基于圆盘近似的动态场景柔和阴影实时绘制

采用圆盘对物体表面进行近似,对每个圆盘形成的遮挡使用椭圆遮挡域表示,预计算过程中根据每个椭圆遮挡域参数进行采样并用球面调和系数表示,同时将其转换为对数,对采样数据采用主元分析方法进行压缩;在绘制的过程中,根据圆盘的法线、半径以及和阴影接收点之间的相对位置确定其椭圆遮挡域的值,然后采用球面调和指数算法进行累积.该方法将圆盘近似算法和球面调和指数算法相结合,能够灵活地描述柔性物体以及较薄的物体形成的柔和阴影.     

基于圆形参数域和重要性采样的三维模型网格重建

给出了一个基于参数化和重要性采样的网格重建算法.算法搜索一条切线将三维模型表面切分成一片碟形表面,并将其参数化到二维单位圆中.根据模型的曲率信息和参数化的扭曲度生成一张控制图.利用蓝噪声采样算法对用控制图调制后的采样密度采样得到离散采样点.通过对采样点进行平面三角化并将其映射回三维空间实现了三维网格重建.实验证明,该方法有效地改善了参数化的扭曲度,并能得到具有自适应性的高度规则网格.     

分片驱动的特征敏感曲面重建

为健壮处理包含尖锐特征或欠采样的数据点集,通过对基于边界推进曲面重建技术的扩展,提出一种分片驱动的、特征敏感的对无方向散乱数据点集进行曲面重建的算法.在一个光滑阈值的控制下,将曲面重建过程分成分片重建和特征缝合2个阶段.在分片重建中,从光滑的种子三角化区域开始进行边界推进三角化,并通过拓扑元素分类与特征检测对边界光顺和特征重定位,以进一步扩展该分片,重复该过程,得到对光滑区域三角化的一系列光滑分片;特征缝合阶段,在边界推进过程中将所有分离的分片或分片中的缝隙缝合在特征区域.这种两阶段的三角化策略可有效地处理含尖锐特征或不规则采样如不充分采样的点集,无需保证拓扑完整性的复杂数学测试,如协变分析和三角形相交检测等,基于局部光滑曲面的求交,可有效地恢复采样点集丢失的特征信息.实验结果表明,采用文中算法能健壮处理不规则采样点集,并生成特征敏感的高质量网格.     

点模型的几何图像简化法

提出一种基于几何图像的曲率自适应点模型简化算法.首先将点模型的球面极坐标映射到平面上,构造其几何图像;然后利用几何图像确定点模型中点的k-最近邻域及其曲面变分;最后结合曲面变分和简化密度对点集曲面重采样,并通过移动最小二乘曲面评估简化的误差.实验结果表明,该算法执行速度快、易于控制采样密度和保持曲面细节,且能够生成高质量的简化曲面.     

基于GPU的球面深度图实时绘制

提出了一种GPU加速的实时基于图像的绘制算法.该算法利用极坐标系生成对物体全方位均匀采样的球面深度图像;然后根据推导的两个预变换公式将单幅球面深度图像预变换到物体包围球的一个与视点相关的切平面上,以生成中间图像;再利用纹理映射生成最终目标图像.利用现代图形硬件的可编程性和并行性,将预变换移植到Vertex Shader来加快绘制速度;利用硬件的光栅化功能来完成图像的插值,以得到连续无洞的结果图像.此外,还在Pixel Shader上进行逐像素的光照以及环境映射的计算,生成高质量的光照效果.最终,文章解决了算法的视点受限问题,并设计了一种动态LOD(Level of Details)算法,实现了一个实时漫游系统,保持了物体间正确的遮挡关系.     

利用控制顶点插值的光滑B样条曲线构造方法

针对非均匀采样点拟合时的光滑B样条曲线构造问题,提出一种基于已知控制顶点和节点向量求解未知控制顶点来构造光滑B样条曲线的方法.首先对所有控制顶点进行基函数极值参数化,再将已知控制顶点作为型值点进行光滑样条曲线插值,并在此曲线上通过利用参数化结果求值得到未知控制顶点.该方法通过控制顶点所在曲线的光顺性提高最终生成曲线的连...     

基于二元混合有理插值的形状渐变方法

为实现多个多边形间的平滑自然渐变,提出基于二元混合向量值有理插值的非线性二维形状渐变方法。将多个多边形的顶点坐标作为平面域上的向量,利用二元Newton-Thiele型向量连分式建立有理插值曲面,通过对插值曲面进行重采样得到一系列渐变中间多边形。实验结果表明,该方法具有计算精度高、适应性强、易于编程实现的特点。     

基于特定边界的组合保角球面参数化

针对亏格为零的流形三角网格模型,提出一种基于特定边界的组合保角球面参数化方法。该方法采用平均值坐标计算出原始三角网格模型各顶点间相关的权数,然后用这个权数去修正球面域上的顶点坐标得到新的具有保角性质的顶点坐标,再利用Möbius变换把新生成的球面域质心转化为球心,使其面积的变形程度降低,最后得到一个精度更高的初始球面域。实验结果表明该方法是有效的,且具有变形小、效率高的特点。     

基于参数化的三维颅面配准

针对颅面配准问题,提出通过对颅面进行参数化将其转换成二维参数域的对应问题。首先,根据人类的生理特征标定6个特征点,利用这些特征点将颅面转换到一个统一的坐标系以实现姿态和大小的统一;其次,以两个外眼角为约束对参考颅面进行最小二乘保角映射,计算出6个特征点的参数值;然后,以这六个生理特征点的参数值为约束,利用最小二乘保角映射将任一待配准模型映射到二维参数域;最后,根据二维参数域确定三维颅面上的对应点,从而实现三维数据配准。为了验证所提方法,以对应点为控制点,利用薄板样条(TPS)变换把参考颅面变形到目标颅面,以变形后两个模型上对应点之间的几何距离的平均为度量,将所提算法和基于主轴分析的迭代最近点(ICP)配准以及基于随机采样控制点的迭代TPS配准方法进行了比较,实验结果表明,所提算法的配准效果优于其他两种方法。     

对偶Voronoi聚类与重网格化

为了以更快的速度得到高质量的多分辨率网格,提出一种基于Voronoi-Delaunay三角化技术的多分辨率表示生成算法.该算法将原三角网格转化为对偶多边形网格再进行Voronoi划分,以自动满足共点聚类块不能超过3个这一约束;根据曲率分布情况来选取基点,以便能更好地捕捉几何特征;最后利用Loop细分规则与局部Laplace 平滑指导参数域上的重采样,再映射回模型空间获取最终采样结果,以提高重采样质量.由于Voronoi划分是重网格化算法的瓶颈,采用文中算法能减少划分时条件检测的耗时,从而显著地降低整个重网格化算法的时间复杂度.     

Curvature-aware adaptive re-sampling for point-sampled geometry

With the emergence of large-scale point-sampled geometry acquired by high-resolution 3D scanning devices, it has become increasingly important to develop efficient algorithms for processing such models which have abundant geometric details and complex topology in general. As a preprocessing step, surface simplification is important and necessary for the subsequent operations and geometric processing. Owing to adaptive mean-shift clustering scheme, a curvature-aware adaptive re-sampling method is proposed for point-sampled geometry simplification. The generated sampling points are non-uniformly distributed and can account for the local geometric feature in a curvature aware manner, i.e. in the simplified model the sampling points are dense in the high curvature regions, and sparse in the low curvature regions. The proposed method has been implemented and demonstrated by several examples.     

High frequency geometric detail manipulation and editing for point-sampled surfaces

In this paper, based on the new definition of high frequency geometric detail for point-sampled surfaces, a new approach for detail manipulation and a detail-preserving editing framework are proposed. Geometric detail scaling and enhancement can always produce fantastic effects by directly manipulating the geometric details of the underlying geometry. Detail-preserving editing is capable of preserving geometric details during the shape deformation of point-sampled model. For efficient editing, the point set of the model is first clustered by a mean shift scheme, according to its anisotropic geometric features and each cluster is abstracted as a simplification sample point (SSP). Our editing operation is implemented by manipulating the SSP first and then diffusing the deformation to all sample points on the underlying geometry. As a postprocessing step, a new up-sampling and relaxation procedure is proposed to refine the deformed model. The effectiveness of the proposed method is demonstrated by several examples.     

点模型的多分辨率形状编辑

提出了一种基于几何细节映射的点模型的形状编辑方法.几何细节是曲面的一个重要属性,定义几何细节为原始曲面及其基曲面之间的向量差,该基曲面由多层次B样条所构成.通过基曲面上的局部仿射坐标,则可以得到与之对应的多分辨率几何细节表示,曲面的低频信息和高频信息易被用户所指定的频段分离.通过调节基曲面的形状,再将这些几何细节映射上去,可以对模型进行保细节的变形;如果将几何细节映射到其他物体上,将可以得到几何细节迁移的结果.为点模型开发了多种特征保持的编辑算子,实验结果表明,所提出的方法是一种有效的点模型造型算法.     

A unified method for appearance and geometry completion of point set surfaces

This paper presents a novel approach for appearance and geometry completion over point-sampled geometry. Based on the result of surface clustering and a given texture sample, we define a global texture energy function on the point set surface for direct texture synthesis. The color texture completion is performed by minimizing a constrained global energy using the existing surface texture on the surface as the input texture sample. We convert the problem of context-based geometry completion into a task of texture completion on the surface. The geometric detail is then peeled and converted into a piece of signed gray-scale texture on the base surface of the point set surface. We fill the holes on the base surface by smoothed extrapolation and the geometric details over these patches are reconstructed by a process of gray-scale texture completion. Experiments show that our method is flexible, efficient and easy to implement. It provides a practical texture synthesis and geometry completion tool for 3D point set surfaces.     

Detail-generating geometry completion for point-sampled geometry

In this paper, we present a novel method for detail-generating geometry completion over point-sampled geometry. The main idea consists of converting the context-based geometry completion into the detail-based texture completion on the surface. According to the influence region of boundary points surrounding a hole, a smooth patch covering the hole is first constructed using radial base functions. By applying region-growing clustering to the patch, the patching units for further completion with geometry details is then produced, and using the trilateral filtering operator formulated by us, the geometry-detail texture of each sample point on the input geometry is determined. The geometry details on the smooth completed patch are finally generated by optimizing a constrained global texture energy function on the point-sampled surfaces. Experimental results demonstrate that the method can achieve efficient completed patches that not only conform with their boundaries, but also contain the plausible 3D surface details.     

点模型的多边滤波器降噪算法

为了更好地去除噪声,并保持模型的突出特征,提出了一种鲁棒的点模型多边滤波器降噪算法,该算法充分考虑了模型表面的法向量、曲率等内蕴几何量和噪声之间的关系。首先通过自适应选取最优邻域控制函数来将滤波窗口限制在顶点法向量相近的区域,以防止滤波后模型的收缩和过光顺;然后运用协方差矩阵分析的方法,在最优邻域内计算出各采样点的法向量和曲率;最后以采样点滤波参考平面为基准,分别平滑采样点的法向量和位置,即先对采样点的法向量进行多边平滑,然后根据新的法向量来多边平滑输出各采样点的位置偏移量,最后在法向方向上移动该顶点,以达到降噪的目的。实验结果表明,多边滤波器在有效地去除噪声的同时,还能较好地保持点模型表面的几何特征。     

细节保持的曲面浅浮雕算法

为保持曲面浮雕的细节特征,改善浮雕与背景曲面的过渡效果,提出一种细节保持的曲面浮雕算法.首先采用Canny算子在梯度域定位内外轮廓,获得连续的梯度域;其次提出非线性函数,并用其压缩梯度幅值实现形状压缩;再通过双边滤波算子来保持和增强浮雕细节;最终通过求解积分方程重建曲面浮雕.该算法将背景曲面作为积分方程的优化条件,使浮雕在轮廓处向曲面光滑过渡;在重建过程中利用Fourier变换的微分性质在频域实现方程的精确求解,无需设置迭代收敛条件;通过调整参数可方便地控制浮雕整体变化范围、细节锐化程度以及浮雕与背景曲面的过渡效果.实验结果表明,文中算法参数的几何意义直观,所得曲面浮雕细节清晰,边界过渡自然.     

一般点模型的交互式布尔运算

提出了一个适用于一般点模型的交互式布尔运算算法，此算法由4个步骤组成．首先将点模型表示为自适应的三色八叉树，然后利用自适应八叉树结构加速内外测试．对于局部采样密度不一致的相交区域或曲率太大容易导致较大求交误差的地方，实行了自适应细分加密采样；重采样相交的部分以获得更精确的求交结果．与已有的点模型布尔运算方法相比，该算法适用于一般的实测点云数据，包括少量噪声的点模型、非均匀采样以及不同分辨率点模型之间的交互式布尔运算。     

Structure-Preserving Reshape for Textured Architectural Scenes

Modeling large architectural environments is a difficult task due to the intricate nature of these models and the complex dependencies between the structures represented. Moreover, textures are an essential part of architectural models. While the number of geometric primitives is usually relatively low (i.e., many walls are at surfaces), textures actually contain many detailed architectural elements.
We present an approach for modeling architectural scenes by reshaping and combining existing textured models, where the manipulation of the geometry and texture are tightly coupled. For geometry, preserving angles such as oor orientation or vertical walls is of key importance. We thus allow the user to interactively modify lengths of edges, while constraining angles. Our texture reshaping solution introduces a measure of directional autosimilarity to focus stretching in areas of stochastic content and to preserve details in such areas.
We show results on several challenging models, and show two applications: Building complex road structures from simple initial pieces and creating complex game-levels from an existing game based on pre-existing model pieces.