基于视点的分块LOD大规模地形生成算法研究

针对虚拟战场环境仿真中构建大规模地形的速度问题,介绍了一种基于视点和分块的LOD大规模地形生成算法,能够快速地构建海量数据的地形模型.该算法首先对海量地形数据分块后以四叉树结构存储;然后应用简单的圆锥视景体快速裁剪地形块,丢弃与视景体相离的地形块,对于与视景体相交地形块剪裁出位于视景体内的四叉树结点.根据视点到节点距离和地形本身的粗糙度,选用合适的LOD渲染地形,最后应用基于视点的方法消除裂缝.在数据相同情况下,该算法比四叉树算法减少约1/2时间,并且更逼真地生成了大规模数据地形.本算法也可用于逆向工程设计中重建物体等问题.     

基于多分辨率数据源镶嵌的大规模地形实时绘制算法

针对不同分辨率的测量地形数据源,提出了镶嵌四叉树模型,用以实现对多分辨率数据源的镶嵌,以及与细节层次(LOD)算法的融合.设计了基于此模型的批LOD算法,给出了相关公式,并设计了一种从CPU到GPU的渐进LOD数据传输方法.分析了镶嵌四叉树的场景浏览模式,设计了基于此模型的双向两级外存数据调度算法,通过水平、高度两个方向的预测和两级数据预取实现数据页的动态更新,实现了三维地形场景的快速构建和实时浏览.实验表明,该算法能够快速处理大规模多分辨率数据源并且能够充分发挥CPU、GPU与I/O的效率,从而在有限环境下进行快速实时绘制.     

基于四叉树的三维地形绘制算法

针对当前三维地形虚拟现实研究中存在的难点,参考比较国内外专家提出的多种模型算法,对其中应用最广泛的层次细节模型（LOD）改进。以四叉树结构为基础组织地形数据,采用视点参数和地形本身粗糙程度两种因素相结合构造节点评价算法,并给出算法实现及其网络扩展方案。实验结果证明,该算法在基本保持视觉效果不变的情况下,极大地提高了虚拟地形场景的绘制速度。     

四叉树网格下水动力及物质输运数值模拟研究进展

为了掌握水体的水流变化及污染物质的输运规律及精确模拟、复演和预测水环境质量时空变化过程,基于四叉树网格生成技术,系统梳理了四叉树网格在水动力和物质输运模拟方面的研究进展及实际应用,总结了现阶段四叉树网格研究过程中存在的不足,并展望了未来的研究方向,为湖泊、河口及近海工程中流场和物质输运的模拟提供了参考。     

Efficient Window Block Retrieval in Quadtree-Based Spatial Databases

An algorithm is presented to answer window queries in a quadtree-based spatial database environment by retrieving all of the quadtree blocks in the underlying spatial database that cover the quadtree blocks that comprise the window. It works by decomposing the window operation into sub-operations over smaller window partitions. These partitions are the quadtree blocks corresponding to the window. Although a block b in the underlying spatial database may cover several of the smaller window partitions, b is only retrieved once rather than multiple times. This is achieved by using an auxiliary main memory data structure called the active border which requires O(n) additional storage for a window query of size n×n. As a result, the algorithm generates an optimal number of disk I/O requests to answer a window query (i.e., one request per covering quadtree block). A proof of correctness and an analysis of the algorithm's execution time and space requirements are given, as are some experimental results.     

Bezier曲面的适应性细分和三角形化的四叉树方法

计算机生成具有浓淡的参数曲面的方法之一是先对曲面进行适应性细分,并对所得到的曲面细分三角形化,得到曲面的三角形网表示,从而可以对每个三角形施行通常的浓淡处理算法。本文介绍了适应性细分双三次Bezier曲面的方法及曲面细分的四叉树表示,在此基础上给出了一个将曲面细分三角形化的算法。该算法防止了由于适应性细分而可能产生的曲面上的裂缝。     

基于自适应四叉树分形图像编码的数字水印技术

提出了一种基于自适应四叉树分形编码的数字水印技术。对原始图像进行四叉树分形编码后，通过给定子块在不同的寻找区域中搜索其最佳匹配父块来嵌入水印。实验结果表明，本方法在嵌入水印编码时间、图像质量和鲁棒性方面均比传统的方法有较大的改进。     

A linear quadtree compression scheme for image encryption

A private key encryption scheme for a two-dimensional image data is proposed in this work. This scheme is designed on the basis of lossless data compression principle. The proposed scheme is developed to have both data encryption and compression performed simultaneously. For the lossless data compression effect, the quadtree data structure is used to represent the image; for the encryption purpose, various scanning sequences of image data are provided. The scanning sequences comprise a private key for encryption. Twenty four possible combinations of scanning sequences are defined for accessing four quadrants, thereby making available 24ⁿ × 4^{n(n − 1)/2} possibilities to encode an image of resolution 2ⁿ × 2ⁿ. The security of the proposed encryption scheme therefore relies on the computational infeasibility of an exhaustive search approach. Three images of 512 × 512 pixels are used to verify the feasibility of the proposed scheme. The testing results and analysis demonstrate the characteristics of the proposed scheme. This scheme can be applied for problems of data storage or transmission in a public network.     

以数字搜索树表示的二值图象的几何变换

本文介绍以数字搜索树表示的二值图象的一种几何变换算法，文中讨论了四分区与正四边区的基本关系，输出ＤＳＴ结点成立的充分条件，给出了完整的几何变换算法并对算法的复杂性作了分析。