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发布时间: 2019-04-16 |
计算机图形学 |
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收稿日期: 2018-08-03; 修回日期: 2018-09-21
基金项目: 国家自然科学基金项目(61861040);甘肃省科技项目(17YF1FA119);甘肃省教育厅科技成果转化项目(2017D-09);兰州市科技项目(2018-4-35)
第一作者简介:
汪凯, 1993年生, 男, 硕士研究生, 主要研究方向为计算机图形学、图形处理。E-mail:616688448@qq.com;
张贵仓, 男, 教授, 博士, 主要研究方向为计算机图形学、图形处理。E-mail:zhanggc@nwnu.edu.cn; 拓明秀, 女, 硕士研究生, 主要研究方向为计算机辅助几何设计。E-mail:1219384006@qq.com.
中图法分类号: TP391.72
文献标识码: A
文章编号: 1006-8961(2019)04-0615-15
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摘要
目的 为了使构造的曲线拥有传统Bézier曲线的良好性质,同时还具备形状可调性、逼近性、保形性以及实用性。方法 首先在拟扩展切比雪夫空间的框架下,构造了一类具有全正性的拟三次三角Bernstein基函数,并给出了该基函数的性质;基于此基函数,构造了相应的拟三次三角Bézier曲线,分析了其曲线的性质,得到了生成曲线的割角算法以及C1,C2光滑拼接条件,同时还提出了一种估计曲线逼近控制多边形程度的三角Bernstein算子;接着在拟三次三角Bernstein基函数的基础上提出一种三角域上带3个指数参数的拟三次三角Bernstein-Bézier基,基于此基生成了一种三角域上的拟三次三角Bernstein-Bézier曲面,该曲面可以构建边界为椭圆弧、抛物线弧以及圆弧的曲面,此外,还提出一种实用的de-Casteljau-type算法,同时还给出了连接两个曲面的G1连续条件。结果 实验表明,本文在拟扩展切比雪夫空间中构造的具有全正性的曲线曲面,能够灵活地进行形状调整,而且具有良好的逼近性以及适用性。结论 本文在拟扩展切比雪夫空间的框架下构造了一类具有全正性的基函数,并以此基函数进行曲线曲面构造。实验表明本文构造的曲线具备传统三次Bézier曲线的所有优良性质,而且具有灵活的形状可调性。随着参数的增大,所生成的曲线能够更加逼近控制多边形,模拟控制多边形的行为。此外,本文在三角域上构造的曲面能够生成边界为椭圆弧的曲面。综上,本文提出的基函数满足几何工业的需要,是一种实用的方法。
关键词
拟扩展切比雪夫空间; 全正性; 割角算法; 三角域曲面; de-Casteljau-tpye算法
Abstract
Objective The construction of basis functions has consistently been a difficult point of computer-aided geometric design (CAGD). The construction of a class of practical basis functions often plays a decisive role in the development of the geometric industry. The traditional Bézier curves and B-splines have been widely used in CAGD. However, when the control points are determined, the generated curve is relatively fixed with respect to the control points and has a certain rigidity. Although the proposed rational B-spline curve can adjust the curve by adjusting the weight factor, the rational methods have difficulty in predicting the influence of the weight factor on the curve due to its own shortcomings. Researchers have exerted efforts in the past two decades to solve this problem. However, most of the improved methods have the basic properties of the traditional Bézier method and the B-spline method, such as affine invariance, convex hull, non-negativity, geometric invariance and flexible shape adjustability. Moreover, the proposed curve can accurately represent special curves used in engineering, such as conic and hyperbolic curves. However, most of the literature does not discuss the variation diminishing the generated curve. The curve with vanishing variation must have convexity. The curve with the total positivity must have diminishing variation. Therefore, the total positivity of the basis functions indicates that these functions are suitable for geometric design. We can easily obtain rectangular patches with shape parameters through these new curves. However, the Bernstein-Bézier patch over the triangular domain is not a tensor product patch exactly. Therefore, we cannot obtain triangular surfaces with an adjustable shape using the method of tensor product. Surface modeling over triangular domain is important for many applications. Thus, the practical methods for generating surfaces over a triangular domain must be explored. The blossom property in quasi extended Chebyshev space is used to construct a group of optimal normalized entirely positive basis for curve and surface construction. This method enables the extended curve and surface, thereby maintaining the good nature of the traditional Bézier and B-spline methods while preserving shape, shape adjustability, and practicability. Method A class of cubic trigonometric quasi Bernstein basis functions with total positivity is constructed under the framework of the extended Chebyshev space, and the properties of the basis functions are provided. The corresponding curve is presented based on this basis function. The properties of the curve are analyzed. The cutting algorithm of the curve and the smooth connecting conditions are obtained. A trigonometric quasi Bernstein operator for estimating the degree of the control polygon is also proposed. Then, based on the cubic trigonometric quasi Bernstein basis function, a class of trigonometric polynomial basis functions with three shape parameters over the triangular domain is proposed. A type of triangular polynomial patch over the triangular domain is proposed based on this basis functions. This patch can be used to construct patches whose boundaries are elliptical arcs, parabolic arcs, and arcs. A practical de-Casteljau-type algorithm is proposed to calculate the proposed triangular polynomial surface efficiently and stably. In addition, G1 continuous conditions for joining two triangular polynomial patches are provided. Result Experimental results show that the proposed total positivity patch in the frame of Chebyshev space not only can adjust the shape flexibly but also has shape preservation and good approximation. Conclusion We construct a class of basis functions with total positivity under the framework of the extended Chebysh ev space, and construct the curve and surface with this basis function. Experimental results show that the curve constructed in this study has all the excellent properties of a traditional cubic Bézier curve and has flexible shape adjustability. As the parameters increase, the generated curve can be closer to the control polygon, thereby simulating its behavior. In addition, the surface constructed on the triangular domain can generate the surface whose boundaries are elliptical arcs. A de Casteljau-type algorithm for calculating the surface is also provided. In summary, the proposed basis function satisfies the requirements of the geometric industry and is a practical method.
Key words
quasi extended Chebyshev space; totally positivity; corner cutting algorithm; triangular patch; de-Casteljau-type algorithm
0 引言
基函数的构造一直是CAGD(computer aided geometric design)的一个难点,一类实用基函数的构造往往对几何工业的发展起着决定性的作用。众所周知,传统Bézier曲线和B样条已经被广泛运用于CAGD中,然而当控制顶点确定的情况下,生成的曲线相对于控制顶点较为固定,具有一定的刚性。尽管提出的有理B样条曲线可以通过调整权因子的方式对曲线进行调整,但是有理化方法因其本身的缺点,使得权因子对曲线的影响很难预测[1-3]。为了解决该类问题,二十多年来,研究人员做出了大量的努力,主要集中在两个方面:1)在经典的三次Bézier曲线以及B样条曲线中加入参数来增强曲线的调整能力[4-7];2)在非多项式空间,如三角空间、双曲空间构造类似经典Bézier以及B样条性质的曲线曲面[8-14]。
上述改进的方法大都具备传统Bézier方法和B样条方法的基本性质,如凸包性、非负性、几何不变性以及灵活的形状可调性,并且提出的曲线能够精确地表示工程上常用的特殊曲线,如圆锥曲线、双曲线等。但是大多数文献并没有讨论生成曲线的变差缩减性,具有变差缩减性的曲线一定具有保凸性,具有全正性的曲线一定具有变差缩减性,因此是否具有全正性是衡量一组基函数是否适合曲线设计的标准之一[15-17]。
众所周知,传统的矩形域曲面已经被广泛运用到CAGD中,具有很高的研究与应用价值。由于传统矩形域曲面是传统Bézier曲线通过张量积方法的直接扩展,因此很容易得到带形状参数的矩形域曲面,然而三角域上的曲面并不是一个张量积曲面,因此无法直接通过张量积的方法获得三角域上的带形状参数的曲面。但是在很多应用中,基于三角域的曲面模型非常重要,因此三角域曲面同样具有很高的研究价值。近年来,大量学者也为此做出了很多努力,具体可参考文献[18-23]以及其他相关文献。在文献[18]中,Cao等人构造了一种基于三角域的带一个形状参数的Bernstein-Bézier曲面,在控制网格固定的情况下,通过改变形状参数的值可以得到不同的曲面。在文献[19]中,Shen等人提出了一种基于三角域的带一个形状参数的线性类Bernstein三角多项式基函数。在文献[24]中,Zhang将文献[20]中的C-Bézier基扩展成了基于三角域的带一个形状参数的类Bézier基,该基可以生成边界为椭圆弧的曲面。在文献[21]中,Yang等人给出了一种带
拟扩展切比雪夫(QEC)空间具有适合构造B基的开花性质,成为适合几何设计的最大一类空间[25-30]。另外,基于三角函数空间的类Bézier基和类B样条基在保形设计中具有很大的潜力[31-34]。为此,本文在函数空间
1 拟三次三角Bézier曲线
1.1 最优规范全正基的构造
对任意的
$ \begin{array}{*{20}{c}} {\mathit{\boldsymbol{ \boldsymbol{\varPhi} }}\left( t \right) = }\\ {\left( {{{\sin }^2}t,{{\left( {1 - \sin t} \right)}^2}{{\rm{e}}^{ - \alpha \sin t}},{{\left( {1 - \cos t} \right)}^2}{{\rm{e}}^{ - \beta \cos t}}} \right)} \end{array} $ | (1) |
由文献[25],只要证明拟三次三角函数空间
$ \begin{array}{*{20}{c}} {{\rm{D}}{\mathit{\boldsymbol{T}}_{\alpha ,\beta }} = \left\{ {2\sin t\cos t,{{\rm{e}}^{ - \alpha \sin t}}\left( {1 - \sin t} \right) \times } \right.}\\ {\cos t\left( { - 2 - \alpha + \alpha \sin t} \right),}\\ {\left. { - {{\rm{e}}^{ - \beta \cos t}}\left( {1 - \cos t} \right)\sin t\left( { - 2 - \beta + \beta \cos t} \right)} \right\}} \end{array} $ |
是闭区间
定理1 对任意的
证明 对任意的
$ \begin{array}{*{20}{c}} {{\xi _0}\left[ {2\sin t\cos t} \right] + }\\ {{\xi _1}\left[ {{{\rm{e}}^{ - \alpha \sin t}}\left( {1 - \sin t} \right)\cos t\left( { - 2 - \alpha + \alpha \sin t} \right)} \right] + }\\ {{\xi _2}\left[ { - {{\rm{e}}^{ - \beta \cos t}}\left( {1 - \cos t} \right)\sin t\left( { - 2 - \beta + \beta \cos t} \right)} \right] = 0} \end{array} $ | (2) |
当
当
最后可得
下面先证明
$ \begin{array}{l} u\left( t \right) = \frac{{{{\rm{e}}^{ - \alpha \sin t}}\cos t}}{{{{\sin }^2}t}}\left[ {2 + {\alpha ^2}{{\left( {1 - \sin t} \right)}^2}\sin t + } \right.\\ \left. {\alpha \left( {1 - {{\sin }^2}t} \right) + 2\alpha \sin t\left( {1 - \sin t} \right)} \right] > 0 \end{array} $ |
$ \begin{array}{l} u'\left( t \right) = - \frac{{{{\rm{e}}^{ - \beta \sin t}}}}{{\sin t}}\left[ {2 + {\alpha ^2}{{\left( {1 - \sin t} \right)}^2}\sin t + } \right.\\ \left. {\alpha \left( {1 - {{\sin }^2}t} \right) + 2\alpha \sin t\left( {1 - \sin t} \right)} \right] - \\ \frac{{{{\rm{e}}^{ - \alpha \sin t}}{{\cos }^2}t}}{{{{\sin }^3}t}}\left\{ {4 + {\alpha ^3}{{\left( {1 - \sin t} \right)}^2}{{\sin }^2}t + \alpha \left( {2 + 4\sin t} \right) + } \right.\\ \left. {2{\alpha ^2}\sin t\left[ {\left( {1 - {{\sin }^2}t} \right) + \sin t\left( {1 - \sin t} \right)} \right]} \right\} < 0 \end{array} $ |
$ \begin{array}{l} v\left( t \right) = \frac{{{{\rm{e}}^{ - \beta \cos 0t}}\sin t}}{{{{\cos }^2}t}}\left[ {2 + {\beta ^2}{{\left( {1 - \cos t} \right)}^2}\cos t + } \right.\\ \left. {\beta \left( {1 - {{\cos }^2}t} \right) + 2\beta \cos t\left( {1 - \cos t} \right)} \right] > 0 \end{array} $ |
$ \begin{array}{l} v'\left( t \right) = \frac{{{{\rm{e}}^{ - \beta \cos t}}}}{{\cos t}}\left[ {2 + {\beta ^2}{{\left( {1 - \cos t} \right)}^2}\cos t + } \right.\\ \left. {\beta \left( {1 - {{\cos }^2}t} \right) + 2\beta \cos t\left( {1 - \cos t} \right)} \right] + \\ \frac{{{{\rm{e}}^{ - \beta \cos t}}{{\sin }^2}t}}{{{{\cos }^3}t}}\left\{ {4 + {\beta ^3}{{\left( {1 - \cos t} \right)}^2}{{\cos }^2}t + \beta \left( {2 + 4\cos t} \right) + } \right.\\ \left. {2{\beta ^2}\cos t\left[ {\left( {1 - {{\cos }^2}t} \right) + \cos t\left( {1 - \cos t} \right)} \right]} \right\} > 0 \end{array} $ |
因此,关于函数
$ \begin{array}{*{20}{c}} {W\left( {u,v} \right)\left( t \right) = u\left( t \right)v'\left( t \right) - u'\left( t \right)v\left( t \right) > 0}\\ {\forall t \in \left[ {0,{\rm{ \mathsf{ π} }}/2} \right]} \end{array} $ |
对
$ \left\{ \begin{array}{l} {w_0}\left( t \right) = 2\sin t\cos t\\ {w_1}\left( t \right) = Au\left( t \right) + Bv\left( t \right)\\ {w_2}\left( t \right) = C\frac{{W\left( {u,v} \right)\left( t \right)}}{{{{\left[ {Au\left( t \right) + Bv\left( t \right)} \right]}^2}}} \end{array} \right. $ |
式中,
$ \left\{ \begin{array}{l} {u_0}\left( t \right) = {w_0}\left( t \right)\\ {u_1}\left( t \right) = {w_0}\left( t \right)\int_a^t {{w_1}\left( {{t_1}} \right){\rm{d}}{t_1}} \\ {u_2}\left( t \right) = {w_0}\left( t \right)\int_a^t {{w_1}\left( {{t_1}} \right)} \int_a^{{t_1}} {{w_2}\left( {{t_2}} \right){\rm{d}}{t_2}{\rm{d}}{t_1}} \end{array} \right. $ |
可以验证,
下面证明
$ \begin{array}{*{20}{c}} {F\left( t \right) = {C_0}\left[ {2\sin t\cos t} \right] + }\\ {{C_1}\left[ {{{\rm{e}}^{ - \alpha \sin t}}\left( {1 - \sin t} \right)\cos t\left( { - 2 - \alpha + \alpha \sin t} \right)} \right]}\\ {{C_2}\left[ { - {{\rm{e}}^{ - \beta \cos t}}\left( {1 - \cos t} \right)\sin t\left( { - 2 - \beta + \beta \cos t} \right)} \right]} \end{array} $ |
式中,
假设
在此情况下,如果
如果
如果
$ \begin{array}{*{20}{c}} {F\left( t \right) = {C_0}\left[ {2\sin t\cos t} \right] + }\\ {{C_2}\left[ { - {{\rm{e}}^{ - \beta \cos t}}\left( {1 - \cos t} \right)\sin t\left( { - 2 - \beta + \beta \cos t} \right)} \right] = }\\ {\sin t\left[ {2{C_0}\cos t + {C_2}{e^{ - \beta \cos t}}\left( {1 - \cos t} \right)\left( {2 + \beta - \beta \cos t} \right)} \right]} \end{array} $ |
很明显
$ {{\rm{e}}^{ - \beta \cos t}}\left( {1 - \cos t} \right)\left( {2 + \beta - \beta \cos t} \right) > 0 $ |
所以
如果
$ g\left( t \right) = 2{C_0}\cos t + {C_2}{{\rm{e}}^{ - \beta \cos t}}\left( {1 - \cos t} \right)\left( {2 + \beta - \beta \cos t} \right) $ |
$ \begin{array}{l} g'\left( t \right) = \sin t\left\{ { - 2{C_0} + } \right.\\ \left. {{{\rm{e}}^{ - \beta \cos t}}{C_2}\left[ {2 + 4b + {b^2} - 2b\left( {2 + b} \right)\cos t + {b^2}{{\cos }^2}t} \right]} \right\} \end{array} $ |
令
可知当
可见
由于
定理2 对于任意的
$ \left\{ \begin{array}{l} {T_0}\left( t \right) = {\left( {1 - \sin t} \right)^2}{{\rm{e}}^{ - \alpha \sin t}}\\ {T_1}\left( t \right) = 1 - {\sin ^2}t - {\left( {1 - \sin t} \right)^2}{{\rm{e}}^{ - \alpha \sin t}}\\ {T_2}\left( t \right) = 1 - {\cos ^2}t - {\left( {1 - \cos t} \right)^2}{{\rm{e}}^{ - \beta \cos t}}\\ {T_3}\left( t \right) = {\left( {1 - \cos t} \right)^2}{{\rm{e}}^{ - \beta \cos t}} \end{array} \right. $ | (3) |
称之为QCT-Bernstein基函数。
证明 对于任意的
$ \mathit{\boldsymbol{ \boldsymbol{\varPhi} }}\left( 0 \right) = \left( {0,1,0} \right) $ |
$ \mathit{\boldsymbol{ \boldsymbol{\varPhi} }}\left( {{\rm{ \mathsf{ π} }}/2} \right) = \left( {1,0,1} \right) $ |
$ \mathit{\boldsymbol{ \boldsymbol{\varPhi} '}}\left( 0 \right) = \left( {2, - \left( {\alpha + 2} \right),0} \right) $ |
$ \mathit{\boldsymbol{ \boldsymbol{\varPhi} '}}\left( {{\rm{ \mathsf{ π} }}/2} \right) = \left( {0,0,2 + \beta } \right) $ |
$ \mathit{\boldsymbol{ \boldsymbol{\varPhi} ''}}\left( 0 \right) = \left( {2,{\alpha ^2} + 4\alpha + 2,0} \right) $ |
$ \mathit{\boldsymbol{ \boldsymbol{\varPhi} ''}}\left( {{\rm{ \mathsf{ π} }}/2} \right) = \left( { - 2,0,{\beta ^2}4\beta + 2} \right) $ |
由此可得
$ {\mathit{\Pi }_0} = \mathit{\boldsymbol{ \boldsymbol{\varPhi} }}\left( 0 \right) = \left( {0,1,0} \right) $ |
$ {\mathit{\Pi }_3} = \mathit{\boldsymbol{ \boldsymbol{\varPhi} }}\left( {{\rm{ \mathsf{ π} }}/2} \right) = \left( {0,0,2 + \beta } \right) $ |
$ \left\{ {{H_1}} \right\} = {O_{S{C_1}}}\mathit{\boldsymbol{ \boldsymbol{\varPhi} }}\left( 0 \right) \cap {O_{S{C_2}}}\mathit{\boldsymbol{ \boldsymbol{\varPhi} }}\left( {{\rm{ \mathsf{ π} }}/2} \right) = \left( {0,0,0} \right) $ |
$ \left\{ {{H_2}} \right\} = {O_{S{C_1}}}\mathit{\boldsymbol{ \boldsymbol{\varPhi} }}\left( 0 \right) \cap {O_{S{C_1}}}\mathit{\boldsymbol{ \boldsymbol{\varPhi} }}\left( {{\rm{ \mathsf{ π} }}/2} \right) = \left( {1,0,0} \right) $ |
对
$ \left\{ \begin{array}{l} {T_2}\left( t \right) + {T_3}\left( t \right) = so{m^2}t\\ {T_0}\left( t \right) = {\left( {1 - \sin t} \right)^2}{{\rm{e}}^{ - \alpha \sin y}}\\ {T_3}\left( t \right) = {\left( {1 - \cos t} \right)^2}{{\rm{e}}^{ - \beta \cos t}} \end{array} \right. $ |
由上式连同
首先,证明
$ \sum\limits_{i = 0}^3 {{\xi _i}{T_i}\left( t \right)} = 0 $ | (4) |
两边对
$ \sum\limits_{i = 0}^3 {{\xi _i}{{T'}_i}\left( t \right)} = 0 $ | (5) |
把
$ \left\{ \begin{array}{l} \xi = 0\\ \left( {\alpha + 1} \right)\left( {{\xi _0} - {\xi _1}} \right) = 0 \end{array} \right. $ |
由此可得
容易验证
因为
对于
因此由文献[25]可知QCT-Bernstein基
为了方便讨论,将定义相应的基函数为
1.2 拟三次三角Bézier曲线
1.2.1 拟三次三角Bézier曲线的定义和性质
定义1 对于给定的控制点
$ T\left( {t;\alpha ,\beta } \right) = \sum\limits_{i = 0}^3 {{P_i}{T_i}\left( {t;\alpha ,\beta } \right)} $ | (6) |
为带2个指数参数
由于QCT-Bernstein基函数具有单位性、非负性和全正性,因此相应的QCT-Bézier曲线具有仿射不变性、凸包性和变差缩减性。此外当
$ \left\{ \begin{array}{l} P\left( {0;\alpha ,\beta } \right) = {P_0}\\ P\left( {{\rm{ \mathsf{ π} }}/2;\alpha ,\beta } \right) = {P_3}\\ P'\left( {0;\alpha ,\beta } \right) = \left( {2 + \alpha } \right)\left( {{P_1} - {P_0}} \right)\\ P'\left( {0;{\rm{ = }}\alpha ,\beta } \right) = \left( {2 + \beta } \right)\left( {{P_3} - {P_3}} \right)6\\ 2\left( {{P_2} - {P_1}} \right)\\ P''\left( {{\rm{ \mathsf{ π} /2;}}\alpha {\rm{,}}\beta } \right) = \left( {\beta + 4\beta + 2} \right)\left( {{P_3} - {P_2}} \right) + \\ 2\left( {{P_1} - {P_2}} \right) \end{array} \right. $ |
1.2.2 QCT-Bézier曲线的形状控制
对于
$ \begin{array}{*{20}{c}} {T\left( {t;\alpha ,\beta } \right) = {P_1}{{\cos }^2}t + {P_2}{{\sin }^2}t + }\\ {{T_0}\left( {t;\alpha } \right)\left( {{P_0} - {P_1}} \right) + {T_3}\left( {t;\beta } \right)\left( {{P_3} - {P_2}} \right)} \end{array} $ | (7) |
显然,对于
1.2.3 QCT-Bézier曲线的割角算法
下面开发一种稳定高效计算QCT-Bézier曲线的割角算法。为此,将QCT-Bézier曲线写成式(8)的形式。图 3展示了利用割角算法计算QCT-Bézier曲线的全过程以及案例。
$ \begin{array}{*{20}{c}} {A\left( {t;\alpha ,\beta } \right) = \left( {\begin{array}{*{20}{c}} {1 - {{\sin }^2}t}&{1 - {{\cos }^2}t} \end{array}} \right) \times }\\ {\left( {\begin{array}{*{20}{c}} {1 - \sin t}&{\sin t}&0\\ 0&{\cos t}&{1 - \cos t} \end{array}} \right) \times }\\ {\left( {\begin{array}{*{20}{c}} {\frac{{{{\rm{e}}^{ - \alpha \sin t}}}}{{1 + \sin t}}}&{\frac{{1 + \sin t - {{\rm{e}}^{ - \alpha \sin t}}}}{{1 + \sin t}}}&0&0\\ 0&{\frac{{\cos t}}{{\sin t + \cos t}}}&{\frac{{\sin t}}{{\sin t + \cos t}}}&0\\ 0&0&{\frac{{1 + \cos t - {{\rm{e}}^{ - \beta \cos t}}}}{{1 + \cos t}}}&{\frac{{{{\rm{e}}^{ - \beta \cos t}}}}{{1 + \cos t}}} \end{array}} \right) \times }\\ {\left( {\begin{array}{*{20}{c}} {{P_0}}\\ {{P_1}}\\ {{P_2}}\\ {{P_3}} \end{array}} \right)} \end{array} $ | (8) |
1.2.4 椭圆和抛物线的精确表示
本文提出的QCT-Bézier曲线可精确表示椭圆和抛物线。对
$ \left\{ \begin{array}{l} {P_0} = \left( {{x_0} + a,{y_0}} \right)\\ {P_1} = \left( {{x_0} + a,{y_0} + b/2} \right)\\ {P_2} = \left( {{x_0} + a/2,{y_0} + b} \right)\\ {P_3} = \left( {{x_0},{y_0} + b} \right) \end{array} \right. $ |
则得式(6)中
$ \left\{ \begin{array}{l} x\left( t \right) = {x_0} + a\cos t\\ y\left( t \right) = {y_0} + b\sin t \end{array} \right.\;\;\;t \in \left[ {0,{\rm{ \mathsf{ π} }}/2} \right] $ |
这表明
此外,对
$ \left\{ \begin{array}{l} x\left( t \right) = \left( {b - a} \right)\cos t + a\\ y\left( t \right) = {c_2}{\left[ {\left( {b - a} \right)\cos t + a} \right]^2} + \\ {c_1}\left[ {\left( {b - a} \right)\cos t + a} \right] + {c_0} \end{array} \right. $ |
式中,
1.2.5 QCT-Bézier曲线的拼接
在实际应用过程中,需要通过拼接QCT-Bézier曲线来生成几何造型复杂的曲线。设两段QCT-Bézier曲线分别为
$ {F_1}\left( {t;{\alpha _1},{\beta _1}} \right) = \sum\limits_{i = 0}^3 {{P_i}{A_i}\left( {t;{\alpha _1},{\beta _1}} \right)} $ | (9) |
$ {F_1}\left( {t;{\alpha _2},{\beta _2}} \right) = \sum\limits_{i = 0}^3 {{P_i}{A_i}\left( {t;{\alpha _2},{\beta _2}} \right)} $ | (10) |
显然,若控制顶点满足
为了方便讨论,对于节点
$ F\left( u \right) = \left\{ \begin{array}{l} {F_1}\left( {\frac{{\rm{ \mathsf{ π} }}}{2} \times \frac{{u - {u_1}}}{{{h_1}}};{\alpha _1},{\beta _1}} \right)\;\;\;\;\;\;u \in \left[ {{u_1},{u_2}} \right]\\ {F_2}\left( {\frac{{\rm{ \mathsf{ π} }}}{2} \times \frac{{u - {u_2}}}{{{h_2}}};{\alpha _2},{\beta _2}} \right)\;\;\;\;\;u \in \left[ {{u_2},{u_3}} \right] \end{array} \right. $ | (11) |
式中,
定理3 对任意
$ {P_3} = {Q_0} = \frac{{\left( {{\alpha _2} + 2} \right){h_1}{Q_1} + \left( {{\beta _1} + 2} \right){h_2}{P_2}}}{{\left( {{\alpha _2} + 2} \right){h_1} + \left( {{\beta _1} + 2} \right){h_2}}} $ | (12) |
成立。进一步,对任意
$ \begin{array}{*{20}{c}} {{Q_2} = \frac{1}{{2{\alpha _2}h_1^2}}\left\{ {2\beta {h_1}{h_2} + {h_2}\left[ {{\alpha _2}{h_2}\left( {\beta _1^2 + 4{\beta _1} + 2} \right) + } \right.} \right.}\\ {\left. {\left. {{\beta _1}{h_1}\left( {\alpha _2^2 + 4{\alpha _2} + 2} \right)} \right]} \right\}\left( {{P_3} - {P_2}} \right) + \frac{{h_2^2}}{{h_1^2}}\left( {{P_1} - {P_2}} \right) + {P_3}} \end{array} $ |
证明 对任意
$ \left\{ \begin{array}{l} F\left( {u_2^ - } \right) = {P_3}\\ F\left( {u_2^ + } \right) = {Q_0}\\ F'\left( {u_2^ - } \right) = \frac{{\rm{ \mathsf{ π} }}}{2}\frac{{\left( {{\beta _1} + 3} \right)}}{{{h_1}}}\left( {{P_3} - {P_2}} \right)\\ F'\left( {u_2^ + } \right) = \frac{{\rm{ \mathsf{ π} }}}{2}\frac{{\left( {{\alpha _2} + 3} \right)}}{{{h_2}}}\left( {{Q_1} - {Q_0}} \right)\\ F''\left( {u_2^ - } \right) = {\left( {\frac{{\rm{ \mathsf{ π} }}}{{2{h_1}}}} \right)^2}\left[ {\left( {\beta _1^2 + 4{\beta _1} + 2} \right) \times } \right.\\ \left. {\left( {{P_3} - {P_2}} \right) + 2\left( {{P_1} - {P_2}} \right)} \right]\\ F''\left( {u_2^ + } \right) = {\left( {\frac{{\rm{ \mathsf{ π} }}}{{2{h_2}}}} \right)^2}\left[ {\left( {\alpha _2^2 + 4{\alpha _2} + 2} \right) \times } \right.\\ \left. {\left( {{Q_0} - {Q_1}} \right) + 2\left( {{Q_2} - {Q_1}} \right)} \right] \end{array} \right. $ |
由此可得
图 5显示的是QCT-Bézier的拼接。对于
1.2.6 拟三角Bernstein算子
拟三角Bernstein算子对测量向量函数空间的近似性质是非常有用的,算子的第3大特征值和1的差值可以粗略地估计曲线与控制多边形的近似程度[37],差值越小说明曲线和控制多边形越接近。本小节将构造一种拟三角Bernstein算子来分析给出的曲线式(6)和相应控制多边形的逼近程度。
对于任意的
$ \begin{array}{*{20}{c}} {{f_1}\left( {t;\alpha } \right) = \frac{1}{\alpha }{T_1}\left( {t;\alpha } \right) + \left( {1 - \frac{1}{\alpha }} \right){T_2}\left( {t;\alpha } \right) + }\\ {{T_3}\left( {t;\alpha } \right)} \end{array} $ | (13) |
式中,
直接计算有
$ \begin{array}{l} \frac{{{\rm{d}}{f_1}\left( {t;\alpha } \right)}}{{{\rm{d}}t}} = 2\left( {1 - \frac{2}{\alpha }} \right)\sin t\cos t + \\ {{\rm{e}}^{ - \alpha \sin t}}\cos t\left( {1 - \sin t} \right)\left( {2 + \alpha - \alpha \sin t} \right) + \\ {{\rm{e}}^{ - \alpha \cos t}}\sin t\left( {1 - \cos t} \right)\left( {2 + \alpha - \alpha \cos t} \right) > 0 \end{array} $ |
不难发现,对任意的
众所周知,经典的Bernstein算子具有
$ \begin{array}{l} B\left( f \right) = f\left( 0 \right){T_0}\left( {t;\alpha } \right) + f\left( {t_\alpha ^ * } \right){T_1}\left( {t;\alpha } \right) + \\ f\left( {{\rm{ \mathsf{ π} /2}} - t_\alpha ^ * } \right){T_2}\left( {t;\alpha } \right) + f\left( {{\rm{ \mathsf{ π} /2}}} \right){T_3}\left( {t;\alpha } \right) \end{array} $ | (14) |
式中,
明显有
$ B\left( {{f_0}} \right) = {f_0}\left( t \right),B\left( {{f_1}} \right) = {f_1}\left( {t;\alpha } \right) $ |
这意味着特征方程
对于任意的
$ {f_2}\left( {t;\alpha } \right) = 1 - {T_0}\left( {t;\alpha } \right) - {T_3}\left( {t;\alpha } \right) $ | (15) |
该函数满足
$ \begin{array}{*{20}{c}} {B\left( {{f_2}} \right) = {f_2}\left( {t_\alpha ^ * ;\alpha } \right){T_1}\left( {t;\alpha } \right) + }\\ {{f_2}\left( {\left( {{\rm{ \mathsf{ π} /2}} - t_\alpha ^ * } \right);\alpha } \right){T_2}\left( {t;\alpha } \right) = }\\ {{f_2}\left( {t_\alpha ^ * ;\alpha } \right)\left[ {{T_1}\left( {t;\alpha } \right) + {T_2}\left( {t;\alpha } \right)} \right] = }\\ {{f_2}\left( {t_\alpha ^ * ;\alpha } \right){f_2}\left( {t;\alpha } \right)} \end{array} $ |
这表明
此外,对于任意的
$ {f_3}\left( {t;\alpha } \right) = {T_1}\left( {t;\alpha } \right) - {T_2}\left( {t;\alpha } \right) $ | (16) |
该函数满足
$ \begin{array}{*{20}{c}} {B\left( {{f_3}} \right) = {f_3}\left( {t_\alpha ^ * ;\alpha } \right){T_1}\left( {t;\alpha } \right) + }\\ {{f_3}\left( {\left( {{\rm{ \mathsf{ π} /2}} - t_\alpha ^ * } \right);\alpha } \right){T_2}\left( {t;\alpha } \right) = }\\ {{f_3}\left( {t_\alpha ^ * ;\alpha } \right)\left[ {{T_1}\left( {t;\alpha } \right) - {T_2}\left( {t;\alpha } \right)} \right] = }\\ {{f_3}\left( {t_\alpha ^ * ;\alpha } \right){f_3}\left( {t;\alpha } \right)} \end{array} $ |
这表明
下面进一步说明
$ \begin{array}{l} {\lambda _{2,\alpha }} - {\lambda _{3,\lambda }} = {f_2}\left( {t_\alpha ^ * ;\alpha } \right) - {f_3}\left( {t_\alpha ^ * ;\alpha } \right) = \\ 1 - {T_0}\left( {t_\alpha ^ * ;\alpha } \right) - {T_3}\left( {t_\alpha ^ * ;\alpha } \right) - {T_1}\left( {t_\alpha ^ * ;\alpha } \right) + \\ {T_2}\left( {t_\alpha ^ * ;\alpha } \right) = 2{T_2}\left( {t_\alpha ^ * ;\alpha } \right) > 0 \end{array} $ |
最后,对于任意
$ \left| {\begin{array}{*{20}{c}} {{T_1}\left( {t_\alpha ^ * ;\alpha } \right)}&{{T_2}\left( {t_\alpha ^ * ;\alpha } \right)}\\ {{T_1}\left( {{\rm{ \mathsf{ π} }}/2;\alpha } \right)}&{{T_2}\left( {{\rm{ \mathsf{ π} }}/2;\alpha } \right)} \end{array}} \right| = {\lambda _{3,\lambda }} \ge 0 $ |
从上式可以得出
从上面的讨论中,可以发现
$ \mathop {\lim }\limits_{\alpha \to + \infty } 1 - {\lambda _{2,\alpha }} = 0 $ |
因此,随着参数
2 三角域上拟三次三角Bernstein-Bézier基
本节将扩展QCT-Bernstein基(式(3))形成三角域上的拟三次三角Bernstein-Bézier(QTC-Bernstein-Bézier)基。
2.1 三角域上QCT-Bernstein-Bézier基的构造
定义2 对任意的
$ \left\{ \begin{array}{l} T_{3,0,0}^3\left( {u,v,w;\alpha ,\beta ,\gamma } \right) = {\left( {1 - \cos u} \right)^2}{{\rm{e}}^{ - \alpha \cos u}}\\ T_{0,3,0}^3\left( {u,v,w;\alpha ,\beta ,\gamma } \right) = {\left( {1 - \cos v} \right)^2}{{\rm{e}}^{ - \beta \cos v}}\\ T_{0,0,3}^3\left( {u,v,w;\alpha ,\beta ,\gamma } \right) = {\left( {1 - \cos w} \right)^2}{{\rm{e}}^{ - \gamma \cos w}}\\ T_{2,1,0}^3\left( {u,v,w;\alpha ,\beta ,\gamma } \right) = \cos w\sin v\left( {1 - \cos u} \right) \times \\ \frac{{1 + \cos u - \left( {1 - \cos u} \right){{\rm{e}}^{ - \alpha \cos u}}}}{{\cos u}}\\ T_{2,0,1}^3\left( {u,v,w;\alpha ,\beta ,\gamma } \right) = \cos v\sin w\left( {1 - \cos u} \right) \times \\ \frac{{1 + \cos u - \left( {1 - \cos u} \right){{\rm{e}}^{ - \alpha \cos u}}}}{{\cos u}}\\ T_{1,2,0}^3\left( {u,v,w;\alpha ,\beta ,\gamma } \right) = \cos w\sin u\left( {1 - \cos v} \right) \times \\ \frac{{1 + \cos v - \left( {1 - \cos v} \right){{\rm{e}}^{ - \beta \cos v}}}}{{\cos v}}\\ T_{0,2,1}^3\left( {u,v,w;\alpha ,\beta ,\gamma } \right) = \cos u\sin w\left( {1 - \cos v} \right) \times \\ \frac{{1 + \cos v - \left( {1 - \cos v} \right){{\rm{e}}^{ - \beta \cos v}}}}{{\cos v}}\\ T_{1,0,2}^3\left( {u,v,w;\alpha ,\beta ,\gamma } \right) = \cos v\sin u\left( {1 - \cos w} \right) \times \\ \frac{{1 + \cos w - \left( {1 - \cos w} \right){{\rm{e}}^{ - \gamma \cos w}}}}{{\cos w}}\\ T_{0,1,2}^3\left( {u,v,w;\alpha ,\beta ,\gamma } \right) = \cos u\sin v\left( {1 - \cos w} \right) \times \\ \frac{{1 + \cos w - \left( {1 - \cos w} \right){{\rm{e}}^{ - \gamma \cos w}}}}{{\cos w}}\\ T_{1,1,1}^3\left( {u,v,w;\alpha ,\beta ,\gamma } \right) = 1 - \sum\limits_{\begin{array}{*{20}{c}} {i + j + k = 3,}\\ {i \cdot j \cdot k \ne 1} \end{array}} {T_{i,j,k}^3\left( {u,v,w;\alpha ,\beta ,\gamma } \right)} \end{array} \right. $ | (17) |
注意,对任意的
$ \begin{array}{l} \mathop {\lim }\limits_{u \to {\rm{ \mathsf{ π} }}/2} \frac{{1 + \cos u - \left( {1 - \cos u} \right){{\rm{e}}^{ - \alpha \cos u}}}}{{\cos u}} = \\ \mathop {\lim }\limits_{u \to {\rm{ \mathsf{ π} }}/2} \frac{{ - \sin u - \sin u\left[ {1 + \alpha \left( {1 - \cos u} \right)} \right]{{\rm{e}}^{ - \alpha \cos u}}}}{{ - \sin u}} = \\ \mathop {\lim }\limits_{u \to {\rm{ \mathsf{ π} }}/2} 1 + \left[ {1 + \alpha \left( {1 - \cos u} \right)} \right]{{\rm{e}}^{ - \alpha \cos u}} = \alpha + 2 \end{array} $ |
类似地,
$ \mathop {\lim }\limits_{u \to {\rm{ \mathsf{ π} }}/2} \frac{{1 + \cos v - \left( {1 - \cos v} \right){{\rm{e}}^{ - \beta \cos v}}}}{{\cos v}} = \beta + 2 $ |
$ \mathop {\lim }\limits_{u \to {\rm{ \mathsf{ π} }}/2} \frac{{1 + \cos w - \left( {1 - \cos w} \right){{\rm{e}}^{ - \gamma \cos w}}}}{{\cos w}} = \gamma + 2 $ |
因此对于不同的指数参数,在三角域
引理1 对
$ 1 - \left( {{{\sin }^2}u + {{\sin }^2}v + {{\sin }^2}w} \right) = 2\sin u\sin v\sin w $ |
证明 对
$ \begin{array}{l} 1 - \left( {{{\sin }^2}u + {{\sin }^2}v + {{\sin }^2}w} \right) = \\ \frac{1}{2}\left( {\cos 2u + \cos 2v + \cos 2w - 1} \right) = \\ \cos \left( {u + v} \right)\cos \left( {u - v} \right) - {\sin ^2}w = \\ \cos \left( {u - v} \right)\sin w - \cos \left( {u + v} \right)\sin w = \\ \left[ {\cos \left( {u - v} \right) - \cos \left( {u + v} \right)} \right]\sin w = \\ 2\sin u\sin v\sin w \end{array} $ |
证毕。
2.2 三角域上QCT-Bernstein-Bézier基的性质
定理4 三角域上QCT-Bernstein-Bézier基具有如下性质
1) 单位性:
2) 非负性:对于任意的
3) 对称性:
$ \begin{array}{l} T_{i,j,k}^3\left( {u,v,w;\alpha ,\beta ,\gamma } \right) = T_{j,i,k}^3\left( {v,u,w;\beta ,\alpha ,\gamma } \right) = \\ T_{j,k,i}^3\left( {v,w,u;\beta ,\gamma ,\alpha } \right) = T_{i,k,j}^3\left( {u,w,v;\alpha ,\gamma ,\beta } \right) = \\ T_{k,i,j}^3\left( {w,u,v;\gamma ,\alpha ,\beta } \right) = T_{i,j,k}^3\left( {w,v,u;\gamma ,\beta ,\alpha } \right) \end{array} $ |
4) 边界性:当3个变量
5) 线性无关性:
证明 下面证明性质2)和5)。其余性质容易证明。
性质2),对任意
$ T_{i,j,k}^3\left( {u,v,w;\alpha ,\beta ,\gamma } \right) \ge 0 $ |
此外,对于
$ \sum\limits_{i + j + k = 3} {{\lambda _{i,j,k}}T_{i,j,k}^3\left( {u,v,w;\alpha ,\beta ,\gamma } \right)} = 0 $ | (18) |
令
$ \sum\limits_{i = 0}^3 {{\xi _{i,\left( {3 - i} \right),0}}{T_i}\left( {u;\alpha ,\beta } \right)} = 0 $ | (19) |
因此,由QCT-Bézier基的线性无关性,可得
图 7给出了部分QCT-Bernstein-Bézier基的图形,指数参数值为
2.3 三角域上QCT-Bernstein-Bézier曲面
定义3 对任意的实数
$ \begin{array}{*{20}{c}} {R\left( {u,v,w} \right) = \sum\limits_{i + j + k = 3} {T_{i,j,k}^3\left( {u,v,w;\alpha ,\beta ,\gamma } \right)} }\\ {{P_{i,j,k}},\left( {u,v,w} \right) \in \mathit{\boldsymbol{D}}} \end{array} $ | (20) |
为三角域上的QCT-Bernstein-Bézier曲面。
由三角域上QCT-Bernstein-Bézier基的性质,易推出相应曲面的性质:
1) 仿射不变性和凸包性。由于基函数具有单位性和非负性,故相应的曲面具有仿射不变性和凸包性。
2) 插值角点性质。直接计算可得
$ \begin{array}{*{20}{c}} {R\left( {{\rm{ \mathsf{ π} }}/2,0,0} \right) = {P_{3,0,0}},R\left( {0,{\rm{ \mathsf{ π} }}/2,0} \right) = {P_{0,3,0}},}\\ {R\left( {0,0,{\rm{ \mathsf{ π} }}/2} \right) = {P_{0,0,3}}} \end{array} $ |
3) 角点切平面性质。令
$ \frac{{{\rm{d}}R\left( {u,v,w} \right)}}{{{\rm{d}}u}}\left| {_{\left( {{\rm{ \mathsf{ π} }}/2,0,0} \right)}} \right. = \left( {2 + \alpha } \right)\left( {{P_{3,0,0}} - {P_{2,0,1}}} \right) $ |
$ \frac{{{\rm{d}}R\left( {u,v,w} \right)}}{{{\rm{d}}v}}\left| {_{\left( {{\rm{ \mathsf{ π} }}/2,0,0} \right)}} \right. = \left( {2 + \alpha } \right)\left( {{P_{2,1,0}} - {P_{2,0,1}}} \right) $ |
$ \frac{{{\rm{d}}R\left( {u,v,w} \right)}}{{{\rm{d}}u}}\left| {_{\left( {0,{\rm{ \mathsf{ π} }}/2,0} \right)}} \right. = \left( {2 + \beta } \right)\left( {{P_{1,2,0}} - {P_{0,2,1}}} \right) $ |
$ \frac{{{\rm{d}}R\left( {u,v,w} \right)}}{{{\rm{d}}v}}\left| {_{\left( {0,{\rm{ \mathsf{ π} }}/2,0} \right)}} \right. = \left( {2 + \beta } \right)\left( {{P_{0,3,0}} - {P_{0,2,1}}} \right) $ |
$ \frac{{{\rm{d}}R\left( {u,v,w} \right)}}{{{\rm{d}}u}}\left| {_{\left( {0,0,{\rm{ \mathsf{ π} }}/2} \right)}} \right. = \left( {2 + \gamma } \right)\left( {{P_{1,0,2}} - {P_{0,0,3}}} \right) $ |
$ \frac{{{\rm{d}}R\left( {u,v,w} \right)}}{{{\rm{d}}v}}\left| {_{\left( {0,0,{\rm{ \mathsf{ π} }}/2} \right)}} \right. = \left( {2 + \gamma } \right)\left( {{P_{0,1,2}} - {P_{0,0,3}}} \right) $ |
这表明QCT-Bernstein-Bézier曲面在3个角点
4) 边界性质。当
$ R\left( {u,v,0} \right) = \sum\limits_{i = 0}^3 {{P_{i,3 - i,0}}{A_i}\left( {u;\alpha ,\beta } \right)} $ | (21) |
类似地,
5) 形状调整性质。由于QCT-Bernstein-Bézier曲面式(20)含有3个指数参数
2.4 de Casteljau-type算法
下面给出一种生成QCT-Bernstein-Bézier曲面高效稳定的de-Casteljau-type算法。对任意
$ \left\{ \begin{array}{l} {f_1}\left( {u,v,w} \right) = \frac{{\sin u\cos w\left( {{{\sin }^2}u + {{\sin }^2}v + {{\sin }^2}w} \right)}}{{\cos w\left( {\sin u + \sin v} \right)\left( {{{\sin }^2}u + {{\sin }^2}v + {{\sin }^2}w} \right) + \sin w\left( {{{\sin }^2}u + {{\sin }^2}v} \right)}}\\ {f_2}\left( {u,v,w} \right) = \frac{{\sin u\cos w\left( {{{\sin }^2}u + {{\sin }^2}v + {{\sin }^2}w} \right)}}{{\cos w\left( {\sin u + \sin v} \right)\left( {{{\sin }^2}u + {{\sin }^2}v + {{\sin }^2}w} \right) + \sin w\left( {{{\sin }^2}u + {{\sin }^2}v} \right)}}\\ {f_3}\left( {u,v,w} \right) = \frac{{\sin w\left( {{{\sin }^2}u + {{\sin }^2}v} \right)}}{{\cos w\left( {\sin u + \sin v} \right)\left( {{{\sin }^2}u + {{\sin }^2}v + {{\sin }^2}w} \right) + \sin w\left( {{{\sin }^2}u + {{\sin }^2}v} \right)}} \end{array} \right. $ |
式中,
$ \left\{ \begin{array}{l} {g_1}\left( {u,v,w} \right) = \left( {1 - \cos u} \right)\left( {{{\sin }^2}u + {{\sin }^2}v + {{\sin }^2}w} \right)\\ {g_2}\left( {u,v,w} \right) = \sin v\cos w\left( {{{\sin }^2}u + {{\sin }^2}v + {{\sin }^2}w} \right) + \\ \sin u\sin v\sin w\\ {g_3}\left( {u,v,w} \right) = \cos v\sin w\left( {{{\sin }^2}u + {{\sin }^2}v + {{\sin }^2}w} \right) + \\ \sin u\sin v\sin w \end{array} \right. $ |
和
$ \left\{ \begin{array}{l} P_{2,0,0}^1 = \\ \frac{{{{\rm{e}}^{ - \alpha \cos u}}}}{{1 + \cos u}}{P_{3,0,0}} + \frac{{\left( {1 + \cos u - {{\rm{e}}^{ - \alpha \cos u}}} \right)\sin v\cos w}}{{\left( {1 + \cos u} \right)\cos u}}{P_{2,1,0}} + \\ \frac{{\left( {1 + \cos u - {{\rm{e}}^{ - \alpha \cos u}}} \right)\sin w\cos v}}{{\left( {1 + \cos u} \right)\cos u}}{P_{2,0,1}}\\ P_{0,2,0}^1 = \\ \frac{{{{\rm{e}}^{ - \beta \cos v}}}}{{1 + \cos v}}{P_{0,3,0}} + \frac{{\left( {1 + \cos v - {{\rm{e}}^{ - \beta \cos v}}} \right)\sin u\cos w}}{{\left( {1 + \cos v} \right)\cos v}}{P_{1,2,0}} + \\ \frac{{\left( {1 + \cos v - {{\rm{e}}^{ - \beta \cos v}}} \right)\sin w\cos u}}{{\left( {1 + \cos v} \right)\cos v}}{P_{0,2,1}}\\ P_{2,0,0}^1 = \\ \frac{{{{\rm{e}}^{ - \gamma \cos w}}}}{{1 + \cos w}}{P_{0,0,3}} + \frac{{\left( {1 + \cos w - {{\rm{e}}^{ - \gamma \cos w}}} \right)\sin v\cos u}}{{\left( {1 + \cos w} \right)\cos w}}{P_{1,0,2}} + \\ \frac{{\left( {1 + \cos w - {{\rm{e}}^{ - \gamma \cos w}}} \right)\sin v\cos u}}{{\left( {1 + \cos w} \right)\cos w}}{P_{0,1,2}} \end{array} \right. $ |
则可以改写QCT-Bernstein-Bézier曲面的表达式为
$ \begin{array}{*{20}{c}} {R\left( {u,v,w} \right) = }\\ {\frac{{1 - {{\cos }^2}u}}{{{{\sin }^2}u + {{\sin }^2}v + {{\sin }^2}w}}\left[ {{g_1}\left( {u,v,w} \right)P_{2,0,0}^1 + } \right.}\\ {\left. {{g_2}\left( {u,v,w} \right)P_{1,1,0}^1 + {g_3}\left( {u,v,w} \right)P_{1,0,1}^1} \right] + }\\ {\frac{{1 - {{\cos }^2}v}}{{{{\sin }^2}u + {{\sin }^2}v + {{\sin }^2}w}}\left[ {{g_1}\left( {v,u,w} \right)P_{0,2,0}^1 + } \right.}\\ {\left. {{g_2}\left( {u,v,w} \right)P_{1,1,0}^1 + {g_3}\left( {u,v,w} \right)P_{0,1,1}^1} \right] + }\\ {\frac{{1 - {{\cos }^2}w}}{{{{\sin }^2}u + {{\sin }^2}v + {{\sin }^2}w}}\left[ {{g_1}\left( {w,v,u} \right)P_{0,0,2}^1 + } \right.}\\ {\left. {{g_2}\left( {w,u,v} \right)P_{0,1,1}^1 + {g_3}\left( {w,v,u} \right)P_{1,0,1}^1} \right]} \end{array} $ | (22) |
再记
$ \begin{array}{l} P_{1,0,0}^2 = {g_1}\left( {u,v,w} \right)P_{2,0,0}^1 + \\ {g_2}\left( {u,v,w} \right)P_{1,1,0}^1 + {g_3}\left( {u,v,w} \right)P_{1,0,1}^1 \end{array} $ |
$ \begin{array}{l} P_{0,1,0}^2 = {g_2}\left( {v,u,w} \right)P_{1,1,0}^1 + \\ {g_1}\left( {v,u,w} \right)P_{0,2,0}^1 + {g_3}\left( {v,u,w} \right)P_{0,1,1}^1 \end{array} $ |
$ \begin{array}{l} P_{0,0,1}^2 = {g_3}\left( {w,v,u} \right)P_{1,0,1}^1 + \\ {g_2}\left( {w,v,u} \right)P_{0,1,1}^1 + {g_1}\left( {w,v,u} \right)P_{0,0,2}^1 \end{array} $ |
则可进一步改写QCT-Bernstein-Bézier曲面的表达式为
$ \begin{array}{l} R\left( {v,u,w} \right) = \frac{{1 - {{\cos }^2}u}}{{{{\sin }^2}u + {{\sin }^2}v + {{\sin }^2}w}}P_{1,0,0}^2 + \\ \frac{{1 - {{\cos }^2}v}}{{{{\sin }^2}u + {{\sin }^2}v + {{\sin }^2}w}}P_{0,1,0}^2 + \\ \frac{{1 - {{\cos }^2}w}}{{{{\sin }^2}u + {{\sin }^2}v + {{\sin }^2}w}}P_{0,0,1}^2 = P_{0,0,0}^2 \end{array} $ | (23) |
对任意
2.5 曲面的拼接
定义两张QCT-Bernstein-Bézier曲面分别为
$ \begin{array}{*{20}{c}} {{R_1}\left( {v,u,w} \right) = \sum\limits_{i + j + k = 3} {T_{i,j,k}^3\left( {u,v,w;{\alpha _1},\beta ,\gamma } \right)} }\\ {{P_{i,j,k}},\left( {u,v,w} \right) \in \mathit{\boldsymbol{D}}} \end{array} $ | (24) |
$ \begin{array}{*{20}{c}} {{R_2}\left( {u,v,w} \right) = \sum\limits_{i + j + k = 3} {T_{i,j,k}^3\left( {u,v,w;{\alpha _2},\beta ,\gamma } \right)} }\\ {{Q_{i,j,k}},\left( {u,v,w} \right) \in \mathit{\boldsymbol{D}}} \end{array} $ | (25) |
显然,若控制点满足
$ {P_{0,j,k}} = {Q_{0,j,k}}\;\;\;j,k \in {\bf{N}},j + k = 3 $ | (26) |
则曲面式(24)和式(25)存在公共的边界:
对公共边界曲线
$ \begin{array}{l} \frac{{{\rm{d}}{R_1}\left( {0,v,{\rm{ \mathsf{ π} }}/2 - v} \right)}}{{{\rm{d}}v}} = \\ \left[ { - {{\rm{e}}^{ - \beta \cos t}}\left( {1 - \cos t} \right)\sin t\left( { - 2 - \beta + \beta \cos t} \right)} \right] \times \\ \left( {{P_{0,3,0}} - {P_{0,2,1}}} \right) + 2\sin v\cos v\left( {{P_{0,2,1}} - {P_{0,1,2}}} \right) + \\ \left[ {{{\rm{e}}^{ - \gamma \sin t}}\left( {1 - \sin t} \right)\cos t\left( { - 2 - \gamma + \gamma \sin t} \right)} \right] \times \\ \left( {{P_{0,1,2}} - {P_{0,0,3}}} \right) \end{array} $ | (27) |
对曲面
$ \begin{array}{l} \frac{{{\rm{d}}{R_1}\left( {0,v,{\rm{ \mathsf{ π} }}/2 - u - v} \right)}}{{{\rm{d}}v}}\left| {_{u = 0}} \right. = \\ \left[ { - {{\rm{e}}^{ - \beta \cos t}}\left( {1 - \cos t} \right)\sin t\left( { - 2 - \beta + \beta \cos t} \right)} \right] \times \\ \left( {{P_{1,2,0}} - {P_{0,2,1}}} \right) + 2\sin v\cos v\left( {{P_{1,1,1}} - {P_{0,1,2}}} \right) + \\ \left[ {{{\rm{e}}^{ - \gamma \sin t}}\left( {1 - \sin t} \right)\cos t\left( { - 2 - \gamma + \gamma \sin t} \right)} \right] \times \\ \left( {{P_{1,0,2}} - {P_{0,0,3}}} \right) \end{array} $ | (28) |
$ \begin{array}{l} \frac{{{\rm{d}}{R_2}\left( {0,v,{\rm{ \mathsf{ π} }}/2 - u - v} \right)}}{{{\rm{d}}u}}\left| {_{u = 0}} \right. = \\ \left[ { - {{\rm{e}}^{ - \beta \cos t}}\left( {1 - \cos t} \right)\sin t\left( { - 2 - \beta + \beta \cos t} \right)} \right] \times \\ \left( {{Q_{1,2,0}} - {Q_{0,2,1}}} \right) + 2\sin v\cos v\left( {{Q_{1,1,1}} - {Q_{0,1,2}}} \right) + \\ \left[ {{{\rm{e}}^{ - \gamma \sin t}}\left( {1 - \sin t} \right)\cos t\left( { - 2 - \gamma + \gamma \sin t} \right)} \right] \times \\ \left( {{Q_{1,0,2}} - {Q_{0,0,3}}} \right) \end{array} $ | (29) |
两张QCT-Bernstein-Bézier曲面
$ \begin{array}{*{20}{c}} {\frac{{{\rm{d}}{R_2}\left( {u,v,{\rm{ \mathsf{ π} }}/2 - u - v} \right)}}{{{\rm{d}}u}}\left| {_{u = 0}} \right. = \varphi \frac{{{\rm{d}}{R_1}\left( {0,v,{\rm{ \mathsf{ π} }}/2 - v} \right)}}{{{\rm{d}}v}} + }\\ {\phi \frac{{{\rm{d}}{R_1}\left( {u,v,{\rm{ \mathsf{ π} }}/2 - u - v} \right)}}{{{\rm{d}}u}}\left| {_{u = 0}} \right.} \end{array} $ |
式中,
$ \left\{ \begin{array}{l} {Q_{1,2,0}} - {Q_{0,2,1}} = \varphi \left( {{P_{0,3,0}} - {P_{0,2,1}}} \right) + \phi \left( {{P_{1,2,0}} - {P_{0,2,1}}} \right)\\ {Q_{1,1,1}} - {Q_{0,1,2}} = \varphi \left( {{P_{0,2,1}} - {P_{0,1,2}}} \right) + \phi \left( {{P_{1,1,1}} - {P_{0,1,2}}} \right)\\ {Q_{1,0,2}} - {Q_{0,0,3}} = \varphi \left( {{P_{0,1,2}} - {P_{0,0,3}}} \right) + \phi \left( {{P_{1,0,2}} - {P_{0,0,3}}} \right) \end{array} \right. $ | (30) |
总结上述讨论,可以得到定理5。
定理5 对任意
由定理5可知,曲面式(24)和式(25)的
3 结论
本文提出了一种具有指数参数的三角基函数进行曲线曲面构造,该参数具有张力作用效果,随着参数的增大,生成的曲线曲面会越来越逼近控制网格,从而能够更加准确地描述控制网格的行为。在光滑连续方面,本文给出了QCT-Bézier曲线的
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