Mathematical Problems in Engineering

Volume 2019, Article ID 9026187, 16 pages

https://doi.org/10.1155/2019/9026187

## A Class of Trigonometric Bernstein-Type Basis Functions with Four Shape Parameters

^{1}School of Mathematics, South China University of Technology, Guangzhou, 510641, China^{2}Tianan Hi-Tech Ecological Park, Panyu District, Guangzhou 511400, China

Correspondence should be addressed to Yuanpeng Zhu; nc.ude.tucs@uhzpy

Received 24 October 2018; Revised 3 December 2018; Accepted 1 April 2019; Published 17 April 2019

Academic Editor: Nhon Nguyen-Thanh

Copyright © 2019 Yuanpeng Zhu and Zhuo Liu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

In this work, a family of four new trigonometric Bernstein-type basis functions with four shape parameters is constructed, which form a normalized basis with optimal total positivity. Based on the new basis functions, a kind of trigonometric Bézier-type curves with four shape parameters, analogous to the cubic Bézier curves, is constructed. With appropriate choices of control points and shape parameters, the resulting trigonometric Bézier-type curves can represent exactly any arc of an ellipse or parabola. The four shape parameters have tension control roles on adjusting the shape of resulting curves. Moreover, a new corner cutting algorithm is also proposed for calculating the trigonometric Bézier-type curves stably and efficiently.

#### 1. Introduction

In computer aided geometric design and computer graphics, parametric curves and surfaces are often expressed by linearly combining control points and basis functions. Generally speaking, basis functions with good properties play a vital role in parametric curves and surfaces design. For instance, if the basis functions have partition of unity, nonnegativity, and total positivity, the resulting parametric curves will possess affine invariance property, convex hull property, and variation diminishing property, which are important in curves design. In engineering, the classical B-spline basis functions have been widely applied in modeling parametric curves; see [1, 2]. However, with the fixed knot vectors, the shape of B-spline curves is determined totally by their control points. One may use the weights in the nonuniform rational B-spline curves to modify the shape of the resulting parametric curves; however, rational form may be unstable and its derivatives and integrals are hard to compute.

In order to adjust the shape of the parametric curves flexibly, some basis functions with shape parameters have been proposed; see [3–5]. These methods have a common idea that new basis functions are constructed by incorporating shape parameters into the classical Bézier or B-spline basis functions. In [6, 7], quadratic and cubic trigonometric Bernstein-type basis functions with shape parameters were shown. In [8], a kind of cubic trigonometric Bernstein-type basis functions possessing two shape parameters was presented, which includes the cubic trigonometric Bernstein-type basis functions with a shape parameter given in [6] as a special case. In [9], shape analysis of the cubic trigonometric Bézier-type curve with a shape parameter given in [6] was presented by using the theory of envelop and topological mapping. Later, in [10], the totally positive property of the cubic trigonometric Bernstein-type basis functions with two shape parameters given in [8] was proved, which implies that the cubic trigonometric Bernstein-type basis with two shape parameters is suitable for conformal design. Recently, a class of cubic trigonometric nonuniform B-spline basis functions having a local shape parameter was proposed in [11], which is an extension of the cubic trigonometric nonuniform spline basis functions with a global shape parameter given in [6]. In [12], a class of C-Bézier basis of the space span was constructed, where the length of the interval serves as shape parameter. Later, in [13], geometric interpretation of the change of the shape parameter on C-Bézier curves was given. In [14], it was proved that the critical length for the span is , which implies that in the span, Extended Complete Chebyshev- (ECC-) system exists only on interval of length less than . Later, in [15], it was shown that this restriction can be overcome by replacing ECC-system with the Canonical Complete Chebyshev- (CCC-) system.

For controlling the parametric curves efficiently, basis functions with tension shape parameters have aroused great interest among the researchers. In [16], a class of polynomial splines with variable degree was constructed in the space spanned by span, in which and serve as tension shape parameters. In [17, 18], it was proved that the polynomial splines with variable degree form a Quasi Extended Chebyshev- (QEC-) system. Later, in [19], the approximation power, the existence of a normalized B-basis, and the structure of a degree-raising process for spaces of the form were given. Within the general framework of QEC-system, the dimension elevation algorithm for the space was studied via blossom theory; see [20–23]. Recently, in [24], the total positivity of the polynomial splines with variable degree was proved based on the theory of CCC-systems. The variable degree polynomial splines have been widely used for constructing shape preserving interpolation and approximation splines; see [25–27]. In [28], based on some truncated polynomial functions, the explicit representations of changeable degree spline basis functions were given. In [29], a kind of five trigonometric blending functions with two exponential shape parameters and was proposed in the space spanned by span. Later, in [30], a generalization of these five trigonometric blending functions was presented. Some exponential splines and rational splines with tension shape parameters have also developed for curve design, see [31–33] for example. Recently, four trigonometric Bernstein-type basis functions of the space spanned by span were constructed in [34], which form a normalized basis with optimal total positivity. In [35], a family of rational trigonometric basis functions with denominator shape parameters of the space spanned by span was shown.

The purpose of this paper is to present four new trigonometric Bernstein-type basis functions constructed in the space spanned by span, which form a normalized optimal totally positive basis and include the bases given in [6–8, 34] as special cases. The parametric curves constructed by this new basis have shape preserving property. The four shape parameters , , , and have tension control property on modifying the shape of parametric curves. Compared with the four polynomial Bernstein-type basis functions with variable degree constructed in the space span ( see [16]), the new constructed trigonometric Bernstein-type basis functions have more computation complexity for the same degree, while any arc of an ellipse or parabola can be represented exactly by using the new trigonometric Bernstein-type basis functions. And compared with the four rational trigonometric basis functions with two denominator shape parameters constructed in the space spanned by span (see [35]), the new constructed trigonometric Bernstein-type basis functions possess four shape parameters and thus have more flexibility in free-form curves shape design.

The rest of this paper is organized as follows. In Section 2, the construction and properties of the trigonometric Bernstein-type basis functions are given. Section 3 gives the definition and properties of the trigonometric Bézier-type curves. For computing the trigonometric Bézier-type curves stably and efficiently, a new corner cutting algorithm is developed. Comparison between the trigonometric Bézier-type curves and the variable degree Bézier polynomial curves given in [16] is shown. And tensor product Bézier-type patches are also shown. Conclusions are given in Section 4.

#### 2. Trigonometric Bernstein-Type Basis Functions

##### 2.1. Construction of Trigonometric Bernstein-Type Basis Functions

For arbitrary real numbers , , a new family of trigonometric Bernstein-type basis functions will be constructed in the space The corresponding mother-function is given as follows:

We shall prove the totally positive property of the new basis functions by using the theory of Quasi Extended Chebyshev (QEC) space. The related concepts concerning ECC-space, QEC-space, blossom, and Quasi Bernstein-type basis can be found in [17, 18, 20–23, 34].

In the following theorem, we will show that the following space is a -dimensional QEC-space on .

Theorem 1. *For any real numbers , the space is a -dimensional QEC-space on .*

*Proof. *For any , , we consider a linear combinationFrom (4), for and , we have and , respectively. It follows that . Thus we can see that the space is a -dimensional space.

We will prove that the space forms a QEC-space on by two steps. In the first step, we prove that the space forms an ECC-space in . For any , let andBoth the two functions and are positive on any subinterval . In fact, for and , consider the following quadratic function: For , , notice that ; we have Furthermore, for , , , direct computation gives that From this together with , we can see that for any . These imply that for any and . Thus we can see that and for any .

For , by directly computing, we get It follows that the Wronskian of and is positive in , that is, For any , , , and , we consider the following three weight functions: Obviously, all the three weight functions , are , positive and bounded on . For the following ECC-space defined by the three weight functions , , after some simple computation, we can see that all the three functions , , and can be expressed as the forms of some linear combinations of the three functions , , , which implies that the space is an ECC-space on . Since are arbitrary subinterval of , we can further conclude that the space is an ECC-space in .

In the second step, we further prove that the space forms a QEC-space on . To this end, we need to prove that any nonzero element of the space has at most roots on (keep it in mind that in a QEC-space, we count multiplicities as far as possible up to ). Consider any nonzero function where . Since the space is an ECC-space in , has at most two roots in . Suppose that the function has a root at ; then we get . In this case, if , then has a singular root at and a singular root at . If , we can check that is a double root of (we count multiplicities as far as possible up to ). If , has singular one root at and it does not vanish anywhere on . If , has singular one root at and it does not vanish at . Moreover, for the following function by directly computing, we obtain and it follows that is a monotone increasing function on . From these together with , we can see that has exactly singular one root in ; thus we can immediately conclude that (notice that for the current case) has exactly one root in . Similarly, for the case that has a root at , we can also derive that the function has at most roots on (we count multiplicities as far as possible up to ). Summarizing the above analysis, we can conclude that the space is a QEC-space on .

Since the space forms a QEC-space on , from Theorem 3.1 of [21], we can conclude that blossom exists in , which indicates that the new space is suited for curve design. In addition, from Theorem 2.18 of [21], we can also know that the space has a normalized basis of Quasi Bernstein-type on . In the next Theorem 3, we will compute the associated Chebyshev-Bézier points of the mother-function defined in (2) and construct the associated trigonometric Bernstein-type basis of the space . Before further discussion, we want to prove the following lemma, which will be used to discuss the positivity of the trigonometric Bernstein-type basis.

Lemma 2. *For any , the function .*

*Proof. *For any , direct computation that , , and . For any , the function . In fact, and . Therefore, for , we have Thus increases with the increase of for any fixed . From these, for any , we have

Theorem 3. *For any , the four Chebyshev-Bézier points of the mother-function defined in (2) are given byThe four associated trigonometric Bernstein-type (TB-type for short) basis functions of the space are given byAnd the system of functions forms a normalized basis of the space with optimal total positivity.*

*Proof. *For arbitrary , from the expression of the mother-function given in (2), we get Thus, by simply computing, we get For any , from , we haveTherefore, from (22) together with , we can easily derive the expressions of the new basis functions , .

It can be easily checked that the new basis functions have the following important end-point property.

(i) , and vanishes times at (we count multiplicities as far as possible up to .

(ii) , and vanishes times at (we count multiplicities as far as possible up to .

(iii) For , vanishes exactly times at and exactly times at .

For any , , it is obvious that . And for , , by using the Lemma 2, we have In addition, for each , the function is strictly positive in . Thus, from Definition 2.10 of [21], we can see that the normalized trigonometric Bernstein-type basis (19) is precisely the Quasi Bernstein-type basis of the space . Moreover, from Theorem 2.18 of [21], we can conclude that the normalized trigonometric Bernstein-type basis (19) is exactly the normalized basis of the space restricted to with optimal total positivity.

*Remark 4. *It is easy to check that, for , , the four TB-type basis functions (19) will return to the four TB-type basis functions with a shape parameter given in [6]. For , the four TB-type basis functions (19) will return to the four TB-type basis functions with two shape parameters given in [7]. And for , the four TB-type basis functions (19) will return to the four TB-type basis functions with two shape parameters given in [8]. Moreover, for , the four TB-type basis functions (19) will return to the four TB-type basis functions with two exponential shape parameters given in [34].

From Lemma 2, it is easy to see that for , the four TB-type basis functions (19) also satisfy for any .

For convenience, we shall also denote the four TB-type basis functions as , , or , , , . Figure 1 shows some TB-type basis functions with different shape parameters.