three procedures that run concurrently on a single GPU. The first is the main rendering procedure. The second procedure computes new cache entries, and the third one corrects the errors that may arise after creation of new cache entries. We propose two distinct approaches to allow massive parallelism of cache entry creation. In addition, we show a novel extrapolation approach which outputs high quality irradiance approximations and a suitable prioritization scheme to increase the convergence rate by dedicating more computational power to more complex rendering areas.     

一种非规则数据场的体绘制算法

非规则数据场的体绘制是可视化的一个热点和难点。常用直接体绘制算法有光线投射法、单元投影法、快速体绘制算法。本文吸取了上述三种算法的优点,采用体元面投影的方法来确定光线路径;同时考虑到非规则数据场的一些特性,采取三个有效的措施大大加快了投影和求交速度;在采样中用分段积分法代替等距采样,从而进一步提高了图象质量。     

体绘制的任意曲面切割方法

为了在体绘制的过程中,响应用户的实时交互以及对体数据进行切割。论文提出了一种预计算外部轮廓,基于ConstructiveSolidGeometry(CSG)的方法来进行切割的方法。预先计算的外部轮廓在整个体绘制过程中完成界定体元绘制的边界和进行切割交互的功能,因此提高了体元绘制和切割的效率。引入外部轮廓几何体之后,采用基于CSG的方法来进行交互式切割,保证了原有外部轮廓和切割体各自内部的拓扑关系。最后,通过对纹理坐标的实时计算,实现3D纹理的映射。论文借助于现有可编程图形流水线的功能实现该方法,完成了高效率的实时交互和绘制。     

八叉树编码体数据的快速体绘制算法

提出一种基于并行投影Shear-Warp分解的射线模板快速RayCasting体绘制算法．体数据采用有效的八叉树存储和表达方案．八叉树的适应性均一化分级策略使得算法在简化了不必要的处理过程的同时,为构建用于RayCasting算法的射线模板和后续的交叉计算提供了有力的应用基础．针对体绘制中八叉树编码体数据构造基本的交叉运算如节点单元定位、区域定能和邻接节点单元搜索的需要,相应地提出了,简单而且有效的方法．这些方法是非递归的,在减少比较运算的同时不需要构建中间结果列表,进而减少了对内存的占用和避免了繁重的交叉计算．所提出的体绘制算法能在标准PC平台下快速实现超大型数据集的处理和高质量的人体内部结构图像的绘制．