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发布时间: 2018-06-16 |
计算机图形学 |
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收稿日期: 2017-11-10; 修回日期: 2017-12-26
基金项目: 国家自然科学基金项目(11261003);湖南省自然科学基金项目(2017JJ3124);装发/航天联合基金项目(6141B0602102)
第一作者简介:
李军成(1982-), 男, 副教授, 2014年于南京航空航天大学获模式识别与智能系统专业工学博士学位, 主要研究方向为计算机辅助几何设计及其应用, 已发表论文近40篇。E-mail:lijuncheng82@126.com.
中图法分类号: TP391.72
文献标识码: A
文章编号: 1006-8961(2018)06-0896-00
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摘要
目的 虽然Ball曲线具有很好的几何特性,但当控制顶点保持不变时,曲线的形状却无法进行调整,这无疑限制了其在几何造型中的应用。为了使得任意次Ball曲线在控制顶点保持不变的情形下具有形状可调性,提出了一种构造带参数的同次Ball曲线的简单方法。方法 首先通过将传统三次Ball基的定义区间由[0,1]扩展为[0,α],构造了一种带参数α的三次Ball基,并称之为三次α-Ball基;然后基于三次α-Ball基定义了相应的三次α-Ball曲线,并讨论了三次α-Ball曲线的拼接、参数对曲线的影响以及参数的3种选取方案;最后借助传统高次Ball基的递推性构造了任意次α-Ball基及其对应的α-Ball曲线,并给出了任意次α-Ball基与α-Ball曲线的性质。结果 实例表明,所构造的α-Ball曲线是传统Ball曲线的同次扩展,不仅保留了传统Ball曲线的性质,而且还由于带有参数α使得曲线具有更好的表现能力。利用所给出的3种参数选取方案可构造出满足相应要求的α-Ball曲线。结论 所提出的α-Ball曲线克服了传统Ball曲线在形状调整方面的不足,是一种构造形状可调的任意次Ball曲线的有效方法。
关键词
Ball基; Ball曲线; 任意次; 同次扩展; 形状调整; 参数选取
Abstract
Objective The Ball curve has excellent geometric properties. However, its shape cannot be adjusted when the control points remain unchanged. This condition undoubtedly limits its application in geometrical modeling. A simple method for constructing the Ball curve of the same degree with a parameter is presented to enable the Ball curve with arbitrary degree to obtain shape adjustment capability under fixed control points. Method The cubic Ball basis, referred to as cubic α-Ball basis, is constructed by extending the definition interval of the traditional cubic Ball basis from [0, 1] to [0, α]. Then, the corresponding cubic α-Ball curve is defined base on the cubic α-Ball basis. The splicing of the curves, the influence of the parameter on the curve, and the three selection schemes for the parameter are discussed. Finally, the α-Ball basis and α-Ball curve with arbitrary degree are established by the recursion of the transitional high-degree Ball basis, and the properties of the α-Ball basis and α-Ball curve with arbitrary degree are provided. Result Examples show that the proposed α-Ball curve is an extension of the same degree to the traditional Ball curve. The curve not only preserves the properties of the traditional Ball curve, such as convex hull, symmetry, geometric invariance, variance reduction, and convexity, but also has better performance because of the parameter α. The α-Ball curve can be constructed to satisfy the requirements by using the three selection schemes for the parameter, including the scheme for the curve with the shortest arc length, the curve with minimum energy, and the curve with the shortest arc length and minimum energy. Conclusion The α-Ball curve overcomes the disadvantage of the traditional Ball curve in shape adjustment, which is an effective method for constructing the shape-adjustable Ball curve with arbitrary degree.
Key words
Ball basis; Ball curve; arbitrary degree extension of the same degree; shape adjustment; parameter selection
0 引言
自Ball曲线[1]在1974年被提出以来,由于该曲线不仅与三次Bézier曲线一样具有良好的保形性,而且在求值及升降阶等计算速度上优于三次Bézier曲线,因此被人们广泛研究。为了构造高次的Ball曲线,王国瑾[2]与Said[3]分别独立地将三次Ball曲线推广到任意次数,并被Hu等人[4]分别命名为Wang-Ball曲线与Said-Ball曲线;丁友东等人[5]讨论了任意次Ball曲线的性质及其应用;Delgado等人[6]构造了保形的DP-Ball基;邬弘毅[7]则提出了分别介于Wang-Ball曲线与Said-Ball曲线之间以及介于Bézier曲线与Said-Ball曲线之间的两类广义Ball曲线;沈莞蔷等人[8]与汪志华等人[9]也分别构造了介于Wang-Ball曲线与Said-Ball曲线的曲线族、介于Bézier曲线与Said-Ball曲线之间的曲线族。随后,沈莞蔷等人[10]又通过加入多个带约束条件的参加,利用递归的方法构造出一类广义Ball基及其相应曲线。
另一方面,为了使得Ball曲线在控制顶点保持不变的情形下具有形状可调性,人们又试图构造带参数的Ball曲线。王成伟[11]给出了一种带单个参数的四次Ball曲线,该曲线是三次Ball曲线的一种扩展;严兰兰等人[12, 13]构造了两种带参数的四/五次Ball曲线,实现了从四/五Wang-Ball曲线到Said-Ball曲线以及四/五Said-Ball到Bézier的过渡;严兰兰等人[14]又构造了次数分别为三次与四次且带有参数的Ball曲线,这两种曲线都是三次Ball曲线的扩展;刘华勇等人[15]构造了一种带两个参数的四次Ball曲线,该曲线与三次Ball曲线具有相似的结构与特性。注意到,这些方法的主要目的在Ball曲线中引入参数,通过参数的改变实现对Ball曲线形状的调整,以此来克服Ball曲线在形状调控方面的不足。另外,这些方法都是针对特定次数的Ball曲线进行研究,并没有考虑如何在任意次Ball曲线中引入参数。
为此,本文提出了一种在任意次Ball曲线中引入参数的简单方法。首先将三次Ball基的定义区间由[0, 1]扩展为
1 三次 $\alpha $ -Ball基的构造及其性质
对于0≤
$ \left\{ \begin{array}{l} {b_0}\left( t \right) = {\left( {1 - t} \right)^2}\\ {b_1}\left( t \right) = 2{\left( {1 - t} \right)^2}t\\ {b_2}\left( t \right) = 2\left( {1 - t} \right){t^2}\\ {b_3}\left( t \right) = {t^2} \end{array} \right. $ | (1) |
三次Ball基在端点处满足
$ \left\{ \begin{array}{l} {b_0}\left( 0 \right) = 1,\;\;\;\;\;\;{b_0}\left( 1 \right) = 0\\ {b_1}\left( 0 \right) = 0,\;\;\;\;\;\;{b_1}\left( 1 \right) = 0\\ {b_2}\left( 0 \right) = 0,\;\;\;\;\;\;{b_2}\left( 1 \right) = 0\\ {b_3}\left( 0 \right) = 0,\;\;\;\;\;\;{b_3}\left( 1 \right) = 1\\ {{b'}_0}\left( 0 \right) = - 2,\;\;\;\;{{b'}_0}\left( 1 \right) = 0\\ {{b'}_1}\left( 0 \right) = 2,\;\;\;\;\;\;{{b'}_1}\left( 1 \right) = 0\\ {{b'}_2}\left( 0 \right) = 0,\;\;\;\;\;\;{{b'}_2}\left( 1 \right) = - 2\\ {{b'}_3}\left( 0 \right) = 0,\;\;\;\;\;\;{{b'}_3}\left( 1 \right) = 2 \end{array} \right. $ | (2) |
为了在三次Ball基中直接引入参数
设所要构造的新三次Ball基
$ \begin{array}{*{20}{c}} {\left( {\begin{array}{*{20}{c}} {{B_{3,0}}\left( t \right)}&{{B_{3,1}}\left( t \right)}&{{B_{3,2}}\left( t \right)}&{{B_{3,3}}\left( t \right)} \end{array}} \right) = }\\ {\left( {\begin{array}{*{20}{c}} 1&t&{{t^2}}&{{t^3}} \end{array}} \right)\mathit{\boldsymbol{A}}} \end{array} $ | (3) |
式中,
由式(3)可得
$ \begin{array}{*{20}{c}} {\left( {\begin{array}{*{20}{c}} {{{B'}_{3,0}}\left( t \right)}&{{{B'}_{3,1}}\left( t \right)}&{{{B'}_{3,2}}\left( t \right)}&{{{B'}_{3,3}}\left( t \right)} \end{array}} \right) = }\\ {\left( {\begin{array}{*{20}{c}} 0&1&{2t}&{3{t^2}} \end{array}} \right)\mathit{\boldsymbol{A}}} \end{array} $ | (4) |
为了使得新三次Ball基在端点处的性质与式(2)类同,依据式(3)与式(4)则有
$ \left( {\begin{array}{*{20}{c}} 1&0&0&0\\ 0&0&0&1\\ { - 2}&2&0&0\\ 0&0&{ - 2}&2 \end{array}} \right) = \left( {\begin{array}{*{20}{c}} 1&0&0&0\\ 1&\alpha &{{\alpha ^2}}&{{\alpha ^3}}\\ 0&1&0&0\\ 0&1&{2\alpha }&{3{\alpha ^2}} \end{array}} \right)\mathit{\boldsymbol{A}} $ | (5) |
由式(5)可得
$ \mathit{\boldsymbol{A}} = \left( {\begin{array}{*{20}{c}} 1&0&0&0\\ { - 2}&2&0&0\\ {\frac{{4\alpha - 3}}{{{\alpha ^2}}}}&{ - \frac{4}{\alpha }}&{\frac{2}{\alpha }}&{\frac{{3 - 2\alpha }}{{{\alpha ^2}}}}\\ {\frac{{2 - 2\alpha }}{{{\alpha ^3}}}}&{\frac{2}{{{\alpha ^2}}}}&{ - \frac{2}{{{\alpha ^2}}}}&{\frac{{2\alpha - 2}}{{{\alpha ^3}}}} \end{array}} \right) $ | (6) |
将式(6)代入式(3),可得所要构造的新三次Ball基为
$ \left\{ \begin{array}{l} {B_{3,0}}\left( t \right) = 1 - 2t + \frac{{4\alpha - 3}}{{{\alpha ^2}}}{t^2} + \frac{{2 - 2\alpha }}{{{\alpha ^3}}}{t^3}\\ {B_{3,1}}\left( t \right) = 2t - \frac{4}{\alpha }{t^2} + \frac{2}{{{\alpha ^2}}}{t^3}\\ {B_{3,2}}\left( t \right) = \frac{2}{\alpha }{t^2} - \frac{2}{{{\alpha ^2}}}{t^3}\\ {B_{3,3}}\left( t \right) = \frac{{3 - 2\alpha }}{{{\alpha ^2}}}{t^2} + \frac{{2\alpha - 2}}{{{\alpha ^3}}}{t^3} \end{array} \right. $ | (7) |
式中,
若令
定义1对于
$ \left\{ \begin{array}{l} {B_{3,0}}\left( u \right) = {\left( {1 - u} \right)^2}\left( {1 + 2\left( {1 - \alpha } \right)u} \right)\\ {B_{3,1}}\left( u \right) = 2\alpha {\left( {1 - u} \right)^2}u\\ {B_{3,2}}\left( u \right) = 2\alpha \left( {1 - u} \right){u^2}\\ {B_{3,3}}\left( u \right) = {u^2}\left( {1 + 2\left( {1 - \alpha } \right)\left( {1 - u} \right)} \right) \end{array} \right. $ | (8) |
称为带参数
由三次
定理1 三次
1) 非负性:
2) 单位分解性:
3) 对称性:
4) 单峰性:
5) 关于参数
6) 端点性:在端点处有
$ \left\{ \begin{array}{l} {B_{3,0}}\left( 0 \right) = 1,\;\;\;\;\;\;\;\;\;{B_{3,0}}\left( 1 \right) = 0\\ {B_{3,1}}\left( 0 \right) = 0,\;\;\;\;\;\;\;\;\;{B_{3,1}}\left( 1 \right) = 0\\ {B_{3,2}}\left( 0 \right) = 0,\;\;\;\;\;\;\;\;\;{B_{3,2}}\left( 1 \right) = 0\\ {B_{3,3}}\left( 0 \right) = 0,\;\;\;\;\;\;\;\;\;{B_{3,3}}\left( 1 \right) = 1\\ {{B'}_{3,0}}\left( 0 \right) = - 2\alpha ,\;\;\;\;{{B'}_{3,0}}\left( 1 \right) = 0\\ {{B'}_{3,1}}\left( 0 \right) = 2\alpha ,\;\;\;\;\;\;{{B'}_{3,1}}\left( 1 \right) = 0\\ {{B'}_{3,2}}\left( 0 \right) = 0,\;\;\;\;\;\;\;\;{{B'}_{3,2}}\left( 1 \right) = - 2\alpha \\ {{B'}_{3,3}}\left( 0 \right) = 0,\;\;\;\;\;\;\;\;{{B'}_{3,3}}\left( 1 \right) = 2\alpha \end{array} \right. $ | (9) |
证明 1)由
2) 由式(8)不难可得
$ \begin{array}{*{20}{c}} {\sum\limits_{i = 0}^3 {{B_{3,i}}\left( u \right)} = {{\left( {1 - u} \right)}^2} + 2{{\left( {1 - u} \right)}^2}u + }\\ {2{u^2}\left( {1 - u} \right) + {u^2} \equiv 1。} \end{array} $ |
3) 由式(8)易知
$ \begin{array}{*{20}{c}} {{B_{3,0}}\left( {1 - u} \right) = {u^2}\left( {1 + 2\left( {1 - \alpha } \right)\left( {1 - u} \right)} \right) = }\\ {{B_{3,3}}\left( u \right),{B_{3,1}}\left( {1 - u} \right) = 2\alpha {u^2}\left( {1 - u} \right) = {B_{3,2}}\left( u \right)。} \end{array} $ |
4) 由式(8)可得
$ \left\{ \begin{array}{l} {{B'}_{3,0}}\left( u \right) = - 2\left( {1 - u} \right)\left( {\alpha + 3\left( {1 - \alpha } \right)u} \right)\\ {{B'}_{3,1}}\left( u \right) = 2\alpha \left( {1 - u} \right)\left( {1 - 3u} \right)\\ {{B'}_{3,2}}\left( u \right) = 2\alpha \left( {2 - 3u} \right)u\\ {{B'}_{3,3}}\left( u \right) = - 6\left( {1 - \alpha } \right){u^2} + 2\left( {3 - 2\alpha } \right)u \end{array} \right. $ | (10) |
当
(1)
(2) 当
(3) 当
(4) 由于
5) 固定变量
$ \frac{{{\rm{d}}{B_{3,0}}\left( u \right)}}{{{\rm{d}}\alpha }} = - 2u{\left( {1 - u} \right)^2} \le 0 $ |
$ \frac{{{\rm{d}}{B_{3,1}}\left( u \right)}}{{{\rm{d}}\alpha }} = 2u{\left( {1 - u} \right)^2} \ge 0 $ |
$ \frac{{{\rm{d}}{B_{3,2}}\left( u \right)}}{{{\rm{d}}\alpha }} = 2{u^2}\left( {1 - u} \right) \ge 0 $ |
$ \frac{{{\rm{d}}{B_{3,3}}\left( u \right)}}{{{\rm{d}}\alpha }} = - 2{u^2}\left( {1 - u} \right) \le 0 $ |
从而,
6) 由式(8)与式(10)经简单计算,可得式(9)成立。证毕。
2 三次 $\alpha $ -Ball曲线
2.1 三次 $\alpha $ -Ball曲线的定义及其性质
基于三次
定义2 给定
$ {\mathit{\boldsymbol{R}}_3}\left( u \right) = \sum\limits_{i = 0}^3 {{B_{3,i}}\left( u \right){\mathit{\boldsymbol{p}}_i}} $ | (11) |
为带形状参数
由式(11)易知,当
由定理1不难可得,三次
性质1 凸包性
由三次
性质2 对称性
由三次
性质3 几何不变性与仿射不变性
由于式(11)为参数式方程,故当参数
性质4 形状可调性
由于带有参数
性质5 变差缩减性与保凸性
平面内的任何一条直线与三次
性质6 端点性
由式(9)经简单计算可得,曲线在端点处满足
$ \left\{ \begin{array}{l} {\mathit{\boldsymbol{R}}_3}\left( 0 \right) = {\mathit{\boldsymbol{p}}_0}\\ {\mathit{\boldsymbol{R}}_3}\left( 0 \right) = {\mathit{\boldsymbol{p}}_3}\\ {{\mathit{\boldsymbol{R'}}}_3}\left( 0 \right) = 2\alpha \left( {{\mathit{\boldsymbol{p}}_1} - {\mathit{\boldsymbol{p}}_0}} \right)\\ {{\mathit{\boldsymbol{R'}}}_3}\left( 1 \right) = 2\alpha \left( {{\mathit{\boldsymbol{p}}_3} - {\mathit{\boldsymbol{p}}_2}} \right) \end{array} \right. $ | (12) |
2.2 曲线的拼接
记第
$ {\mathit{\boldsymbol{R}}_{i,3}}\left( u \right) = \sum\limits_{j = 0}^3 {{B_{3,j}}\left( u \right){\mathit{\boldsymbol{p}}_{i,j}}} $ | (13) |
式中,
$ \left\{ \begin{array}{l} {\mathit{\boldsymbol{R}}_{i,3}}\left( 1 \right) = {\mathit{\boldsymbol{R}}_{i + 1,3}}\left( 0 \right)\\ {{\mathit{\boldsymbol{R'}}}_{i,3}}\left( 1 \right) = {\lambda _i}{{\mathit{\boldsymbol{R'}}}_{i + 1,3}}\left( 0 \right) \end{array} \right. $ |
则称这两段相邻曲线满足
定理2 若相邻两段三次
$ {\mathit{\boldsymbol{p}}_{i,3}} = {\mathit{\boldsymbol{p}}_{i + 1,0}} $ |
证明 由式(12)与式(13)可得
$ \left\{ \begin{array}{l} {\mathit{\boldsymbol{R}}_{i,3}}\left( 1 \right) = {\mathit{\boldsymbol{p}}_{i,3}}\\ {{\mathit{\boldsymbol{R'}}}_{i,3}}\left( 1 \right) = 2{\alpha _i}\left( {{\mathit{\boldsymbol{p}}_{i,3}} - {\mathit{\boldsymbol{p}}_{i,2}}} \right)\\ {{\mathit{\boldsymbol{R'}}}_{i + 1,3}}\left( 1 \right) = {\mathit{\boldsymbol{p}}_{i + 1,0}}\\ {{\mathit{\boldsymbol{R'}}}_{i + 1,3}}\left( 0 \right) = 2{\alpha _{i + 1}}\left( {{\mathit{\boldsymbol{p}}_{i + 1,1}} - {\mathit{\boldsymbol{p}}_{i + 1,0}}} \right) \end{array} \right. $ | (14) |
由于
$ \left\{ \begin{array}{l} {\mathit{\boldsymbol{R}}_{i,3}}\left( 1 \right) = {\mathit{\boldsymbol{R}}_{i + 1,3}}\left( 0 \right)\\ {{\mathit{\boldsymbol{R'}}}_{i,3}}\left( 1 \right) = {\lambda _i}{{\mathit{\boldsymbol{R'}}}_{i + 1,3}}\left( 0 \right) \end{array} \right. $ | (15) |
式中,
式(15)即表明相邻两段三次
$ \left\{ \begin{array}{l} {\mathit{\boldsymbol{R}}_{i,3}}\left( 1 \right) = {\mathit{\boldsymbol{R}}_{i + 1,3}}\left( 0 \right)\\ {{\mathit{\boldsymbol{R'}}}_{i,3}}\left( 1 \right) = {{\mathit{\boldsymbol{R'}}}_{i + 1,3}}\left( 0 \right) \end{array} \right. $ | (16) |
式(16)即表明相邻两段三次
注1 事实上,相邻两段三次
2.3 参数对曲线的影响
当控制顶点保持不变时,三次Ball曲线的形状将会随之确定,而三次
定理3 当控制顶点保持不变时,参数
证明 将式(11)改写为矩阵形式为
$ \begin{array}{*{20}{c}} {{\mathit{\boldsymbol{R}}_3}\left( u \right) = \left( {\begin{array}{*{20}{c}} 1&u&{{u^2}}&{{u^3}} \end{array}} \right) \times }\\ {\left( {\begin{array}{*{20}{c}} \begin{array}{l} 1\\ - 2\alpha \\ 4\alpha - 3\\ 2 - 2\alpha \end{array}&\begin{array}{l} 0\\ 2\alpha \\ - 4\alpha \\ 2\alpha \end{array}&\begin{array}{l} 0\\ 0\\ 2\alpha \\ - 2\alpha \end{array}&\begin{array}{l} 0\\ 0\\ 3 - 2\alpha \\ 2\alpha - 2 \end{array} \end{array}} \right)\left( {\begin{array}{*{20}{c}} {{\mathit{\boldsymbol{p}}_0}}\\ {{\mathit{\boldsymbol{p}}_1}}\\ {{\mathit{\boldsymbol{p}}_2}}\\ {{\mathit{\boldsymbol{p}}_3}} \end{array}} \right)} \end{array} $ | (17) |
而由控制顶点
$ \begin{array}{*{20}{c}} {{\mathit{\boldsymbol{R}}_3}\left( u \right) = \left( {\begin{array}{*{20}{c}} 1&u&{{u^2}}&{{u^3}} \end{array}} \right)}\\ {\left( {\begin{array}{*{20}{c}} \begin{array}{l} 1\\ - 3\\ 3\\ - 1 \end{array}&\begin{array}{l} 0\\ 3\\ - 6\\ 3 \end{array}&\begin{array}{l} 0\\ 0\\ 3\\ - 3 \end{array}&\begin{array}{l} 0\\ 0\\ 0\\ 1 \end{array} \end{array}} \right)\left( {\begin{array}{*{20}{c}} {{\mathit{\boldsymbol{q}}_0}}\\ {{\mathit{\boldsymbol{q}}_1}}\\ {{\mathit{\boldsymbol{q}}_2}}\\ {{\mathit{\boldsymbol{q}}_3}} \end{array}} \right)} \end{array} $ | (18) |
对比式(17)与式(18)可得三次Bézier曲线与三次
$ \left\{ \begin{array}{l} {\mathit{\boldsymbol{q}}_0} = {\mathit{\boldsymbol{p}}_0}\\ {\mathit{\boldsymbol{q}}_1} = {\mathit{\boldsymbol{p}}_0} + \frac{{2\alpha }}{3}\left( {{\mathit{\boldsymbol{p}}_1} - {\mathit{\boldsymbol{p}}_0}} \right)\\ {\mathit{\boldsymbol{q}}_2} = {\mathit{\boldsymbol{p}}_3} + \frac{{2\alpha }}{3}\left( {{\mathit{\boldsymbol{p}}_2} - {\mathit{\boldsymbol{p}}_3}} \right)\\ {\mathit{\boldsymbol{q}}_3} = {\mathit{\boldsymbol{p}}_3} \end{array} \right. $ | (19) |
由式(19)可知,参数
图 1为参数
进一步地,由定理2可知,取定合适的控制顶点后,当
若取
类似地,由定理2可知,取定合适的控制顶点后,当
$ {\mathit{\boldsymbol{p}}_{i,3}} - {\mathit{\boldsymbol{p}}_{i,2}} = {\mathit{\boldsymbol{p}}_{i + 1,1}} - {\mathit{\boldsymbol{p}}_{i + 1,0}}\left( {i = 1,2,3} \right) $ |
故由定理2可知,为了保证整条曲线满足
需要说明的是,虽然三次
2.4 参数的选取准则
设整条
准则1 曲线的弧长最短
一般地,三次
$ 有优化模型\left\{ \begin{array}{l} \min \;\;\;L\left( {{\alpha _i}} \right) = \int_0^1 {\left| {{{\mathit{\boldsymbol{R'}}}_{i,3}}\left( u \right)} \right|{\rm{d}}u} \\ {\rm{s}}.\;{\rm{t}}{\rm{.}}\;\;\;\;0 < {\alpha _i} \le 1 \end{array} \right. $ | (20) |
为方便计算,可将式(21)近似转化为
$ \left\{ \begin{array}{l} \min \;\;\;L\left( {{\alpha _i}} \right) = \int_0^1 {{{\left( {{{\mathit{\boldsymbol{R'}}}_{i,3}}\left( u \right)} \right)}^2}{\rm{d}}u} \\ {\rm{s}}.\;{\rm{t}}{\rm{.}}\;\;\;\;0 < {\alpha _i} \le 1 \end{array} \right. $ | (21) |
依据式(8),可将式(13)改写为
$ {\mathit{\boldsymbol{R}}_{i,3}}\left( u \right) = {\mathit{\boldsymbol{L}}_i}\left( u \right){\alpha _i} + {\mathit{\boldsymbol{M}}_i}\left( u \right) $ | (22) |
式中
$ \left\{ \begin{array}{l} {\mathit{\boldsymbol{L}}_i}\left( u \right) = 2u{\left( {1 - u} \right)^2}\left( {{\mathit{\boldsymbol{p}}_{i,1}} - {\mathit{\boldsymbol{p}}_{i,0}}} \right) - \\ 2{u^2}\left( {1 - u} \right)\left( {{\mathit{\boldsymbol{p}}_{i,3}} - {\mathit{\boldsymbol{p}}_{i,2}}} \right)\\ {\mathit{\boldsymbol{M}}_i}\left( u \right) = {\left( {1 - u} \right)^2}\left( {1 + 2u} \right){\mathit{\boldsymbol{p}}_{i,0}} + {u^2}\left( {3 - 2u} \right){\mathit{\boldsymbol{p}}_{i,3}} \end{array} \right. $ |
将式(22)代入式(21)经计算整理可得
$ \left\{ \begin{array}{l} \min \;\;\;L\left( {{\alpha _i}} \right) = {A_{1i}}\alpha _i^2 + 2{B_{1i}}{\alpha _i} + {C_{1i}}\\ {\rm{s}}.\;{\rm{t}}.\;\;\;\;0 < {\alpha _i} \le 1 \end{array} \right. $ | (23) |
式中,
$ {A_{1i}} = \int_0^1 {{{\left( {{{\mathit{\boldsymbol{L'}}}_i}\left( u \right)} \right)}^2}{\rm{d}}u} $ |
$ {B_{1i}} = \int_0^1 {\left( {{{\mathit{\boldsymbol{L'}}}_i}\left( u \right) \cdot {{\mathit{\boldsymbol{M'}}}_i}\left( u \right)} \right){\rm{d}}u} $ |
$ {C_{1i}} = \int_0^1 {{{\left( {{{\mathit{\boldsymbol{M'}}}_i}\left( u \right)} \right)}^2}{\rm{d}}u} $ |
求解式(23)即可得能使三次
准则2 曲线的能量值最小
一般地,三次
$ \left\{ \begin{array}{l} \min \;\;\;E\left( {{\alpha _i}} \right) = \int_0^1 {{{\left( {{{\mathit{\boldsymbol{R''}}}_{i,3}}\left( u \right)} \right)}^2}{\rm{d}}u} \\ {\rm{s}}.\;{\rm{t}}.\;\;\;\;0 < {\alpha _i} \le 1 \end{array} \right. $ | (24) |
将式(22)代入式(21)经计算整理可得
$ \left\{ \begin{array}{l} \min \;\;\;E\left( {{\alpha _i}} \right) = {A_{2i}}\alpha _i^2 + 2{B_{2i}}{\alpha _i} + {C_{2i}}\\ {\rm{s}}.\;{\rm{t}}.\;\;\;\;0 < {\alpha _i} \le 1 \end{array} \right. $ | (25) |
式中,
$ {A_{2i}} = \int_0^1 {{{\left( {{{\mathit{\boldsymbol{L''}}}_i}\left( u \right)} \right)}^2}{\rm{d}}u} ,{B_{2i}} = \int_0^1 {\left( {{{\mathit{\boldsymbol{L''}}}_i}\left( u \right) \cdot {{\mathit{\boldsymbol{M''}}}_i}\left( u \right)} \right){\rm{d}}u} $ |
$ {C_{2i}} = \int_0^1 {{{\left( {{{\mathit{\boldsymbol{M''}}}_i}\left( u \right)} \right)}^2}{\rm{d}}u} $ |
求解式(25)即可得能使三次
准则3 曲线兼具弧长最短与能量值最小
给定控制顶点
$ \left\{ \begin{array}{l} \min \;\;\;F\left( {{\alpha _i}} \right) = L\left( {{\alpha _i}} \right) + E\left( {{\alpha _i}} \right)\\ {\rm{s}}.\;{\rm{t}}.\;\;\;\;0 < {\alpha _i} \le 1 \end{array} \right. $ | (26) |
求解式(26)即可得能使三次
注2 对于由
3 高次 $\alpha $ -Ball基与 $\alpha $ -Ball曲线
在三次
定义3 对于
1) 当
$ \left\{ \begin{array}{l} {B_{n,i}}\left( u \right) = {B_{n - 1,i}}\left( u \right)\;\;\;\;\left( {i = 0,1, \cdots ,n/2 - 2} \right)\\ {B_{n,j}}\left( u \right) = {B_{n - 1,j - 1}}\left( u \right)\;\;\;\;\left( {j = n/2 + 2,n/2 + 3, \cdots ,n} \right)\\ {B_{n,n/2 - 1}}\left( u \right) = \left( {1 - u} \right){B_{n - 1,n/2 - 1}}\left( u \right)\\ {B_{n,n/2}}\left( u \right) = u{B_{n - 1,n/2 - 1}}\left( u \right) + \left( {1 - u} \right){B_{\left. {n - 1,n/2} \right]}}\left( u \right)\\ {B_{n,n/2 + 1}}\left( u \right) = u{B_{n - 1,n/2}}\left( u \right) \end{array} \right. $ | (27) |
2) 当
$ \left\{ \begin{array}{l} {B_{n,i}}\left( u \right) = {B_{n - 1,i}}\left( u \right)\;\;\;\;\left( {i = 0,1, \cdots ,\left( {n - 3} \right)/2} \right)\\ {B_{n,j}}\left( u \right) = {B_{n - 1,j - 1}}\left( u \right)\;\;\;\;\left( {j = \left( {n + 3} \right)/2,} \right.\\ \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\left. {\left( {n + 5} \right)/2, \cdots ,n} \right)\\ {B_{n,\left( {n - 1} \right)/2}}\left( u \right) = \left( {1 - u} \right){B_{n - 1,\left( {n - 1} \right)/2}}\left( u \right)\\ {B_{n,\left( {n + 1} \right)/2}}\left( u \right) = u{B_{n - 1,\left( {n - 1} \right)/2}}\left( u \right) \end{array} \right. $ | (28) |
式中,
基于三次
例如,当
$ \left\{ \begin{array}{l} {B_{4,0}}\left( u \right) = {\left( {1 - u} \right)^2}\left( {1 + 2\left( {1 - \alpha } \right)u} \right)\\ {B_{4,1}}\left( u \right) = 2\alpha {\left( {1 - u} \right)^3}u\\ {B_{4,2}}\left( u \right) = 4\alpha {\left( {1 - u} \right)^2}{u^2}\\ {B_{4,3}}\left( u \right) = 2\alpha \left( {1 - u} \right){u^3}\\ {B_{4,4}}\left( u \right) = {u^2}\left( {1 + 2\left( {1 - \alpha } \right)\left( {1 - u} \right)} \right) \end{array} \right. $ |
当
$ \left\{ \begin{array}{l} {B_{5,0}}\left( u \right) = {\left( {1 - u} \right)^2}\left( {1 + 2\left( {1 - \alpha } \right)u} \right)\\ {B_{5,1}}\left( u \right) = 2\alpha {\left( {1 - u} \right)^3}u\\ {B_{5,2}}\left( u \right) = 4\alpha {\left( {1 - u} \right)^3}{u^2}\\ {B_{5,3}}\left( u \right) = 4\alpha {\left( {1 - u} \right)^2}{u^3}\\ {B_{5,4}}\left( u \right) = 2\alpha \left( {1 - u} \right){u^3}\\ {B_{5,5}}\left( u \right) = {u^2}\left( {1 + 2\left( {1 - \alpha } \right)\left( {1 - u} \right)} \right) \end{array} \right. $ |
根据定理1,利用数学归纳法可证明任意
1) 非负性:
2) 单位分解性:
3) 对称性:
4) 单峰性:
5) 关于参数
6) 端点性:在端点处有
$ {B_{n,i}}\left( 0 \right) = \left\{ \begin{array}{l} 1\;\;\;\;i = 0\\ 0\;\;\;i \ne 0 \end{array} \right. $ |
$ {B_{n,i}}\left( 1 \right) = \left\{ \begin{array}{l} 1\;\;\;\;i = n\\ 0\;\;\;i \ne n \end{array} \right. $ |
$ {{B'}_{n,i}}\left( 0 \right) = \left\{ \begin{array}{l} - 2\alpha \;\;\;\;i = 0\\ 2\alpha \;\;\;\;\;\;i = 1\\ 0\;\;\;\;\;\;\;\;i \ne 0,1 \end{array} \right. $ |
$ {{B'}_{n,i}}\left( 1 \right) = \left\{ \begin{array}{l} 2\alpha \;\;\;\;\;\;\;\;i = n\\ - 2\alpha \;\;\;\;\;\;i = n - 1\\ 0\;\;\;\;\;\;\;\;\;\;\;i \ne n - 1,n \end{array} \right. $ |
基于
定义4 给定
$ {\mathit{\boldsymbol{R}}_n}\left( u \right) = \sum\limits_{i = 0}^n {{B_{n,i}}\left( u \right){\mathit{\boldsymbol{p}}_n}} $ | (29) |
为带形状参数
由
1) 凸包性。
2) 对称性。
3) 几何不变性与仿射不变性。
4) 形状可调性。
5) 变差缩减性与保凸性。
6) 端点性。曲线在端点处满足
$ \left\{ \begin{array}{l} {\mathit{\boldsymbol{R}}_n}\left( 0 \right) = {\mathit{\boldsymbol{p}}_0}\\ {\mathit{\boldsymbol{R}}_n}\left( 0 \right) = {\mathit{\boldsymbol{p}}_n}\\ {{\mathit{\boldsymbol{R'}}}_n}\left( 0 \right) = 2\alpha \left( {{\mathit{\boldsymbol{p}}_1} - {\mathit{\boldsymbol{p}}_0}} \right)\\ {{\mathit{\boldsymbol{R'}}}_n}\left( 1 \right) = 2\alpha \left( {{\mathit{\boldsymbol{p}}_n} - {\mathit{\boldsymbol{p}}_{n - 1}}} \right) \end{array} \right. $ |
与三次
图 8为当参数
4 结论
为了构造出带参数的同次Ball基,首先将三次Ball基的定义区间由[0, 1]扩展为
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