三次均匀B样条曲线的一种新扩展

构造了一组带形状参数的三次B样条曲线,该曲线与经典三次B样条曲线具有相同的基本性质,且可在不改变控制顶点的情况下,通过改变形状参数的取值实现对曲线形状的调整;选取适当的控制顶点,并对形状参数选取适当的取值,构造的三次λ-B样条曲线可以很好的逼近圆和椭圆;提供了插值于已知数据点的λ-B样条曲线的构造方法;最后,通过图例体现了新方法的有效性.     

重型值点阵的样条插值统一求解算法

统一求解算法从分析重型值点对控制顶点的影响入手,合理修改插值样条控制顶点方程组的系数矩阵,无需以型值点为界分段求解,而是一次性求出所有控制顶点,比分段求解算法简单,并引入了光顺因子,使曲线曲面在重型值点处的光顺程度可灵活控制,利用该算法还能构造出夹尖点、棱角以及平面片的复杂曲面.     

保凸插值样条曲线

1 引言 曲线设计是设计工程的重要课题。用向量样条,可使曲线经过型值点且有很好的逼近性质,但没有保凸性;Bzier曲线,B-样条曲线等一类向量线性正算子,有很好保凸性,但一般只通过首尾两个型值点。在实际设计中提出这样的问题:能否使所作曲线既过型值点,又有保凸性呢?大家知道,逼近论与样条理论最基础的是研究一元函数逼近与插值,所以[8][9]很自然地推广至逼近与插值向量值函数F(t):[a,b]→X(X为Banach空间)。但是,[8][9]的方法不能用于把平面曲线逼近与拟合算法[2—7]向多维推广。     

Bézier曲线降多阶逼近的一种方法

文献[1,2]讨论了Bezier曲线一次降多阶逼近问题,得到了很好的结果.文献[1]利用广义逆矩阵得到不保端点插值的降多阶逼近曲线的控制顶点的表达式.但却没有得到带端点任意阶插值条件的降多阶逼近曲线的控制顶点的表达式.文献[2]得到了带端点任意阶插值的降多阶逼近曲线的控制顶点的解析表达式.本文首先给出两Bezier曲线间距离的定义;然后根据降阶曲线与原曲线间的距离最小,分别得到了用矩阵表示的不保端点插值和保端点任意阶插值的降多阶逼近曲线的控制顶点的显示表达式.所给数值例子显示,用本文方法得到的降多阶逼近曲线对原曲线有很好的逼近效果.     

一类非端点插值B样条曲线降阶的方法

降阶算法是B样条曲线和曲面设计的一个基本算法,它广泛应用于组合曲线,蒙皮或扫描曲面等设计中.Piegl与Tiller曾给出B样条曲线的降阶方法.本文给出了解决更一般的非端点插值B样条曲线降阶的方法.新的方法主要是通过对现有的节点插入方法进行分析,给出了一种端点插值递推公式,并利用此公式对Piegl与Tiller降阶方法加以改进,使之能够解决非端点插值均匀及非均匀B样条曲线的降阶问题.     

有理曲线的多项式逼近

利用曲线摄动的思想给出了用多项式曲线逼近有理曲线的一种新方法．其基本步骤是对有理曲线的控制顶点进行摄动，使之产生一多项式曲线，并使摄动误差在某种范数意义之下达到最小．同时，通过适当控制摄动曲线的顶点，使逼近多项式曲线与有理曲线在两端点保持一定的连续性．这一结果可以与细分（ｓｕｂｄｉｖｉｓｉｏｎ）技术结合给出有理曲线的整体光滑的分片多项式逼近．实例表明，在某些情况下本文中的方法要优于传统的Ｈｅｒｍｉｔｅ插值方法及Ｔ．Ｗ．Ｓｅｄｅｒｂｅｒｇ和Ｍ．Ｋａｋｉｍｏｔｏ（１９９１）提出的杂交曲线逼近算法．     

关于两类圆弧插值样条

[1]中考察了两类圆弧插值样条,我们依次简称为C~0类与C~1类。本文指出,C~0类圆弧插值样条与C~1类比较,虽然光滑性差,但是逼近阶一般较好。对于这两类样条,本文都给出了比较精确的逼近度。 一、C~0类圆弧插值样条 设平面上的曲线段T与圆弧样条S分别由n个曲线段T_1,…,T_n与n个圆弧S_1,…,S_n组成,其中T_i与S_i均由P_(2i-2)点出发,经过P_(2i-1)点而至P_(2i)点(i=1,…,n)。当该曲线段T(或该点列P_0,P_1,…,P_(2n))确定时,该圆弧样条S显然唯一确定。这时,我们称该     

一类新的(2n-1)点二重动态逼近细分

利用正弦函数构造了一类新的带有形状参数ω的(2n-1)点二重动态逼近细分格式.从理论上分析了随n值变化时这类细分格式的C~k连续性和支集长度;算法的一个特色是随着细分格式中参数ω的取值不同,相应生成的极限曲线的表现张力也有所不同,而且这一类算法所对应的静态算法涵盖了Chaikin,Hormann,Dyn,Daniel和Hassan的算法.文末附出大量数值实例,在给定相同的初始控制顶点,且极限曲线达到同一连续性的前提下和现有几种算法做了比较,数值实例表明这类算法生成的极限曲线更加饱满,表现力更强.     

局部形状可调整的三次有理B样条插值曲线

利用三次非均匀有理B样条,给出了一种构造局部插值曲线的方法,生成的插值曲线是C2连续的.曲线表示式中带有一个局部形状参数,随着一个局部形状参数值的增大,所给曲线将局部地接近插值点构成的控制多边形.基于三次非均匀有理B样条函数的局部单调性和一种保单调性的准则,给出了所给插值曲线的保单调性的条件.     

一种C~1连续的二元三次有理插值样条及其若干性质

<正>1引言曲线与曲面的构造方法及其数学描述是CAGD中的一个重要课题.已有许多方法[1-13]来研究这一问题,如多项式样条方法,Bezier样条方法及NURBS方法.然而大多数多项式方法都是插值方法,当插值数据给定时,局部插值曲面的形状无法进行修改.NURBS方法与Bezier方法是非插值方法,即所构建的曲线、曲面并不满足给定的插值数据,其中给定的点是控制顶点.因此,在构造CAGD中的插值函数时,需要考虑下面两种情形:1)     

Bézier曲线和B样条曲线光顺拟合法

§1.引言 在计算机辅助几何设计(CAGD)工作中,适用于曲线造型的方法主要有样条函数、Bezier曲线和B样条曲线等。在实际工作中,几何外形设计又大致可以分成两类: (1)从头设计。按照给定的几个原始设计参数,决定曲线的特征多边形顶点,继而决定曲面的特征网格。在[1],[2]中所作的叶片和船体曲面造型,就是一种从头设计方案。 (2)模型设计。例如,传统的汽车车身设计,首先由美工师塑造一只车身的油泥模     

基于离散点和法矢的曲线重构算法

This paper presents a curve reconstruction algorithm based on discrete data points and normal vectors using B-splines.The proposed algorithm has been improved in three steps:parameterization of the discrete data points with tangent vectors,the B-spline knot vector determination by the selected dominant points based on normal vectors,and the determination of the weight to balancing the two errors of the data points and normal vectors in fitting model.Therefore,we transform the B-spline fitting problem into three sub-problems,and can obtain the B-spline curve adaptively.Compared with the usual fitting method which is based on dominant points selected only by data points,the B-spline curves reconstructed by our approach can retain better geometric shape of the original curves when the given data set contains high strength noises.     

NURBS曲线曲面拟合数据点的迭代算法

本文推广了文献[1]的结果,将文献[1]中关于B样条曲线曲面拟合数据点的迭代算法推广至有理形式,给出了无需求解方程组反求控制点及权因子即可得到拟合NURBS曲线曲面的迭代方法．该算法和文献[1]的算法本质上是统一的,而后者恰是前者的一种退化形式．文章还给出了收敛性证明以及一些定性分析．文末的数值实例说明该算法简单实用．     

Curve fitting with a Sobolev gradient method

Consider the problem of constructing a mathematical representation of a curve that satisfies constraints such as interpolation of specified points. This problem arises frequently in the context of both data fitting and Computer Aided Design. We treat the most general problem: the curve may or may not be constrained to lie in a plane; the constraints may involve specified points, tangent vectors, normal vectors, and/or curvature vectors, periodicity, or nonlinear inequalities representing shapepreservation criteria. Rather than the usual piecewise parametric polynomial (B-spline) or rational (NURB) formulation, we represent the curve by a discrete sequence of vertices along with first, second, and third derivative vectors at each vertex, where derivatives are with respect to arc length. This provides third-order geometric continuity and maximizes flexibility with an arbitrarily large number of degrees of freedom. The free parameters are chosen to minimize a fairness measure defined as a weighted sum of curve length, total curvature, and variation of curvature. We thus obtain a very challenging constrained optimization problem for which standard methods are ineffective. A Sobolev gradient method, however, is particularly effective. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

B-spline curve fitting with invasive weed optimization

B-spline curves and surfaces are generally used in computer aided design (CAD), data visualization, virtual reality, surface modeling and many other fields. Especially, data fitting with B-splines is a challenging problem in reverse engineering. In addition to this, B-splines are the most preferred approximating curve because they are very flexible and have powerful mathematical properties and, can represent a large variety of shapes efficiently [1]. The selection of the knots in B-spline approximation has an important and considerable effect on the behavior of the final approximation. Recently, in literature, there has been a considerable attention paid to employing algorithms inspired by natural processes or events to solve optimization problems such as genetic algorithms, simulated annealing, ant colony optimization and particle swarm optimization. Invasive weed optimization (IWO) is a novel optimization method inspired from ecological events and is a phenomenon used in agriculture. In this paper, optimal knots are selected for B-spline curve fitting through invasive weed optimization method. Test functions which are selected from the literature are used to measure performance. Results are compared with other approaches used in B-spline curve fitting such as Lasso, particle swarm optimization, the improved clustering algorithm, genetic algorithms and artificial immune system. The experimental results illustrate that results from IWO are generally better than results from other methods.     

一种Logistic模型参数估计的新方法及应用

介绍了logistic曲线参数估计的一种新方法,它是利用三次样条插值函数求导代替logistic曲线在这一点的导数值,进而利用最小二乘法得出参数的估计值,通过实例分析表明本文提出的方法比一般的三点法估计的参数值k再用线性化方法估计的参数值b,c,拟合精度更高.     

Determining knots by optimizing the bending and stretching energies

For a given set of data points in the plane, a new method is presented for computing a parameter value (knot) for each data point. Associated with each data point, a quadratic polynomial curve passing through three adjacent consecutive data points is constructed. The curve has one degree of freedom which can be used to optimize the shape of the curve. To obtain a better shape of the curve, the degree of freedom is determined by optimizing the bending and stretching energies of the curve so that variation of the curve is as small as possible. Between each pair of adjacent data points, two local knot intervals are constructed, and the final knot interval corresponding to these two points is determined by a combination of the two local knot intervals. Experiments show that the curves constructed using the knots by the new method generally have better interpolation precision than the ones constructed using the knots by the existing local methods.     

有理B样条曲线的光顺拟合法

研究了用三次均匀有理Ｂ样条样曲线光顺拟合一组平面点列的问题,其中光顺性由曲线的能量积分与扰动的权平均来确定。     

NURBS曲线的一个插值性质

本介绍了非均匀有理B样条曲线，并给出了非均匀有理B样条曲线的一个插值性质。