基于四点插值细分小波的SAR图像去噪

针对SAR图像去噪过程中存在降低相干斑与保持有效细节这一矛盾,提出了一种基于四点插值细分小波的SAR图像去噪算法,该方法将小波和细分方法相融合,将四点插值细分规则应用到细分小波中,提出了图像去噪的新方法.该算法先用四点插值细分小波对原始图像进行分解,然后用Bayes自适应阈值及阈值函数对图像进行去噪,最后对去噪的小波系数进行重构,并通过等效视数、边缘保持指数等评价指标对去噪结果进行了评价.实验结果表明,算法的等效视数、边缘保持指数都有所提高,去噪效果得到了优化.     

构造曲线的插值型细分法--非均匀四点法

本文提出了一种构造曲线的插值型细分法－非均匀四点法,四点法可作为这个方法的一个特例。用这种方法可以构造出Ｇ＾１连续的插值曲线,该法引入了一些偏移参数来控制细分过程,偏移参数参曲线形状的影响是局部的。     

变参数四点法的理论及其应用

四点插值细分法（简称四点法）是一种离散插值方法，在曲线和曲面造型中有着广泛的应用．本文主要讨论当参数可变时，四点法的收敛性和连续性以及变参数四点法的应用．     

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.     

带参数的Catmull-Clark细分曲面

在均匀B样条曲线的Lane-Riesenfeld细分算法中,每一步细分可看成是对原控制多边形的"切角"操作.文章通过引入一个参数来控制切角的程度,提出加权的Lane-Riesenfeld算法,并从均匀三次B样条曲线出发,得到光滑性为C~1的单参数曲线细分格式.进一步将该算法推广到任意拓扑的四边形网格上,得到除奇异点外处处C~1的细分曲面(称之为带参数的Catmull-Clark(C-C)细分曲面).格式中的参数在一定范围内调整时,可以使细分曲线/曲面不同程度地逼近控制多边形/控制网格,具有较好的灵活性.     

五点二重融合型细分法

提出了一种新的细分算法——五点二重融合型细分法.利用生成多项式对该细分法的一致收敛性和C~k连续性进行了分析,通过对融合型细分法中参数的不同取值,可以分别生成C~1～C~6连续的极限曲线.数值实例表明,与现有一些格式相比,细分算法生成的极限曲线不仅可以保持较高光滑性,并且更接近初始控制多边形.     

一类融合逼近和插值的曲线细分

采用生成多项式为主的方法对一类融合逼近和插值三重细分格式的支撑区间、多项式生成、连续性、多项式再生及分形性质进行了分析,给出并证明了极限曲线C^k连续的充分条件.通过对融合型细分规则中参数变量的适当选择来实现对极限曲线的形状调整,从而衍生出具有良好性质的新格式,并将这类新格式与现有格式进行比较.数值实例表明这类新格式生成的极限曲线具有较好的保形性.     

扩散方程九点格式中节点未知量的一种新的插值算法

本文研究了二维扩散方程九点格式中节点辅助未知量的插值问题.利用多点通量逼近的边未知量插值算法和一个特殊的极限技巧,获得了节点辅助未知量的一个新的插值算法,并在给定假设下严格分析了该算法中局部线性系统的可解性.新算法满足线性精确准则,具有较高的精度.     

Lagrange四边形单位分解有限元法的最优误差分析

用构造最优局部逼近空间的方法对Lagrange型四边形单位分解有限元法进行了最优误差分析.单位分解取Lagrange型四边形上的标准双线性基函数,构造了一个特殊的局部多项式逼近空间,给出了具有2阶再生性的Lagrange型四边形单位分解有限元插值格式,从而得到了高于局部逼近阶的最优插值误差.     

两类新的广义Ball曲线曲面的求值算法及其应用

本文研究两类新的广义Ball曲线曲面的求值算法及其应用.其一是把Bezier曲线曲面的求值转换到这两类曲线曲面的求值,大大加快了计算速度.其二是给出Bezier曲线与这两类广义Ball曲线的统一表示,并利用这种表示给出它们之间相互转换的递归算法.     

Exact evaluation of limits and tangents for interpolatory subdivision surfaces at rational points

This paper presents a new method for exact evaluation of a limit surface generated by stationary interpolatory subdivision schemes and its associated tangent vectors at arbitrary rational points. The algorithm is designed on the basis of the parametric m-ary expansion and construction of the associated matrix sequence. The evaluation stencil of the control points on the initial mesh is obtained, through computation, by multiplying the finite matrices in a sequence corresponding to the expansion sequence and eigendecomposition of the contractive matrix related to the period of rational numbers. The method proposed in this paper works for other non-polynomial subdivision schemes as well.     

Computation of interpolatory splines via triadic subdivision

We present an algorithm for the computation of interpolatory splines of arbitrary order at triadic rational points. The algorithm is based on triadic subdivision of splines. Explicit expressions for the subdivision symbols are established. These are rational functions. The computations are implemented by recursive filtering.     

Smoothness of subdivision surfaces at extraordinary points

A stationary subdivision scheme such as Catmull and Clark's is described by a matrix iteration around an extraordinary point. We show how higher order smoothness of a limiting surface obtained by a stationary subdivision algorithm for tri- or quadrilateral nets depends on the spectral properties of the matrix and give necessary and sufficient conditions. The results are also useful to construct subdivision algorithms for surfaces of any smoothness order at extraordinary points. This revised version was published online in August 2006 with corrections to the Cover Date.     

Limit points of the iterative scaling procedure

The iterative scaling procedure (ISP) is an algorithm which computes a sequence of matrices, starting from some given matrix. The objective is to find a matrix ’proportional’ to the given matrix, having given row and column sums. In many cases, for example if the initial matrix is strictly positive, the sequence is convergent. It is known that the sequence has at most two limit points. When these are distinct, convergence to these two points can be slow. We give an efficient algorithm which finds the limit points, invoking the ISP only on subproblems for which the procedure is convergent.     

整数扩展欧几里得矩阵序列的选择项算法的修正

Wang和Pan提出了一个计算整数扩展欧几里得矩阵序列的选择项的算法,并把此算法应用于模有理数重构问题和数值有理数重构问题.这个算法仅消耗接近线性的时间复杂度,与目前已知的整数gcd算法的最佳时间复杂度相一致,而整数gcd算法只是此算法的一个特殊情形.分析了这个算法,指出了算法中由于考虑的不够全面而存在的错误,补充了矩阵序列性质的理论部分,并修正这个算法.     

Approximating conic sections by constrained Bézier curves of arbitrary degree

In this paper, an algorithm for approximating conic sections by constrained Bézier curves of arbitrary degree is proposed. First, using the eigenvalues of recurrence equations and the method of undetermined coefficients, some exact integral formulas for the product of two Bernstein basis functions and the denominator of rational quadratic form expressing conic section are given. Then, using the least squares method, a matrix-based representation of the control points of the optimal Bézier approximation curve is deduced. This algorithm yields an explicit, arbitrary-degree Bézier approximation of conic sections which has function value and derivatives at the endpoints that match the function value and the derivatives of the conic section up to second order and is optimal in the _L2$L_2$ norm. To reduce error, the method can be combined with a curve subdivision scheme. Computational examples are presented to validate the feasibility and effectiveness of the algorithm for a whole curve or its part generated by a subdivision.     

Approximation of the linear combination of $\varphi$-functions using the block shift-and-invert Krylov subspace method

In this paper, we develop an algorithm in which the block shift-and-invert Krylov subspace method can be employed for approximating the linear combination of the matrix exponential and related exponential-type functions. Such evaluation plays a major role in a class of numerical methods known as exponential integrators. We derive a low-dimensional matrix exponential to approximate the objective function based on the block shift-and-invert Krylov subspace methods. We obtain the error expansion of the approximation, and show that the variants of its first term can be used as reliable a posteriori error estimates and correctors. Numerical experiments illustrate that the error estimates are efficient and the proposed algorithm is worthy of further study.     

A generalization of rational Bernstein–Bézier curves

This paper is concerned with a generalization of Bernstein–Bézier curves. A one parameter family of rational Bernstein–Bézier curves is introduced based on a de Casteljau type algorithm. A subdivision procedure is discussed, and matrix representation and degree elevation formulas are obtained. We also represent conic sections using rational q-Bernstein–Bézier curves. AMS subject classification (2000)  65D17