Rectangular v-Splines

This article describes and presents examples of some techniques for the representation and interactive design of surfaces based on a parametric surface representation that user v-spline curves. These v-spline curves, similar in mathematical structure to v-splines, were developed as a more computationally efficient alternative to splines in tension. Although splines in tension can be modified to allow tension to be applied at each control point, the procedure is computationally expensive. The v-spline curve, however, uses more computationally tractable piecewise cubic curves segments, resulting in curves that are just as smoothly joined as those of a standard cubic spline. After presenting a review of v-splines and some new properties, this article extends their application to a rectangular grid of control points. Three techniques and some application examples are presented.     

A Convexity-Preserving Grid Refinement Algorithm for Interpolation of Bivariate Functions

This article presents an algorithms to refine bevariate grid data that is convex (and monotonic) along the grid lines so that the refined data exhibits the same convexity (and monotonicity). The algorithm is based on some observations about univariate data and an algorithm for shape-preserving quadratic splines for such data. It can be used as is or with standard surface-path techniques.     

A one-pass algorithm for shape-preserving quadratic spline interpolation

Schumaker (1983) and McAllister and Roulier (1981) have proposed algorithms for shape-preserving interpolation using quadratic splines. The former requires the user to provide and perhaps to adjust estimates of the slope at the data points. Here we show that, for a particular slope estimation technique, the two methods are identical, and that in this case the Schumaker algorithm automatically generates shape-preserving interpolants. Furthermore, in case of convex data the slopes are improved iteratively to produce more visually pleasing curves.     

A blending function method with interior constraints

A blending function technique with a monovariate interpolating scheme is proposed to provide an initial approximation to the solution of the Poisson problem. Such an approximation combines the interpolatory properties of exact matching of boundary conditions with some interior values of the solution, somehow known. The bilinear blending functions and a shape-preserving interpolation scheme are used in the numerical tests.     

WireWarping: A fast surface flattening approach with length-preserved feature curves

This paper presents a novel approach — WireWarping for computing a flattened planar piece with length-preserved feature curves from a 3D piecewise linear surface patch. The property of length-preservation on feature curves is very important to industrial applications for controlling the shape and dimension of products fabricated from planar pieces. WireWarping simulates warping a given 3D surface patch onto plane with the feature curves as tendon wires to preserve the length of their edges. During warping, the surface-angle variations between edges on wires are minimized so that the shape of a planar piece is similar to its corresponding 3D patch. Two schemes — the progressive warping and the global warping schemes are developed, where the progressive scheme is flexible for local shape control and the global scheme gives good performance on highly distorted patches. Experimental results show that WireWarping can successfully flatten surface patches into planar pieces while preserving the length of edges on feature curves.     

Subdivision Schemes for Thin Plate Splines

Thin plate splines are a well known entity of geometric design. They are defined as the minimizer of a variational problem whose differential operators approximate a simple notion of bending energy. Therefore, thin plate splines approximate surfaces with minimal bending energy and they are widely considered as the standard "fair" surface model. Such surfaces are desired for many modeling and design applications.
Traditionally, the way to construct such surfaces is to solve the associated variational problem using finite elements or by using analytic solutions based on radial basis functions. This paper presents a novel approach for defining and computing thin plate splines using subdivision methods. We present two methods for the construction of thin plate splines based on subdivision: A globally supported subdivision scheme which exactly minimizes the energy functional as well as a family of strictly local subdivision schemes which only utilize a small, finite number of distinct subdivision rules and approximately solve the variational problem. A tradeoff between the accuracy of the approximation and the locality of the subdivision scheme is used to pick a particular member of this family of subdivision schemes.
Later, we show applications of these approximating subdivision schemes to scattered data interpolation and the design of fair surfaces. In particular we suggest an efficient methodology for finding control points for the local subdivision scheme that will lead to an interpolating limit surface and demonstrate how the schemes can be used for the effective and efficient design of fair surfaces.     

带形状参数的三次三角Hermite插值样条曲线

给出了一种带形状参数的三次三角Hermite插值样条曲线,具有标准三次Hermite插值样条曲线完全相同的性质。给定插值条件时,样条曲线的形状可通过改变形状参数的取值进行调控。在适当条件下,该样条曲线对应的Ferguson曲线可精确表示椭圆、抛物线等工程曲线。通过选择合适的形状参数,该插值样条曲线能达到[C2]连续,而且其整体逼近效果要好于标准三次Hermite插值样条曲线。     

Quasi-Interpolants with Tension Properties from and in CAGD

We present C² quasi-interpolating schemes with tension properties. The B-splines like functions used in the quasi-interpolanting schemes are parametric cubic curves and their shape can be easily controlled via tension parameters which have an immediate geometric interpretation. Applications to the problem of approximation of curves with shape-constraints are discussed.     

Urn Models and Beta-Splines

Splines were originally studied in approximation theory where the focus is on approximating explicit functions of the form y = f(x) or z = f(x,y). These splines were later adopted by mathematicians and computer sicentists for use in computer-aided geometric design (CAGD) where the emphasis was shifted to parametric curves and surfaces. Initially the continuity conditions for splines developed in approximation theory were retained in CAGD, but it was soon realized that the old constraints were unnecessarily restrictive in this new context and that they could be relaxed without losing the essential property of smoothness. Beta-splines were developed to take advantage of this new freedom by introducing shape parameters into the constraint equations. These parameters could then be manipulated by a designer to change the shape of a curve of surface in an intuitively meaningful and useful way. Another seemingly unrelated context in which shape parameters appear is in blending functions constructed from discrete urn models. The purpose of this article is to begin to unify these two independent approaches to shape parameters, and in the process apply the techniques of urn models to gain some insight into the properties of Beta-splines.     

On Shape of Plane Elastic Curves

We study shapes of planar arcs and closed contours modeled on elastic curves obtained by bending, stretching or compressing line segments non-uniformly along their extensions. Shapes are represented as elements of a quotient space of curves obtained by identifying those that differ by shape-preserving transformations. The elastic properties of the curves are encoded in Riemannian metrics on these spaces. Geodesics in shape spaces are used to quantify shape divergence and to develop morphing techniques. The shape spaces and metrics constructed are novel and offer an environment for the study of shape statistics. Elasticity leads to shape correspondences and deformations that are more natural and intuitive than those obtained in several existing models. Applications of shape geodesics to the definition and calculation of mean shapes and to the development of shape clustering techniques are also investigated.     

A subdivision scheme for Poisson curves and surfaces

The de Casteljau evaluation algorithm applied to a finite sequence of control points defines a Bézier curve. This evaluation procedure also generates a subdivision algorithm and the limit of the subdivision process is this same Bézier curve. Extending the de Casteljau subdivision algorithm to an infinite sequence of control points defines a new family of curves. Here, limits of this stationary non-uniform subdivision process are shown to be equivalent to curves whose control points are the original data points and whose blending functions are given by the Poisson distribution. Thus this approach generalizes standard subdivision techniques from polynomials to arbitrary analytic functions. Extensions of this new subdivision scheme from curves to tensor product surfaces are also discussed.     

静态ternary逼近细分格式的连续性分析和构造

提出了一般的三点三重、四点三重逼近细分格式,利用稳定细分格式Ck连续的充要条件,分析了细分法各阶连续时参数的取值范围。利用提出的一般细分法,可以造型光滑逼近曲线;当某些细分参数取特殊值时,还可以用来造型插值曲线。为便于应用,还对Hassan的3点ternary逼近细分法进行了改进,使其带有一个全局张力参数,通过它更易控制曲线的形状。在全局张力参数的一定范围内可以生成C1,C2连续的极限曲线。     

A geometric programming approach for bivariate cubic L1 splines

Bivariate cubic L₁ splines provide C¹-smooth, shape-preserving interpolation of arbitrary data, including data with abrupt changes in spacing and magnitude. The minimization principle for bivariate cubic L₁ splines results in a nondifferentiable convex optimization problem. This problem is reformulated as a generalized geometric programming problem. A geometric dual with a linear objective function and convex cubic constraints is derived. A linear system for dual-to-primal conversion is established. The results of computational experiments are presented.     

Polynomial blending in a mesh hole-filling application

In this paper, a feature-preserving mesh hole-filling algorithm is realized by the polynomial blending technique. We first search for feature points in the neighborhood of the hole. These feature points allow us to define the feature curves with missing parts in the hole. A polynomial blending curve is constructed to complete the missing parts of the feature curves. These feature curves divide the original complex hole into small simple sub-holes. We use the Bézier-Lagrange hybrid patch to fill each sub-hole. The experimental results show that our mesh hole-filling algorithm can effectively restore the original shape of the hole.     

Fitting smoothing splines to time-indexed,noisy points on nonlinear manifolds

We address the problem of estimating full curves/paths on certain nonlinear manifolds using only a set of time-indexed points, for use in interpolation, smoothing, and prediction of dynamic systems. These curves are analogous to smoothing splines in Euclidean spaces as they are optimal under a similar objective function, which is a weighted sum of a fitting-related (data term) and a regularity-related (smoothing term) cost functions. The search for smoothing splines on manifolds is based on a Palais metric-based steepest-decent algorithm developed in Samir et al. [38]. Using three representative manifolds: the rotation group for pose tracking, the space of symmetric positive-definite matrices for DTI image analysis, and Kendall's shape space for video-based activity recognition, we demonstrate the effectiveness of the proposed algorithm for optimal curve fitting. This paper derives certain geometrical elements, namely the exponential map and its inverse, parallel transport of tangents, and the curvature tensor, on these manifolds, that are needed in the gradient-based search for smoothing splines. These ideas are illustrated using experimental results involving both simulated and real data, and comparing the results to some current algorithms such as piecewise geodesic curves and splines on tangent spaces, including the method by Kume et al. [24].     

Shape-preserving computation in economic growth models

We point out the shape characteristics — monotonicity and concavity — of the value functions of optimal economic growth problems. We introduce the concept of shape preservation in approximating the value functions. We also present a shape-preserving algorithm to compute the solutions of continuous-state optimal economic growth problems. Numerical results show that shape-preserving interpolation methods are superior to others with less-sophisticated interpolation in the sense of smaller approximation errors.Scope and PurposeUnder standard conditions on a risk-averse utility, the value function of an optimal economic growth problem is known to possess the shape characteristics-monotonicity and concavity. As the closed form solutions are rarely available, the only way to solve for the value function is numerically. However, there are no numerical methods which guarantee to preserve the shape features in the course of approximation. In this article, we introduce the usage of shape preservation and present a shape-preserving interpolation in numerical dynamic programming.     

基于推广B 样条细分方法的曲面混合

提出用推广B 样条细分曲面来混合多张曲面的方法,既适用于一般网格曲面,又适 用于推广B 样条参数曲面混合。根据需要选择阶数和张力参数,可全局调整整张混合曲面的形状。 中心点和谷点的计算都设置了形状参数,可局部调整混合部分形状。推导出二次曲面细分初始网 格计算公式,并将3 阶推广B 样条细分曲面混合方法用于多张二次曲面混合,与已有的二次曲面 混合方法相比具有明显的优势。     

An algorithm for computing shape-preserving cubic spline interpolation to data

We present an algorithm for the construction of shape-preserving cubic splines interpolating a set of data point. The method is based upon some existence properties recently developed. Graphical examples are given.