The quartic H-Bézier curve is used for the approximation of circular arcs. It has five control points and one

positive real free parameter. The four control points are carried out b

Hussain, Malik Zawwar

Hussain, Maria

Nazir, Farah

Wen, Eng Ong

Repository@USM

Pak.j.stat.oper.res.  Vol.XIII  No.2 2017  pp417-423 Circular Arc Approximation by Quartic H-Bézier Curve  Maria Hussain Department of Mathematics Lahore College for Women University, Pakistan maria.hussain@lcwu.edu.pk  W. E. Ong School of Mathematical Sciences Universiti Sains Malaysia, Malaysia weneng@usm.my  Farah Nazir Department of Mathematics University of the Punjab, Lahore, Pakistan farah714@live.com  Malik Zawwar Hussain Department of Mathematics University of the Punjab, Lahore, Pakistan malikzawwr.math@pu.edu.pk Abstract The quartic H-Bézier curve is used for the approximation of circular arcs. It has five control points and one positive real free parameter. The four control points are carried out by 𝐺1-approximation constraints and the remaining control point is dividing the line segment joining the second and fourth control points in the ratio 1:2. Optimized value of free parameter 𝛼 is obtained by minimizing the maximum value of absolute radius error of the recommended approximation scheme. The developed approximation scheme is found considerably better than the existing approximation schemes for these computed values of control points and optimized value of the free parameter.  Keywords:   Quartic H-Bézier curve, Control points, Free parameter, 𝐺1-approximation constraints, Absolute radius error. AMS Subject Classification 2010:  65D17, 68U07 1.   Introduction Circles and circular arcs are widely applicable in the field of CAD for the designing of various objects. The most common applications can be seen in the designing of highway and railway routs and in the construction of suspension bridges (Lu, 2012). Since the designers can not directly use the parametric equations of circles in CAD rather they use the approximations of circles. So it is of keen interest of many authors to find the optimal approximations of circular arcs. Ahn and Kim (1997) used Bernstein-Bézier curves of degree four and five for 𝐺𝐶𝑘, 𝑘 = 2,3, approximation of circular arcs. Order of approximation of these schemes was eight and ten respectively. Fang (1998) discussed five circular arcs approximation methods by using polynomial curves of degree 5. The convergence rate of these methods was either 8 or 10. Floater (1995) approximated the conic sections by quadratic splines with continuous curvature. Hur and Kim (2011) used Maria Hussain, W. E. Ong, Farah Nazir, Malik Zawwar Hussain Pak.j.stat.oper.res.  Vol.XIII  No.2 2017  pp417-423 418 cubic and quartic Bézier curves for the 𝐺𝑘, 𝑘 = 1,2, approximation of circular arcs. The Hausdorff distance between the approximating Bézier curves and circle was least for the proposed approximation schemes. Lee et al. (2006) presented the 𝐺0-approximation of circular arc by Bézier curve of degree 2. In (Piegl and Tiller, 2003), the integral B-spline curve of appropriate degree was used as an interpolant for the approximation of circular arcs. The approximation was carried out by interpolating the derivatives at terminal points and a few interior points. Riškus (2006) approximated circular arc by using cubic Bézier curve. The proposed scheme was valuable in CAD system as it supported Bézier curve to interchange data through any data formats.   H-Bézier curves (Lee and Ahn, 2015) have hyperbolic basis functions. These basis functions are known as H-basis functions. H-Bézier curves preserve the favourable characteristics of ordinary Bézier curves like end points and end tangents interpolation property, partition of unity, invariance under affine transformation. H-Bézier curves give better smooth shape-preserving curves as compared to ordinary Bézier curves. In the proposed study, the circular arc approximation scheme by the quartic H-Bézier curve is presented. The existing approximation schemes of circular arcs use 𝐺2-constraints for computing the control points of quartic and quintic approximating polynomials (Fang, 1998). Here the control points 𝑏𝑖 , 𝑖 = 0,1,3,4, are evaluated by 𝐺1-approximation constraints. The control point 𝑏2 is chosen as a point dividing the line segment 𝑏1𝑏3̅̅ ̅̅ ̅̅  in the ratio 1:2. The value of free parameter 𝛼 is evaluated by minimizing the maximum value of absolute radius error of approximation. The absolute radius error of the cultivated circular arc approximation scheme is considerably smaller than the existing schemes (Fang 1998; Lu 2012). 2.   Quartic H-Bézier Curve  The quartic H-Bézier curve (Lee and Ahn, 2015)  𝑞(𝑡) = ∑ 𝑍𝑖4 (𝑡)𝑏𝑖4𝑖=0 , 𝑡 ∈ [0,1].      (1)  It has five control points 𝑏𝑖, 𝑖 = 0,1,2,3,4, a free parameter 𝛼 (𝛼 > 0), 𝑡 is H-Bézier parameter, and 𝑍𝑖4(𝑡), 𝑖 = 0,1,2,3,4,  are the quartic H-Basis functions given as: 𝑍04(𝑡) =𝐿𝐴,  𝑍14(𝑡) =𝑅𝐴−𝑀1−𝑀2𝐵𝐶, 𝑍24(𝑡) =𝑀1−𝑀2𝐵𝐶−𝑁1−𝑁2𝐵𝐶,   𝑍34(𝑡) =𝑁1−𝑁2𝐵𝐶−𝑄𝐴,  𝑍44(𝑡) =𝑄𝐴, where 𝐿 = 𝛼2(1 + 𝑡2 ) + 2(1 − 𝑐𝑜𝑠ℎ 𝛼(1 − 𝑡) − 2𝛼2𝑡), 𝐴 = 𝛼2 + 2 − 2 cosh 𝛼, 𝑅 =  2𝛼2𝑡 − 𝛼2𝑡2 + 2 cosh 𝛼(1 − 𝑡) − 2 cosh 𝛼, 𝑀1 = (𝛼2𝑡2 − 2 cosh 𝛼(1 − 𝑡) − 2𝛼𝑡𝑠𝑖𝑛ℎ 𝛼 + 2 cosh 𝛼)(𝛼𝑐𝑜𝑠ℎ 𝛼 + 𝛼 − 2 sinh 𝛼), 𝑀2 = (𝛼 − sinh 𝛼)(−2 cosh 𝛼(1 − 𝑡) + 𝛼2𝑡2 cosh 𝛼  + 𝛼2𝑡2 − 2 cosh 𝛼𝑡+ 2 cosh 𝛼 −2𝛼𝑡sinh 𝛼 +  2), 𝑁1 = −2 cosh 𝛼(1 − 𝑡) + 𝛼2𝑡2 cosh 𝛼 + 𝛼2𝑡2 − 2 cosh 𝛼𝑡 − 2𝛼𝑡𝑠𝑖𝑛ℎ 𝛼 + 2, 𝑁2 = (𝛼2𝑡2 − 2 cosh 𝛼𝑡 + 2)(𝛼 cosh 𝛼 + 𝛼 − 2 sinh 𝛼), 𝑄 =  𝛼2𝑡2 − 2 cosh 𝛼𝑡 + 2, Circular Arc Approximation by Quartic H-Bézier Curve  Pak.j.stat.oper.res.  Vol.XIII  No.2 2017  pp417-423 419 𝐵 = 𝛼 cosh 𝛼 + 𝛼 − 2 sinh 𝛼, 𝐶 = 2 cosh 𝛼 − 2 − 𝛼 sinh 𝛼. Numerical experiments suggest that the basis functions are either positive or negative depending upon the value of 𝛼.  3.   Approximation of Circular Arc by Quartic H-Bézier Curve In this section, the numerical approximation scheme of circular arc by quartic H-Bézier curve is computed. The circular arc is considered in standard position i.e. center at origin 𝑂(0,0), radius 𝑟, the initial point of the arc is along the positive horizontal axis and the final point is making a counter clockwise angle 𝜃 with the same axis. Any circular arc with arbitrary center can be transformed to this position by affine transformations.   The following 𝐺1-approximation constraints are used for the approximation: 𝑞(𝑡)|𝑡=0 = 𝑐0,  𝑞(𝑡)|𝑡=1 = 𝑐1,       (2) 𝑇0 = 𝑡0,  𝑇1 = 𝑡1.         (3)  Here, 𝑐𝑖′𝑠 are the end points and 𝑡𝑖′𝑠 are the end unit tangent vectors of the circular arc. By using (1), we have 𝑞(0) = 𝑏0 and  𝑞(1) = 𝑏4. The end unit tangents of quartic H-Bézier curves are denoted by 𝑇0 and 𝑇1 and are computed by the relation 𝑇𝑖 =𝑞′(𝑖)‖𝑞′(𝑖)‖,  for 𝑖 = 0, 1, which implies 𝑇0 =𝑏1−𝑏0𝛾1 and 𝑇1 =𝑏3−𝑏2𝛾2. Now by using these values of end points and end unit tangents of the quartic H-Bézier curve into (2) and (3) respectively, the following equations are obtained 𝑏0 = 𝑐0, 𝑏4 = 𝑐1,         (4) 𝑏1−𝑏0𝛾1= 𝑡0,   𝑏4−𝑏3𝛾2= 𝑡1.        (5)  The values of  𝛾1 = ‖𝑏1 − 𝑏0‖ and 𝛾2 = ‖𝑏4 − 𝑏3‖ are positive real numbers. The end points and the end unit tangents of the concerned circular arc 𝑐0𝑐1 are 𝑐0(𝑟, 0), 𝑐1 (𝑟𝑐𝑜𝑠𝜃 , 𝑟𝑠𝑖𝑛𝜃), 𝑡0(0,1), 𝑡1(−𝑠𝑖𝑛 𝜃, 𝑐𝑜𝑠 𝜃). Substituting these values in (4) and (5), the control points of quartic H-Bézier curve are given by 𝑏0 = (𝑟, 0), 𝑏1 = (𝑟, 𝛾1), 𝑏2 = (𝑏20, 𝑏21),𝑏3 = (𝑏30 = 𝑟𝑐𝑜𝑠𝜃 + 𝛾2𝑠𝑖𝑛𝜃, 𝑟𝑠𝑖𝑛𝜃 − 𝛾2𝑐𝑜𝑠𝜃), 𝑏4 = (𝑟𝑐𝑜𝑠𝜃, 𝑟𝑠𝑖𝑛𝜃) }  (6)  The values of parameters 𝛾1 and 𝛾2 are assumed as: 𝛾1 = ‖𝑏1 − 𝑏0‖ =‖𝑏4−𝑏0‖4, 𝛾2 = ‖𝑏4 − 𝑏3‖ =‖𝑏4−𝑏0‖4. It gives, 𝛾1 = 𝛾2 =√(𝑟𝑐𝑜𝑠𝜃−𝑟)2+(𝑟𝑠𝑖𝑛𝜃)24.      (7)  In (6), the control point 𝑏2 can be evaluated by various methods. In this study, 𝑏2  is chosen as a point which divides the line segment 𝑏1𝑏3̅̅ ̅̅ ̅̅  in the ration 1:2. The coordinates of 𝑏2 are 𝑏20 =𝑟𝑐𝑜𝑠𝜃+𝛾2𝑠𝑖𝑛𝜃+2𝑟3, 𝑏21 =𝑟𝑠𝑖𝑛𝜃−𝛾2𝑐𝑜𝑠𝜃+2𝛾13.   Maria Hussain, W. E. Ong, Farah Nazir, Malik Zawwar Hussain Pak.j.stat.oper.res.  Vol.XIII  No.2 2017  pp417-423 420 In (1) by substituting the values of control points from (6), the following parametric equations of quartic H-Bézier curve are obtained: 𝑥(𝑡) = 𝑍04(𝑡)𝑥0 +  𝑍14(𝑡)𝑥1 + 𝑍24(𝑡)𝑥2 +  𝑍34(𝑡)𝑥3 + 𝑍44(𝑡)𝑥4,   (8) 𝑦(𝑡) =  𝑍04(𝑡)𝑦0 + 𝑍14( 𝑡)𝑦1 + 𝑍24(𝑡)𝑦2 + 𝑍34(𝑡)𝑦3 + 𝑍44(𝑡)𝑦4,   (9) where  𝑥0 = 𝑟, 𝑥1 = 𝑟, 𝑥2 =𝑟𝑐𝑜𝑠𝜃+𝛾2𝑠𝑖𝑛𝜃+2𝑟3, 𝑥3 = 𝑟𝑐𝑜𝑠𝜃 + 𝛾2𝑠𝑖𝑛𝜃, 𝑥4 = 𝑟𝑐𝑜𝑠𝜃, 𝑦0 = 0, 𝑦1 = 𝛾1, 𝑦2 =𝑟𝑠𝑖𝑛𝜃 − 𝛾2𝑐𝑜𝑠𝜃 + 2𝛾13, 𝑦3 = 𝑟𝑠𝑖𝑛𝜃 − 𝛾2𝑐𝑜𝑠𝜃, 𝑦4 = 𝑟𝑠𝑖𝑛𝜃.  The H-basis functions 𝑍𝑖4(𝑡), 𝑖 = 0,1,2,3,4,  have been already defined in Section 2. The free parameter 𝛼 of quartic H-Bézier curve can assume different values and produces different H-Bézier curves for the approximation of circular arc. Therefore to find optimal approximation the value of 𝛼 must be optimized. Here, the optimized value of 𝛼 is obtained by the following optimization problem-I. Optimization problem-І: min𝛼>0( max0≤𝑡≤1?̃? (𝛼, 𝑡)),        (10) subject to 𝛼 ≥ 𝑢,  where, 𝑢 = 2.2204 × 10−16, ?̃?(𝛼, 𝑡) = |𝑥2(𝑡) + 𝑦2(𝑡) − 𝑟2|. ?̃?(𝛼, 𝑡) is the absolute radius error of the developed approximation scheme for circular arc 𝑐0𝑐1 by quartic H-Bézier curve (1). 𝑟 is the radius of the concerned circular arc, 𝑥(𝑡) and 𝑦(𝑡) are defined in (8) and (9).  The optimization problem-I is solved by the MATLAB 7 built in function fminimax of the MATLAB optimization toolbox. The fminimax is based on the sequential quadratic programming technique (Brayton et al., 1979). Sequential quadratic programming technique is not suitable for discontinuous functions. But the objective function of optimization problem-I is continuous, so the problem is solvable. 4.   Numerical Example In this section, the numerical approximation scheme introduced in Section 3 is implemented on the unit circular arc given in Table 1. Table 1:   Unit circular arc 𝜽 𝒓 𝒄𝟎 𝒄𝟏 𝒕𝟎 𝒕𝟏 𝜋/4 1 (1,0) (0.7071 , 0.7071) (0,1) (−0.7071 , 0.7071)  Circular Arc Approximation by Quartic H-Bézier Curve  Pak.j.stat.oper.res.  Vol.XIII  No.2 2017  pp417-423 421 Control points of the quartic H-Bézier curve corresponding to the unit circular arc of Table 1 are calculated by Theorem 1. These computed values of control points and free parameter 𝛼 are given in Table 2. Table 2:   Control points of the quartic H-Bézier curve  𝒃𝟎 𝒃𝟏 𝒃𝟐 𝒃𝟑 𝒃𝟒 𝜶 (1,0) (1, 0.1913) (0.9475,0.3182) (0.8424,0.5718) (0.7071,0.7071) 9.2149  Graph of quartic H-Bézier curve approximating the circular arc of Table 1 is plotted in Figure 1. The reflection of this circular arc about the line 𝑦 = 𝑥 is given in Figure 2. By combining the graphs of Figures 1 and 2, a quarter circle of unit radius is obtained ( Figure 3). A semi-circle is obtained in Figure 4 by the reflection of the quarter circle of Figure 3 about y-axis. Complete circle is obtained in Figure 5 by the reflection of semi-circle of Figure 4 about x-axis. The maximum value of the absolute radius error of the developed approximation scheme is 8.7 × 10−3. The plot of absolute radius error of the developed quartic H-Bézier approximation scheme is given in Figure 6. 5.   Conclusion In the proposed study, a circular arc quartic H-Bézier curve approximation scheme is introduced. The values of control points are evaluated by 𝐺1-approximation constraints and the value of free parameter is obtained by minimizing the maximum value of absolute radius error of proposed approximation scheme. Absolute radius error of approximation for the proposed scheme is compared to the prevailing schemes (Table 3). It is noted that the absolute radius error of approximation in the proposed scheme is less than (Fang, 1998; Lu 2012).  Table 3:   Absolute radius errors References Methods Absolute radius error Fang, 1998 Fang method ІV 1.1788 × 10−2 Lu, 2012 Lu 1.5213 × 10−2  Section 4  Quartic H-Bézier approximation scheme for circular arc 8.7 × 10−3  Maria Hussain, W. E. Ong, Farah Nazir, Malik Zawwar Hussain Pak.j.stat.oper.res.  Vol.XIII  No.2 2017  pp417-423 422   Figure 1: Unit circular arc making angle  𝜋4                     with x-axis Figure 2: Reflection of Fig. 1 about the line                      𝑦 = 𝑥  Figure 3: Combination of Figures 1 and 2  Figure 4: Reflection of Figure 3 about y-axis  Circular Arc Approximation by Quartic H-Bézier Curve  Pak.j.stat.oper.res.  Vol.XIII  No.2 2017  pp417-423 423  Figure 5: Reflection of  Figure 4 about x-axis  (circle approximation)  Figure 6: Absolute radius error  Acknowledgements Malik Zawwar Hussain acknowledges Universiti Sains Malaysia for providing opportunity to carry out his part of this research at Universiti Sains Malaysia as a visiting professor. References 1. Ahn, YJ. and Kim, H.O. (1997). Approximation of circular arcs by Bézier curves, Journal of Computational and Applied Mathematics, 81, 145–163. 2. Brayton, R.K., Director, S.W., Hachtel G.D. and Vidigal, L. (1979). A new algorithm for statistical circuit design based on Quasi-Newton methods and function splitting, IEEE Transactions on Circuits and Systems, 26, 784-794.  3. Fang, L. (1998). Circular arc approximation by quintic polynomial curves, Computer Aided Geometric Design, 15, 843–861. 4. Floater, M. (1995). High order approximation of conic sections by quadratic splines, Computer Aided Geometric Design, 12, 617–637. 5. Hur, S. and Kim, T.-w. (2011). The best G1 cubic and G2 quartic Bézier approximations of circular arcs, Journal of Computational and Applied Mathematics, 236, 1183–1192. Maria Hussain, W. E. Ong, Farah Nazir, Malik Zawwar Hussain Pak.j.stat.oper.res.  Vol.XIII  No.2 2017  pp417-423 424 6. Lee, I.K., Kim, M.S. and Elber, G. (1996). Planar curve offset based on circular approximation, Computer Aided Design, 28(8), 617-630. 7. Lee, R. and Ahn, Y.J. (2015). Limit curve of H- Bézier curves and rational Bézier curves in standard form with same weight, Journal of Computational and Applied Mathematics, 281, 1-9. 8. Lin, J., Ball, A.A. and Zheng, J.J. (2001).  Approximating circular arcs by Bézier curves and its application to modeling tooling for FE forming simulations, International Journal of Machine Tools & Manufacture, 41, 703–717. 9. Lu, L. (2012). On polynomial approximation of circular arcs and helices, Computers and Mathematics with Applications, 63, 1192–1196. 10. Piegl, L. and Tiller, W. (2003). Circle approximation using integral B-Splines, Computer Aided Design, 35, 601-607. 11. Riškus, A. (2006). Approximation of a cubic Bézier curve by circular arcs and vice versa, Information Technology and Control, 35(4), 371-378. 

Circular Arc Approximation by Quartic H-Bézier Curve

http://eprints.usm.my/36833/1/(Circular_arc_approximation)_1689%2D7188%2D1%2DPB.pdf

Circular Arc Approximation by Quartic H-Bézier Curve

Abstract

Similar works

Full text

Available Versions

Repository@USM