# Chapter 2 A Second-Order Algorithm for Curve Parallel Projection on Parametric Surfaces

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1 Chapter 2 A Second-Order Algorithm for Curve Parallel Projection on Parametric Surfaces Xiongbing Fang and Hai-Yin Xu Abstract A second-order algorithm is presented to calculate the parallel projection of a parametric curve onto a parametric surface in this chapter. The essence of our approach is to transform the problem of computing parallel projection curve on the parametric surface into that of computing parametric projection curve in the two-dimensional parametric domain of the surface. First- and second-order differential geometric characteristics of the parametric projection curve in the parametric domain of the surface are firstly analyzed. A marching method based on secondorder Taylor Approximation is formulated to calculate the parametric projection curve. A first-order correction technique is developed to depress the error caused by the truncated higher order terms in the marching method. Several examples are finally implemented to demonstrate the effectiveness of the proposed scheme. Experimental results indicate that both the computational efficiency and accuracy of the presented method have dominant performance as compared with the firstorder differential equation method. 2.1 Introduction Curves on a surface have a wide range of applications in the fields of Computer Graphics, Computer-Aided Geometric Design, Computer Animation, CNC, etc. For instance, curves on a surface can be used for surface trimming [1], surface blending [2], NC tool path generation [3, 4], and so on. According to the designing manner, curves on a surface can be the intersection curve of two surfaces [4], X. Fang (*) China Ship Development and Design Center, Wuhan , China H.-Y. Xu School of Computer Science and Technology, Huazhong University of Science and Technology, Wuhan , China W.E. Wong and T. Zhu (eds.), Computer Engineering and Networking, Lecture Notes in Electrical Engineering 277, DOI / _2, Springer International Publishing Switzerland

2 10 X. Fang and H.-Y. Xu the offset of a given curve on a surface [3], the projection curve of a spatial curve onto a surface [5 9], the image of a curve in the parametric domain of a parametric surface [1, 2], or the fitting curve of a sequence of points lying on a surface [10]. In this chapter, we focus on computation of parallel projection of a parametric curve onto a parametric surface. Presently, there are two ways to do this problem. One is the first-order differential equation method [6] and the other is discrete method. For the problem of calculating the parallel projection of parametric curves onto parametric surfaces, Wang, et al. transformed the condition of parallel projection into a system of differential equations and then formulated the problem as a firstorder initial value problem. Numerical methods such as Runge Kutta and Adams- Bashforth can be utilized to solve the initial value problem to generate a sequence of points. Under this transformation, difficulties lie in the choice of an accurate initial value and the stability of the adopted numerical method. For the discrete method, the parametric curve should first be discretized into a series of points. Parallel projections of these separated points can be calculated through the technique of intersection of a line with a surface. Usually, the computational efficiency of the discrete method depends on that of the intersection algorithm. Though the current projected point can be taken as the initial value of the next iteration, efficiency of the discrete method is generally slow as it does not fully utilize the differential geometric properties of both the parametric curve and the projection curve. The projection curve can be constructed by fitting the projected points generated by the two aforementioned types of approaches. A second-order algorithm is put forward for tracing the parallel projection of a parametric curve onto a parametric surface. Experimental results show that the proposed scheme has dominant performance in both efficiency and computational accuracy as compared with Wang s approach [6]. The rest of the chapter is organized as follows. An overview for our approach is presented in the next section. A second-order technique with error adjustment for tracing the projection curve is given in Sect Implementation and experimental results of our approach are carried out in Sect. 2.4 and we conclude the chapter in Sect Overview A 3D parametric curve p(t) and a parametric surface S(u,w) given in Fig. 2.1a are represented as and pðþ¼ t ½xt ðþyt ðþzt Š ð2:1þ Sðu; wþ ¼ ½xu; ð wþ yu; ð wþ zu; ð wþš ð2:2þ

3 2 A Second-Order Algorithm for Curve Parallel Projection on Parametric Surfaces 11 Fig. 2.1 Parallel projection of 3D curve onto surface a V p( t) b w q ( t) g( t) S u respectively. Suppose p is a point on the curve p(t) and the q is the corresponding point generated by projecting the point p along with the direction V onto the surface S. While the point p moving along the curve p(t), its parallel projection point q also moves along a curve on the surface, which is termed as the parallel projection curve of p(t) along with the direction V onto the surface S. According to the definition of parallel projection, one has ðqðþ p t ðþ t ÞV ¼ 0 ð2:3þ where denotes the cross product of two vectors. As the point lies on the parametric surface S(u,w), the curve q(t) can be represented as q(t) ¼ [x(u(t), w(t)) y(u(t), w(t)) z(u(t), w(t))]. From the above analysis, there is a curve g(t) ¼ [u(t), w(t)] in the parametric domain of the surface (please see Fig. 2.1b), which has a one-to-one corresponding relationship with the parallel projection curve on the surface. Thus a one-to-one corresponding relationship exists between the 3D curve p(t) and the 2D curve g(t). For the convenience of description, we call g(t) as parametric projection curve in the remainder. In this chapter, we transform the problem of computing parallel projection curve on parametric surface into the one of tracing parametric projection curve in 2D u w parametric domain. In Sect , the first- and second-order differential quantities of the parametric projection curve are analyzed. A second-order iteration method for marching a series of points on the parametric projection curve based on Taylor Approximation is established and two methods for choosing the iterative step size are developed in Sect Considering the error caused by truncated higher order terms, a simple and efficient way to decrease the error is put forward in Sect Parallel Projection Marching The First- and Second-Order Differential Quantities of the Parametric Projection Curve In order to calculate the first-order differential quantities, i.e., u t and w t, of the parameters u and w with respect to the parameter t of the 3D curve, differentiating Eq. (2.3) with respect to t produces

4 12 X. Fang and H.-Y. Xu ðs u VÞu u þ ðs w VÞw t ¼ p t V ð2:4þ Taking the cross product of the S w and both sides of the Eq. (2.4), one has ð½s u VŠS w Þu t ¼ ½p t VŠS w ð2:5þ Substituting [S u V] S w ¼ (S u S w ) V into Eq. (2.5), we have u t ¼ L S w ½S u S w ŠV ð2:6þ where L ¼ p t V. Similarly, dot-multiplying Eq. (2.4) bys u gives L S u w t ¼ ½S u S w ŠV ð2:7þ One can continue to differentiate Eq. (2.3) with respect to the parameter t ds u dt V u t þ ðs u VÞu tt þ ds w V dt w t þ ðs w VÞw tt ¼ p tt V ð2:8þ Since ds u dt ¼ S uu u t þ S uw w t and ds w dt ¼ S uw u t þ S ww w t,eq.(2.8) can be rewritten as h i S uu ðu t Þ 2 þ 2S uw u t w t þ S ww ðw t Þ 2 V þ ðs u VÞu tt þ ðs w VÞw tt ¼ p tt V Dot-multiplying Eq. (2.9) bys w and S u, respectively, gives 8 u tt ¼ J S w >< ðs u S w ÞV J S u >: w tt ¼ ðs u S w ÞV ð2:9þ ð2:10þ where J ¼ [p tt S uu (u t ) 2 2S uw u t w t S ww (w t ) 2 ] V, and u t, w t are computed by Eqs. (2.6) and (2.7), respectively Parametric Projection Curve Marching In this section, we will use a second-order Taylor Approximation method to trace the parametric projection curve g(t) to generate a series of points.

5 2 A Second-Order Algorithm for Curve Parallel Projection on Parametric Surfaces 13 Let g i ¼ (u i,w i ) and g i+1 ¼ (u i+1, w i+1 ) be the current and the next parametric projected point in the u w domain respectively. Then the position of point g i+1 can be iteratively calculated through the current projected point and the first- and second-order differential quantities u t, w t, u tt, w tt at the point as follows ½u iþ1, w iþ1 Š¼½u i, w i Šþ½u t, w t Š½k þ 1=2Šu tt, w tt k 2 ð2:11þ where k is a constant iteration step size. If k ¼ Δt is constant, one can utilize the Eq. (2.11) to get a sequence of points (u i, w i ) on the parametric projection curve g(t), where i ¼ 1, 2,... However, the distribution of the projected points may not be well-proportioned. In order to fully use the differential geometric characteristics of g(t) and p(t), we will give two ways to choose the iteration step size. One can consider marching along the parametric projection curve based on a constant step size v p along the 3D parametric curve. Suppose r is the arc-length parameter of the 3D parametric curve, one has Marching along the 3D parametric curve uses dr ¼ kx t y t z t kdt ¼ kp t kdt ð2:12þ t iþ1 ¼ t i þ k with k ¼ 1 kp t k v p ð2:13þ in addition to using Eqs. (2.6), (2.7), (2.10) and (2.11). For marching along the parametric projection curve based on constant step size v q along the projection curve, one can write ds ¼ kq t kdt ð2:14þ where s is the arc-length parameter of the parallel projection curve and q t ¼ S u u t + S w w t. Marching along the parallel projection curve uses 1 t iþ1 ¼ t i þ k with k ¼ ks u u t þ S w w t k v q ð2:15þ in addition to using Eqs. (2.6), (2.7), (2.10) and (2.11) Error Adjustment Considering the truncated higher order terms, the position of point computed through Eq. (2.11) may depart from the parametric projection curve g(t) in u w domain. Hence, a first-order adjustment technique is presented to reduce the error.

6 14 X. Fang and H.-Y. Xu Suppose ^g iþ1 ¼ ð^u iþ1 ; ^w iþ1 Þ be the parametric projection point computed by Eq. (2.11) ½^u iþ1 ; ^w iþ1 Š ¼ ½u i ; w i Šþ½u t ; w t Šk þ 1=2½u tt ; w tt Šk 2 As the above iteration formula omits higher order terms, the image point of ^g iþ1 may deviate from the parallel projection curve on the parametric surface S, i.e., ^q iþ1 p iþ1 V ¼ ε 6¼ 0 ð2:16þ where ^q iþ1 ¼ Sð^u iþ1 ; ^w iþ1 Þ, p i+1 ¼ p(t i +1 ). Let [Δu, Δw] be the correction vector, the corrected parametric projection point and parallel projection point be g iþ1 ¼ ðu iþ1 ; w iþ1 Þ ¼ ð^u iþ1 þ Δu, ^w iþ1 þ ΔwÞand q i +1 ¼ S(u i +1,w i+1 ), respectively. According to the definition of parallel projection, one has q iþ1 p iþ1 V ¼ 0 ð2:17þ In order to calculate the error adjustment vector [Δu, Δw], we use a first-order Taylor formula to expand the point q i+1 at ^g iþ1, i.e., q iþ1 ¼ Sð^u iþ1 ; ^w iþ1 Þ þs u Δu þ S w Δw. Substituting q i+1 into Eq. (2.17) produces ðs u Δu þ S w ΔwÞV ¼ ε ð2:18þ Dot-multiplying Eq. (2.18) bys w and S u respectively, gives 8 ε S w Δu ¼ >< ðs u S w ÞV ε S u >: Δw ¼ ðs u S w ÞV ð2:19þ Note that all variables in Eq. (2.19) are evaluated at point ^g iþ1 and t i+1. When the deviation kεk calculated by Eq. (2.16) is greater than a given threshold α, i.e., kεk > α, one can adopt the error adjustment vector calculated by Eq. (2.19) to adjust the parametric projection point ^g iþ1. In the following, the quantities kεk and α are called as projection error and projection error threshold, respectively. Suppose g i and g i+1 are two adjacent points after error correction lying on the parametric projection curve g(t) ¼ [u(t) w(t)]. One can use a line segment to connect the two points g i and g i+1 as follows: u ¼ t ðu i t u iþ1 Þ ð2:20þ w ¼ w i þ lt ð u i Þ

7 2 A Second-Order Algorithm for Curve Parallel Projection on Parametric Surfaces 15 where l ¼ (w i+1 w i )/(u i+1 u i ). A G 0 continuous parallel projection curve is acquired by substituting Eq. (2.20) into Eq. (2.2). One can also use the G 1 approximation method [10] to fit the sequence of points S(u i,w i ), i ¼ 1, 2,..., to get a parallel projection curve with G 1 or higher continuity. 2.4 Demonstrations Our parallel projection algorithm framework is outlined in Sect and a number of examples are given to demonstrate the validity of our proposed method in Sect The examples make use of cubic NURBS curves and bicubic NURBS surfaces. In this context, these are viewed as parametric curves and surfaces, respectively Outline of Our Parallel Projection Algorithm Given the control points and knot vectors for a NURBS curve and a NURBS surface, we need to compute a series of parallel projection points on the surface.

8 16 X. Fang and H.-Y. Xu Fig. 2.2 Parallel projection example 1. (a) A butterfly curve onto an undee surface, and (b) parametric projection curve in u w domain Fig. 2.3 Parallel projection example 2. (a) A hand-form curve onto a mouse surface, and (b) parametric projection curve in u w domain Examples In this section we firstly give four examples (please see Figs. 2.2, 2.3, 2.4, and 2.5) obtained by the Algorithm 1. Further detailed comparisons of our scheme with Wang s first-order differential equation method [6] are carried out to testify the computational efficiency and accuracy of our methods (please see Tables 2.1, 2.2, 2.3, and 2.4). All the examples are implemented with Matlab, and run on a PC with 2.80 GHz CPU and 1 GB memory. Figure 2.2a shows an example of projecting a closed butterfly curve onto an undee surface. The corresponding parametric projection curve in the u w parametric domain is shown in Fig. 2.2b. The second example shows the projection of a small hand-form curve onto a mouse surface. The hand-form curve and its parallel

9 2 A Second-Order Algorithm for Curve Parallel Projection on Parametric Surfaces 17 Fig. 2.4 Parallel projection example 3. (a) A closed NURBS curve onto a ridge surface, and (b) parametric projection curve in u w domain Fig. 2.5 Parallel projection example 4. (a) A closed NURBS curve onto a ridge surface, and (b) parametric projection curve in u w domain projection curve on the NURBS surface is show in Fig. 2.3a. Figure 2.3b denotes the parametric projection curve in the u w parametric domain. Figures 2.3a and 2.4a are examples of projecting a same closed NURBS curve onto a same complex ridge NURBS surface along two different directions. The corresponding parametric projection curves in the u w parametric domain are shown in Figs. 2.3b and 2.4b. For the parallel projection examples 1 and 2, we compared our method with Wang s first-order differential equation method [6] focusing on computational accuracy and efficiency (please see Tables 2.1, 2.2, 2.3, and 2.4). For the sake of fairness, the step size is specified by constant parametric increment Δt and the tolerance α of the parallel projection error ε is set as constant (α is set as 1.0e-009 for all the comparisons). Moreover, the initial values for the two methods are the same. As the parallel projection error of Wang s method may overrun the error threshold α, it is solved with the classical fourth-order Runge Kutta method and our error adjustment technique deduced in Sect

11 2 A Second-Order Algorithm for Curve Parallel Projection on Parametric Surfaces 19 Table 2.4 Comparison of efficiency: projection of a hand-form curve onto a mouse surface Wang s method (error adjustment) Our method (error adjustment) Step size, Δt APE CPU time (s) APE CPU time (s) e e e e e e other hand, from the comparisons of Tables 2.1, 2.2 and Tables 2.3, 2.4, one can also find that our first-order error adjustment approach could efficiently improve the computational precision of the Wang s method. 2.5 Conclusion A second-order algorithm based on Taylor Approximation is proposed to compute the parallel projection of a parametric curve onto a parametric surface. Several examples are presented to demonstrate the effectiveness of the presented approach. Experimental results and comparisons indicate that the computational efficiency of our method is about 1.1 times of that of the first-order differential equation method and our method has superior performance in the computational accuracy. References 1. Sederberg, T. W., Finnigan, G. T., Li, X., Lin, H. W., & Ipson, H. (2008). Watertight trimmed NURBS. ACM Transaction on Graphics, 27(3), 79:1 79:8. 2. Chuang, J. H., Lin, C. H., & Hwang, W. C. (1995). Variable-radius blending of parametric surfaces. The Visual Computer, 11(10), Tam, H.-Y., Law, H. W., & Xu, H.-Y. (2004). A geometric approach to the offsetting of profiles on the three-dimensional surfaces. Computer-Aided Design, 36(10), Xu, H.-Y., Tam, H.-Y., Fang, X., & Hu, L. (2009). Quart-parametric interpolations for intersecting paths. Computer-Aided Design, 41(6), Pegna, J., & Wolter, F. E. (1996). Surface curve design by orthogonal projection of space curves onto free-form surface. Journal of Mechanical Design, 118(1), Wang, X., Wei, W., & Zhang, W.-Z. (2010). Projecting curves onto free-form surfaces. International Journal of Computer Applications in Technology, 37(2), Wang, X., An, L. L., Zhou L. S., & Zhang, L. (2010). Constructing G 2 continuous curve on freeform surface with normal projection. Chinese Journal of Aeronautics, 23(1), Xu, H.-Y., Fang, X., Hu, L., Xiao F., & Li, D. (2010). An algorithm for curve orthogonal projections onto implicit surfaces. Journal of Computer-Aided Design and Computer Graphics, 22(12), Xu, H.-Y., Fang, X., Tam, H.-Y., Wu, X., & Hu, L. (2012). A second-order algorithm for curve orthogonal projection onto parametric surface. International Journal of Computer Mathematics, 89(1), Yang, Y.-J., Zeng, W., Yang, C.-L., Meng, X.-X., Yong J.-H., & Deng, B. (2012). G 1 continuous approximate curves on NURBS surfaces. Computer-Aided Design, 44(9),

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