Interactive rendering of dynamic geometry

Fluid simulations typically produce complex three-dimensional (3D) isosurfaces whose geometry and topology change over time. The standard way of representing such "dynamic geometry" is by a set of isosurfaces that are extracted individually at certain time steps. An alternative strategy is to represent the whole sequence as a four-dimensional (4D) tetrahedral mesh. The iso-surface at a specific time step can then be computed by intersecting the tetrahedral mesh with a 3D hyperplane. This not only allows the animation of the surface continuously over time without having to worry about the topological changes, but also enables simplification algorithms to exploit temporal coherence. We show how to interactively render such 4D tetrahedral meshes by improving previous GPU-accelerated techniques and building an out-of-core multi-resolution structure based on quadric error simplification. As a second application, we apply our framework to time-varying surfaces that result from morphing one triangle mesh into another.     

Exploration of local surface geometry with minimum number of contact points and surface normal information

In this paper, we propose a method of exploring the surface geometry of an unknown object by touch. The method is based on the idea that a three-dimensional surface geometry can be reconstructed from two principal curvatures of the object which are estimated from three concurrent curves. First, the process to minimize the number of contact points is addressed for the approximation of an arbitrary curve, which uses normal vectors at the contact points. Then, an algorithm for reconstructing a three-dimensional local surface from four contact points, two of which can be used to compute a normal curvature, is presented. Lastly, our method is applied to cylindrical, spherical and planar objects in simulation and experiments for validation.     

Compression and rendering of iso-surfaces and point sampled geometry

In this paper we present a streaming compression scheme for gigantic point sets including per-point normals. This scheme extends on our previous Duodecim approach [21] in two different ways. First, we show how to use this approach for the compression and rendering of high-resolution iso-surfaces in volumetric data sets. Second, we use deferred shading of point primitives to considerably improve rendering quality. Iso-surface reconstruction is performed in a hexagonal close packing (HCP) grid, into which the initial data set is resampled. Normals are resampled from the initial domain using volumetric gradients. By incremental encoding, only slightly more than 3 bits per surface point and 5 bits per surface normal are required at high fidelity. The compressed data stream can be decoded in the graphics processing unit (GPU). Decoded point positions are saved in graphics memory, and they are then used on the GPU again to render point primitives. In this way high quality gigantic data sets can directly be rendered from their compressed representation in local GPU memory at interactive frame rates (see Fig. 1).     

树木的建模与实时渲染技术

详细论述了树木生成以及渲染的实现过程.描述了一个使用随机L-系统的树木建模方法,以及几何绘制和替代物渲染相结合的树木渲染方法,并讨论了树木阴影的生成.最后,使用VC++和OpenGL设计了一个树木的生成器和一个树木的渲染器,实现了树木的建模与渲染.程序的运行结果表明,该方法能够取得较好的渲染效率和不错的视觉效果.     

Capturing and rendering geometry details for BTF-mapped surfaces

Bidirectional texture functions, or BTFs, accurately model reflectance variation at a fine (meso-) scale as a function of lighting and viewing direction. BTFs also capture view-dependent visibility variation, also called masking or parallax, but only within surface contours. Mesostructure detail is neglected at silhouettes, so BTF-mapped objects retain the coarse shape of the underlying model. We augment BTF rendering to obtain approximate mesoscale silhouettes. Our new representation, the 4D mesostructure distance function (MDF), tabulates the displacement from a reference frame where a ray first intersects the mesoscale geometry beneath as a function of ray direction and ray position along that reference plane. Given an MDF, the mesostructure silhouette can be rendered with a per-pixel depth peeling process on graphics hardware, while shading and local parallax are handled by the BTF. Our approach allows real-time rendering, handles complex, non-height-field mesostructure, requires that no additional geometry be sent to the rasterizer other than the mesh triangles, is more compact than textured visibility representations used previously, and, for the first time, can be easily measured from physical samples. We also adapt the algorithm to capture detailed shadows cast both by and onto BTF-mapped surfaces. We demonstrate the efficiency of our algorithm on a variety of BTF data, including real data acquired using our BTF–MDF measurement system.     

Modeling and rendering of realistic waterfall scenes with dynamic texture sprites

We propose an efficient approach for authoring dynamic and realistic waterfall scenes based on an acquired video sequence. Traditional video based techniques generate new images by synthesizing 2D samples, i.e., texture sprites chosen from a video sequence. However, they are limited to one fixed viewpoint and cannot provide arbitrary walkthrough into 3D scenes. Our approach extends this scheme by synthesizing dynamic 2D texture sprites and projecting them into 3D space. We first generate a set of basis texture sprites, which capture the representative appearance and motions of waterfall scenes contained in the video sequence. To model the shape and motion of a new waterfall scene, we interactively construct a set of flow lines taking account of physical principles. Along each flow line, the basis texture sprites are manipulated and animated dynamically, yielding a sequence of dynamic texture sprites in 3D space. These texture sprites are displayed using the point splatting technique, which can be accelerated efficiently by graphics hardware. By choosing varied basis texture sprites, waterfall scenes with different appearance and shapes can be conveniently simulated. The experimental results demonstrate that our approach achieves realistic effects and real‐time frame rates on consumer PC platforms. Copyright © 2006 John Wiley & Sons, Ltd.     

基于边界曲面零交叉点的体绘制

体绘制中体元二义性使灰度—梯度幅值传递函数空间中出现拱形区域, 可视化工具操作者难以快速、准确地选取出感兴趣目标。针对这个问题, 介绍了一种在传递函数空间引入边界曲面零交叉点特征的算法。该算法结合面绘制移动立方体算法跟踪边缘立方体并记录所有边界曲面的零交叉点, 并利用这些零交叉点的特征信息对灰度—梯度特征空间进行重构并选取感兴趣边界曲面。最后通过实验证明了上述方法对目标边界曲面的选取行之有效。     

图像局部纹理特性的静电力触觉渲染

目的 触摸触觉设备感知物体时,需要实现视觉-力触觉同步反馈,其中图像-力触觉反馈难点在于再现更真实的纹理触感的触觉渲染过程。本文提出了一种基于图像局部纹理特征的静电力触觉渲染模型,实现了更加清晰、触感真实的图像纹理的静电力触觉反馈。方法 首先,采用局部傅里叶变换方法强化局部纹理特征,提取傅里叶变换系数分离出表征形状和局部纹理、边缘的频域分量。其次,对局部纹理特征进行力触觉渲染,建立局部纹理特征与驱动信号的映射模型,采用比例模型将局部纹理特征值转化为同等级的静电力表达。最后,根据静电力与驱动信号的心理学模型,由局部纹理特征控制不同驱动信号的输出产生静电力触觉。结果 进行纹理触觉对比感知实验验证算法有效性,62.5%的实验参与者偏爱基于图像局部纹理的触觉渲染算法反馈的纹理触感,本文算法可以模拟多种图像的纹理、边缘的触感。结论 算法在频域分离图像局部纹理、边缘和形状特征,建立纹理-力触觉渲染模型,针对大多数图片可以有效地增强纹理触感,提升触觉再现交互技术的沉浸感。     

Differential geometry images: remeshing and morphing with local shape preservation

In this paper, we propose a novel conception of differential geometry images (DGIM), encapsulating differential coordinates to traditional geometry images. DGIM preserves the local geometric characteristics in many graphics applications, such as model remeshing and morphing. The traditional geometry images using Cartesian coordinates require normal maps for correctly rendering models, because they neglect the existence of local geometric details in the image structure, which leads us to compute the normals and curvatures imprecisely. Using our differential geometry images, normals can be reconstructed easily and correctly thereafter normal maps are no longer required. In addition, DGIM can be easily applied to mesh morphing due to its regular topology and well-preserved local details. In this paper, we also demonstrate a variety of plausible mesh morphing results based on DGIM in shape space.     

Modeling 3D Euclidean geometry

This article compares five models of 3D Euclidean geometry-not theoretically, but by demonstrating how to implement a simple recursive ray tracer in each of them. It's meant as a tangible case study of the profitability of choosing an appropriate model, discussing the trade-offs between elegance and performance for this particular application. The models we compare are 3D linear algebra, 3D geometric algebra, 4D linear algebra, 4D geometric algebra, and 5D geometric algebra.     

Accumulation of local maximum intensity for feature enhanced volume rendering

Maximum Intensity Difference Accumulation (MIDA) combines the advantage of Direct Volume Rendering (DVR) and Maximum Intensity Projection (MIP). However, many features with local maximum intensity are still missing in the final rendering image. This paper presents a novel approach to focus on features with local maximum intensity within the dataset. Moving Least Squares (MLS) is used to smooth each ray profile during the raycasting in order to eliminate noise in the data and to highlight significant transition points on the profile. We then adopt a local minimum-point searching method to analyze the ray profile, and identify the transition points that mark the local maximum intensity points within the dataset. At the rendering stage, we implement a novel local intensity difference accumulation (LIDA) to accumulate the colors and opacity. Surface shading is introduced to improve the spatial cues of the features. We also employ tone-reduction to preserve the original local contrast. Our approach can highlight local features in the dataset without involving the adjustment of transfer functions. The experiments demonstrate high-quality rendering results at an interactive frame rate.     

Modeling, animating, and rendering complex scenes using volumetrictextures

Complex repetitive scenes containing forests, foliage, grass, hair, or fur, are challenging for common modeling and rendering tools. The amount of data, the tediousness of modeling and animation tasks, and the cost of realistic rendering have caused such kind of scene to see only limited use even in high-end productions. The author describes how the use of volumetric textures is well suited to such scenes. These primitives can greatly simplify modeling and animation tasks. More importantly, they can be very efficiently rendered using ray tracing with few aliasing artifacts. The main idea, initially introduced by Kajiya and Kay (1989), is to represent a pattern of 3D geometry in a reference volume, that is tiled over an underlying surface much like a regular 2D texture. In our contribution, the mapping is independent of the mesh subdivision, the pattern can contain any kind of shape, and it is prefiltered at different scales as for MIP-mapping. Although the model encoding is volumetric, the rendering method differs greatly from traditional volume rendering. A volumetric texture only exists in the neighborhood of a surface, and the repeated instances (called texels) of the reference volume are spatially deformed. Furthermore, each voxel of the reference volume contains a key feature which controls the reflectance function that represents aggregate intravoxel geometry. This allows for ray tracing of highly complex scenes with very few aliasing artifacts, using a single ray per pixel (for the part of the scene using the volumetric texture representation). The major technical considerations of our method lie in the ray-path determination and in the specification of the reflectance function     

基于散乱点的任意拓扑曲面建模及绘制研究

对曲面建模进行了研究,提出了一种基于散乱点的曲面重建方法.给出三维空间的散乱点,采用3个步骤重构出三角化曲面:首先按近似共面程度对点集聚类,把接近共面的点归为同一类,然后根据空间点的邻接关系进行局部调整,最后对点集和点集之间的区域进行三角割分,得到三角化曲面,所得曲面用OpenGL进行绘制.     

局部不变特征点的精度指标

目的 作为计算机视觉的热门研究方向,局部不变特征算法的发展已趋于成熟、稳定,然而目前几乎所有特征点提取算法都没有给出特征点的精度指标.针对这一缺陷,提出一种特征点精度指标-特征点波动区间.方法 性质稳定的点在干扰条件下仍具有较好的精度,即小范围的波动区间,因此,以当前最热门的SIFT(scale-invariant feature transform)特征点为例,在图像加入噪声,发生光照变换,发生模糊变换以及同时进行噪声、光照及模糊处理这四种情况下分别分析同一算法提取的不同特征点的波动情况,进而得到不同特征点的波动区间.结果 实验得到16个稳定检出特征点,其中点2,3,4,11,13这5个点可以在不同干扰条件下的波动范围都较小,而点2则只在模糊条件下波动较小,在其余干扰下波动较大.特征点虽然已经过特征提取,但仍具有不同的波动区间,其优劣也不尽相同.不同的特征点的波动区间并不相同,但仍有一部分特征点在不同干扰条件下均保持较高的提取精度.结论 波动区间能很好地表征每个特征点的提取精度.由于此前只有针对特征点算法的评价指标,而没有针对特征点自身性质的评价方法,因此本文提出的特征点波动区间将为诸如设备标定、视觉测量、精简特征库等相关后续工作打下良好基础.     

B双空间几何中基于消隐点的摄像机标定

对比目前常见的CCD摄像机标定技术,采用了一种基于消隐点的摄像机标定方法。该方法基于射影几何中的B双空间几何概念,在充分利用二维棋盘格标定图像中点、线和面固有的几何关系与属性的基础上,通过改进的角点提取与计算,首先针对摄像机内参数中的横向焦距与纵向焦距进行计算,然后结合非线性优化方法对参数进行优化计算,从而求解出待标定摄像机的全部内参数。该标定方法具有操作过程简单和实验误差小的特点。     

A comparison of local surface geometry estimation methods

The performance of several methods for estimating local surface geometry (the principal frame plus the principal quadric) are examined by applying them to a suite of synthetic and real test data which have been corrupted by various amounts of additive Gaussian noise. Methods considered include finite differences, a facet based approach, and quadric surface fitting. The nonlinear quadric fitting method considered was found to perform the best but has the greatest computational cost. The facet based approach works as well as the other quadric fitting methods and has a much smaller computational cost. Hence it is the recommended method to use in practice.     

Tissue classification based on 3D local intensity structures forvolume rendering

This paper describes a novel approach to tissue classification using three-dimensional (3D) derivative features in the volume rendering pipeline. In conventional tissue classification for a scalar volume, tissues of interest are characterized by an opacity transfer function defined as a one-dimensional (1D) function of the original volume intensity. To overcome the limitations inherent in conventional 1D opacity functions, we propose a tissue classification method that employs a multidimensional opacity function, which is a function of the 3D derivative features calculated from a scalar volume as well as the volume intensity. Tissues of interest are characterized by explicitly defined classification rules based on 3D filter responses highlighting local structures, such as edge, sheet, line, and blob, which typically correspond to tissue boundaries, cortices, vessels, and nodules, respectively, in medical volume data. The 3D local structure filters are formulated using the gradient vector and Hessian matrix of the volume intensity function combined with isotropic Gaussian blurring. These filter responses and the original intensity define a multidimensional feature space in which multichannel tissue classification strategies are designed. The usefulness of the proposed method is demonstrated by comparisons with conventional single-channel classification using both synthesized data and clinical data acquired with CT (computed tomography) and MRI (magnetic resonance imaging) scanners. The improvement in image quality obtained using multichannel classification is confirmed by evaluating the contrast and contrast-to-noise ratio in the resultant volume-rendered images with variable opacity values     

基于多尺度特征点识别与局部谱特征的离散数据匹配

为了实现部分重叠且不同视角的测量数据配准,提出多尺度特征点检测算法,可以从大量的原始数据中提取少量特征点。该算法包括离散曲率计算、双边滤波和特征点计算等步骤,特征点个数可以由尺度参数粗略控制。提出局部形状谱描述器来描述每个特征点的局部形状特性,首先利用局域点的距离和曲率信息构造关系矩阵,然后通过计算关系矩阵的特征值来构造谱描述器,利用该描述器可以方便地计算不同点集中各个特征点的对应关系,进而实现两个数据点集的配准。通过实例验证了该算法有较好的抗噪性和运行速度。