支持森林场景动态生成的树木生长模拟

为了解决传统L文法中树木建模困难和运行效率低的问题,提出了支持森林场景动态生成的树木生长模拟方法.该方法基于分形元建立树木模型,在此基础上对大规模树木实体实例化的简化算法进行研究,将基于视点的多分辨率简化模型融入树木建模方法之中,并选择典型树种进行生长模拟.实验结果表明,该方法能较真实地模拟树木的生长过程,并实现在虚拟森林场景中的实时漫游.     

一种基于环境变化的树木动态生长仿真模型

研究虚拟植物生长的仿真应用较广.针对数字娱乐领域中用于虚拟场景的植物形态-功能仿真的有限需求,为了提高真实性,提出了一种通用的基于环境变化的虚拟树木动态生长的仿真模型,来仿真各种树木的生长过程.采用L系统描述树木的拓扑结构,结合植物学相关知识,引入了环境因素对树木生长的影响,将树木的生物学特性进行抽象与加权处理,建立了树木生长与光照信息交换的简洁算法.仿真实验结果表明,该模型能较好地反映了环境因素变化对植物生长的影响,较真实地仿真再现了树木的动态生长过程.     

面向GIS的动态树木建模研究

树木是动态的实体,具有生长及四季变化的特性.目前,面向GIS的树木建模主要采用贴图法等静态的建模方法,无法表现树木的动态特性.基于时变L系统提出了一种面向GIS的动态树木模型.该模型能模拟树木不同的生长阶段及四季变换,并采用存储L系统公式及参数的方法,存储量小,适合GIS环境下对树木有动态变化的应用需求.     

线面拓扑和度量关系的细分描述和计算方法

由于单独的拓扑关系、度量关系无法区分复杂要素之间的空间关系,在语义描述上存在歧义,提出一种二维空间R~2下空间关系的细分表达模型.该模型将面与线求交后,线面可以表示成n-1个拓扑和度量分量的序列矩阵,在语义上进行更细致的分段描述;采用折线节点微小圆求交的算法,考虑面带洞和目标的不确定性,归纳出线面拓扑关系细分判断的7个规则,以及长度、分割度等度量项的计算方法.最后以土地利用中图斑扣除线状地物计算地类面积为实例,验证了该模型和计算方法对解决实际问题的有效性.     

基于空间分解的三角网格模型求交方法

三角网格模型间的求交问题是计算机辅助设计与制造领域的关键问题之一.分析了已有空间分解技术和包围盒方法等的优缺点,并在此基础上提出了改进的空间分解求交方法. 该算法能够精确定位三角形所占的空间网格,一定程度上减少了需要求交的三角形的数量.实验的结果表明,改进的算法提高了采用空间分解进行三角网格求交的计算效率.     

面向大规模重复结构CAD模型的快速截面可视化方法

截面可视化是揭示复杂三维CAD模型内部结构的有效方法,但目前基于面片求交的截面可视化方法计算量大,难以实现大规模CAD模型的交互可视化,为此提出一种结合面片求交和3D纹理的混合截面交互可视化方法.针对由少量基本对象按规律摆列而成的规则CAD模型-重复结构CAD模型,在预处理阶段,将所有基本对象离散成高精度3D纹理;在可视化对象选取阶段,通过截面可视化裁剪算法选择可视化对象;在截面可视化阶段,通过预估每个对象在屏幕的投影情况选择可视化方法.该方法基于中国科学院核能安全技术研究所FDS团队自主研发的超级蒙卡核模拟软件系统SuperMC进行研究,并使用全堆芯模型以及铅基反应堆模型进行测试,结果表明,该方法视觉效果接近基于面片求交的方法,同时实现了交互可视化.     

基于面积约束的大豆叶片三维建模方法

针对基于图像对大豆叶片进行三维真实感建模的问题,考虑叶面积指数对大豆产量的影响,提出一种受面积约束的三维叶片建模方法.该方法首先从二维图像中提取出叶片特征点和叶片面积,然后采用双三次均匀B样条曲面插值特征点进行建模,并且根据叶片边缘卷曲形态对网格模型进行变形,构造叶片卷曲模型,分别计算不同卷曲模型的最大光照面积,从而选择最优的模型.实验证明,该方法能构造出逼真的叶片模型,并且具有一定的生物学意义和实际应用价值.     

基于视频的树木运动信息提取和动画生成技术研究

为了快速模拟树木在风中的运动和形变,提出了一种基于视频的树木运动信息提取和动画生成方法,可以在虚拟场景中快速绘制具有动态效果的三维树木模型.该方法首先将含有树木运动的视频转化为多个关键帧,以提取树木的形态和运动信息,并通过数据最小二乘拟合方法计算出其运动轨迹,然后对其进行三维模型重建和运动模拟.实验表明,该方法可以快速地建立树木的三维模型,并能较好地模拟树木的运动和形变.     

遮荫环境下的植物生长建模及仿真

为了解决遮荫环境下的植物对光照资源的竞争和分配问题,更真实地仿真自然界中植物的生长情况,而采用了一种基于FON(field-of-neighbourhood)模型的植物生长建模及仿真的方法.方法用FON模型描述植物对光照资源的需求范围,引入植物学中遮荫率因子作为影响光照强度的一个因素,并提出了一种基于计算阴影重叠区域面积的光照资源分配策略.在实现过程中建立了一个支持植物共享光照资源的生长仿真系统,给出了不同竞争地位的植物在共享光照资源上所表现出的生长情况的可视化结果,对计算机仿真植物生长以及指导农林生产方面都具有积极意义.     

基于空间索引与碰撞检测的TIN求交算法

针对三维矿床地质模型构建及后续应用分析中的需求,提出一种基于空间索引与碰撞检测的不规则三角网(TIN)快速求交算法。通过建立TIN模型的空间格网索引,将相交测试与计算限定在映射于同一个空间格网单元内的三角形对之间,在求交计算过程中,应用包围盒碰撞检测方法快速剔除不相交三角形对,并分别采用边-面及改进的边-边求交算法计算异面与共面三角形交线,并根据交线段之间的空间邻接关系完成交线的快速分离。实验及应用结果表明,该算法效率高、运行稳定、计算结果可靠,能够满足大规模TIN快速求交计算的需要。     

Applying segmented right-deep trees to pipelining multiple hashjoins

The pipelined execution of multijoin queries in a multiprocessor-based database system is explored in this paper. Using hash-based joins, multiple joins can be pipelined so that the early results from a join, before the whole join is completed, are sent to the next join for processing. The execution of a query is usually denoted by a query execution tree. To improve the execution of pipelined hash joins, an innovative approach to query execution tree selection is proposed to exploit segmented right-deep trees, which are bushy trees of right-deep subtrees. We first derive an analytical model for the execution of a pipeline segment, and then, in the light of the model, we develop heuristic schemes to determine the query execution plan based on a segmented right-deep tree so that the query can be efficiently executed. As shown by our simulation, the proposed approach, without incurring additional overhead on plan execution, possesses more flexibility in query plan generation, and can lead to query plans of better performance than those achievable by the previous schemes using right-deep trees     

Delineation of individual deciduous trees in plantations with low-density LiDAR data

Delineation of individual deciduous trees with Light Detection and Ranging (LiDAR) data has long been sought for accurate forest inventory in temperate forests. Previous attempts mainly focused on high-density LiDAR data to obtain reliable delineation results, which may have limited applications due to the high cost and low availability of such data. Here, the feasibility of individual deciduous tree delineation with low-density LiDAR data was examined using a point-density-based algorithm. First a high-resolution point density model (PDM) was developed from low-density LiDAR point cloud to locate individual trees through the horizontal spatial distribution of LiDAR points. Then, individual tree crowns and associated attributes were delineated with a 2D marker-controlled watershed segmentation. Additionally, the PDM-based approach was compared with a conventional canopy height model (CHM) based delineation. The results demonstrated that the PDM-based approach produced an 89% detection accuracy to identify deciduous trees in our study area. The tree attributes derived from the PDM-based algorithm explained 81% and 83% of tree height and crown width variations of forest stands, respectively. The conventional CHM-based tree attributes, on the other hand, could explain only 71% and 66% of tree height and crown width, respectively. Our results suggest that the application of the PDM-based individual tree identification in deciduous forests with low-density LiDAR data is feasible and has relatively high accuracy to predict tree height and crown width, which are highly desired in large-scale forest inventory and analysis.     

基于SPRINT方法的并行决策树分类研究

决策树技术的最大问题之一就是它的计算复杂性和训练数据的规模成正比,导致在大的数据集上构造决策树的计算时间太长。并行构造决策树是解决这个问题的一种有效方法。文中基于同步构造决策树的思想,对SPRINT方法的并行性做了详细分析和研究,并提出了进一步研究的方向。     

An efficient algorithm for constructing delay bounded minimum cost multicast trees

Multimedia applications are usually resource intensive, have stringent quality of service requirements, and in many cases involve large groups of participants. Multicasting is poised to play an important role in future deployment of these applications. This paper focuses on developing delay-bounded, minimum-cost multicast trees, linking a source to a set of multicast destination nodes. The approach taken in this paper is efficient, flexible and unique in the sense that it cleverly limits its computation only to paths that originate at multicast nodes, thereby avoiding computing paths that exclusively link non-multicast nodes. The simulation results show that the multicast trees produced by the proposed heuristic are of lower cost than those produced by other well-known heuristics, including those which use an expensive k-shortest-paths procedure to minimize the cost of the multicast tree. Furthermore, the results show that, in comparison to other heuristics, the proposed scheme results in a significant reduction in the computation time required to build the multicast tree.     

The Comparison of Urban Tree Crown Extraction based on Airborne LiDARElevation Difference and High Resolution Imagery

With the support of airborne Light Detection and Ranging (LiDAR) data and high spatial resolution aerial imagery,this paper presents an individual tree extraction method that takes the region of urban as the study area.The elevation difference model originated from LiDAR data was used to extract regions of interest including trees. Then,masking was applied to the high spatial resolution aerial imagery to get the same regions. Besides,image segmentations,based on the marked watershed algorithm,were processed on the high spatial resolution aerial imagery and the elevation difference model separately to extract individual tree crowns. Finally,we took a visual interpretation to delineate tree crowns manually and this result was regarded as the reference crowns map. The extraction accuracies were assessed by comparing the spatial relationships of the reference crowns and the automated delineated tree crowns based on the elevation difference model and the high resolution imagery. The results show that the LiDAR data is developed to improve the efficiency of obtaining forest region that the canopy height model include 85.25% forest information. In addition,the tree crowns extraction accuracy based on the high resolution aerial imagery is 57.14%,while another extraction accuracy based on the elevation difference model is 42.47%,which indicated that the marked watershed algorithm proposed in this paper is effective and the high resolution imagery is better than the elevation difference model to extract tree crowns.     

Tree Growth Modelling Constrained by Growth Equations

Modelling and simulation of tree growth that is faithful to the living environment and numerically consistent to botanic knowledge are important topics for realistic modelling in computer graphics. The realism factors concerned include the effects of complex environment on tree growth and the reliability of the simulation in botanical research, such as horticulture and agriculture. This paper proposes a new approach, namely, integrated growth modelling, to model virtual trees and simulate their growth by enforcing constraints of environmental resources and tree morphological properties. Morphological properties are integrated into a growth equation with different parameters specified in the simulation, including its sensitivity to light, allocation and usage of received resources and effects on its environment. The growth equation guarantees that the simulation procedure numerically matches the natural growth phenomenon of trees. With this technique, the growth procedures of diverse and realistic trees can also be modelled in different environments, such as resource competition among multiple trees.     

基于多词汇树的对话语音识别搜索策略

在查询类对话语音识别系统中,词汇中的少数几类词往往占有词汇量的很大部分,利用这一重要特点,可以对传统的基于单树结构的搜索算法进行改进。本文在采用基于类的语言模型的条件下,在查询类对话语音识别系统中,对词汇中的若干大类单独建树,从而免除了多数词汇的语言模型look-ahead计算时间和存储空间,并实现了语言分的提前精确运用。针对机票预订口语对话系统的实验表明,该搜索策略与单树方法相比,在同样的计算资源条件下,时间效率有显著提高。     

基于均值场计算树的Ising图模型消息族传播算法

提出基于均值场计算树的Ising图模型消息族传播算法。首先定义Ising图模型均值场计算树和均值场剪枝计算树概念来描述Ising图模型均值场推理方法的迭代计算过程。然后基于均值场计算树设计Ising图模型消息族传播算法,指出沿着计算树自底向上逐层进行消息族传播,可计算根节点变量的边缘概率分布族。同时证明基于均值场剪枝计算树的消息族传播算法可计算出变量边缘概率分布的界,即此时的边缘概率分布族包括边缘概率分布精确值。最后通过数值实验验证消息族传播算法的有效性和边缘概率分布界的紧致性。     

Comparing and aggregating partially resolved trees

Partially-resolved-that is, non-binary-trees arise frequently in the analysis of species evolution. Non-binary nodes, also called multifurcations, must be treated carefully, since they can be interpreted as reflecting either lack of information or actual evolutionary history. While several distance measures exist for comparing trees, none of them deal explicitly with this dichotomy. Here we introduce two kinds of distance measures between rooted and unrooted partially-resolved phylogenetic trees over the same set of species; the measures address multifurcations directly. For rooted trees, the measures are based on the topologies the input trees induce on triplets; that is, on three-element subsets of the set of species. For unrooted trees, the measures are based on quartets (four-element subsets). The first class of measures are parametric distances, where there is a parameter that weighs the difference between an unresolved triplet/quartet topology and a resolved one. The second class of measures are based on the Hausdorff distance, where each tree is viewed as a set of all possible ways in which the tree can be refined to eliminate unresolved nodes. We give efficient algorithms for computing parametric distances and give conditions under which Hausdorff distances can be calculated approximately in polynomial time. Additionally, we (i) derive the expected value of the parametric distance between two random trees, (ii) characterize the conditions under which parametric distances are near-metrics or metrics, (iii) study the computational and algorithmic properties of consensus tree methods based on the measures, and (iv) analyze the interrelationships among Hausdorff and parametric distances.     

基于小波方法的数据流查询计算研究与应用

许多领域中大量应用所产生的数据流的处理已成为聚集数据处理的一个重要方面。文章在对数据流查询计算进行深入研究的基础上,分析了该情形下聚集查询语言与重写的特征,给出了一类有效的快速查询计算模型。该计算模型的核心是运用小波分析方法,通过建立有效的小波提纲来提高查询处理的效率。最后,在实际应用环境下对所给出的模型进行了应用分析。