Journal of Applied Mathematics

Volume 2014, Article ID 124240, 13 pages

http://dx.doi.org/10.1155/2014/124240

## Properties of Generalized Offset Curves and Surfaces

^{1}School of Science, Jimei University, Xiamen 361021, China^{2}School of Mathematical Science, Xiamen University, Xiamen 361005, China

Received 25 October 2013; Accepted 11 March 2014; Published 21 May 2014

Academic Editor: Jacek Rokicki

Copyright © 2014 Xuejuan Chen and Qun Lin. 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

This paper proposes a definition of generalized offsets for curves and surfaces, which have the variable offset distance and direction, by using the local coordinate system. Based on this definition, some analytic properties and theorems of generalized offsets are put forward. The regularity and the topological property of generalized offsets are simply given by representing the generalized offset as the standard offset. Some examples are provided as well to show the applications of generalized offsets. The conclusions in this paper can be taken as the foundation for further study on extending the standard offset.

#### 1. Introduction

Offset curves/surfaces, also called parallel curves/surfaces, are defined as locus of the points which are at constant distance along the normal vector from the generator curves/surfaces. In the field of computer aided geometric design (CAGD), offset curves and surfaces have got considerable attention since they are widely used in various practical applications such as tolerance analysis, geometric optics, and robot path-planning [1, 2]. The study on the offset of curve and surface has been one of the hotpots in CAGD [3].

In some of the engineering applications, we need to extend the concept of standard offset, which has constant distance along the normal vector from the generator such as geodesic offset where constant distance is replaced by geodesic distance (distance measured from a curve on a surface along the geodesic curve drawn orthogonally to the curve) and generalized offset where offset direction is not necessarily along the normal direction. Generalized offset surfaces were first introduced by Brechner [3] and have been extended further, from the differential geometric as well as algebraic points of view, by Pottmann [4]. Arrondo et al. [5] presented a formula for computing the genus of irreducible generalized offset curves to projective irreducible plane curves with only affine ordinary singularities over an algebraically closed field. Lin and Rokne [6] defined the variable-radius generalized offset parametric curves and surfaces. The envelopes of these variable offset parametric curves and surfaces are computed explicitly. J. R. Sendra and J. Sendra [7] presented a complete algebraic analysis of degeneration and the existence of simple and special components of generalized offsets to irreducible hypersurfaces over algebraically closed fields of characteristic zero. A notion of a similarity surface offset was introduced by Georgiev [8] and applied to different constructions of rational generalized offsets. There are also some literatures on generalized offsets which primarily focus on solving some concrete problems [4, 9, 10]. But the general definition, properties, and complete analytic conclusions for generalized offsets have not yet been presented.

Some algebraic properties on standard offsets are known to classical geometers. The study of algebraic and geometric properties on offsets has been an active research area since it arises in practical applications. Farouki and Neff [11, 12] analyzed the basic geometric and topological properties of plane offset curves and provided algorithm to compute the implicit equation. We expect that generalized offsets would have more interesting properties and practical applications. In this paper, a strict definition of generalized offsets, which have the variable offset distance and direction, is given. The offset distance and direction are determined by the local coordinate systems. Though using the local coordinate systems to define a curve is not new [13], the definition of offset curves and surfaces by the local coordinate systems has never been presented before. According to this definition, similar to the standard offsets, we are concerned with the enumeration of certain fundamental geometric and algebraic characteristics for generalized offsets. The relationships between generalized and standard offsets are discussed.

This paper studies the generalized offsets of curves and surfaces in two primary segments. In each segment, we firstly give the definition and regularity of generalized offsets which can be explicitly expressed by the local coordinate systems, secondly we analyze the relationship between generalized and standard offsets, then we discuss some major properties of generalized offsets, and finally some examples are given to illustrate the applications of generalized offsets. The results in this paper will be the foundation for further study on extending the standard offset. Most analytic and topological properties of the generalized offset are addressed in this paper, which provide a series of fundamental conclusions for further study in the related field of generalized offsets.

#### 2. Generalized Offset Curves

##### 2.1. The Definition and Regularity of Generalized Offset Curves

For a planar parametric curve , the well-known Frenet [14, 15] equations are given as follows: where is the curvature, is the arc length such that , and and are, respectively, the unit tangent vector and the normal vector at each point of the curve . For the convenience of representation, we define a unit vector such that So we have Based on the local coordinate system in the plane, a generalized curve offset with the variable offset distance and offset direction can be defined.

*Definition 1. *For a planar smooth parametric curve with the regular parameter , its generalized offset curve with the variable offset distance and offset direction is defined by
where and are the functions of .

Thus the offset direction depends on and , and the offset distance is . From the above definition, the related parametric derivatives of can be obtained by Let and let ; we get Therefore we have

Regarding the regularity of generalized offset curves, we have the following theorem.

Theorem 2. *If there exists to satisfy the equation
**
and for any arbitrary small positive number ,
**
then is a nonregular point of the generalized offset curve .*

*Proof. *Let be any non-regular point of the offset curve ; then . We get . Thus
We discuss the following two cases. (i)When , from (10) it follows that
Substituting it into (11), we have
(ii)When , from (11) it follows that
Substituting it into (10), we also have
Hence we prove Theorem 2.

##### 2.2. Relationship between Generalized and Standard Offset Curves

We will prove that the generalized offset curve can be represented as the standard offset curve.

Theorem 3. *The generalized offset can be represented as a standard offset: , where is a new planar smooth parametric curve, is constant, and is the unit normal vector of .*

*Proof. *Let
where and are the functions of ,
In order to establish the above relationship, the following two conditions must be satisfied. (i)The inner product of vectors and must be zero. That is,
(ii) must be constant. That is,

Our goal is to get the values of and by solving the above differential equations.

From (18) and (19), we get
Note that
and we have
Analyzing the following four cases, we can get the values of and . (1)When , and only need to satisfy (19). That is,
where is an arbitrary constant. Thus one of and can be determined arbitrarily. For instance, if is given, then
(2)When and , from (22) it follows that and . Substituting it into (19), we have . Since , then . Therefore , where is an arbitrary nonzero constant.(3)When and , from (22) it follows that and . Substituting it into (19), we have . Since , then . Therefore , where is an arbitrary nonzero constant.(4)When and , by solving (19) and (22), we get
where is an arbitrary constant.

Therefore, in any case there exist two functions and to guarantee that the generalized offset can be expressed as the standard offset , where
That is, we can find so that becomes the standard offset of .

So far we have proved that the generalized offset can be represented as the standard offset. Based on the current results of standard offsets, we can continue the research on the regularity and integral properties of generalized offset curves. This theorem also helps us to obtain the simpler and conciser expressions. The following paragraph explains the details.

##### 2.3. Properties of Generalized Offset Curves

Let and . From the expression of standard offsets , where , and are the functions of , we have Moreover where and are the curvatures of and at each point, respectively. In the above case, there is another expression for the nonregular point of . Since is a nonregular point if , and are not both zero.

Therefore we can study the properties of the generalized offsets by using the similar approaches as what Farouki and Neff [12] had done for the standard offsets.(i)Evolute

We construct where . At the nonregular point , it follows that ; hence we also have On the other hand, we have Moreover, from we can get the relations as follows: (ii)Turning point, inflection, and vertex

Let ; then is called a turning point [16] if and , or and , and is called an inflection if and are called a vertex if .

Theorem 4. *If , and are not both zero, then *(1)*the turning point, inflection, and vertex on are, respectively, in one-to-one correspondence to those on ;*(2)*the turning point on is in one-to-one correspondence to that on as ;*(3)*the inflection on is in one-to-one correspondence to that on as .*

* Proof. *Based upon the following relationships
we can easily prove Theorem 4.

(iii)Length and area

We can calculate the lengths and of the curves and , respectively. Since , then

The area between and its generalized offset is denoted by (Figure 1). and are the area elements. See Figure 4.

At first, we compute Since it follows that Therefore , and (iv)Topological property

The distance between a regular curve and a point in the same plane is defined as follows: For the standard offset , we have the following theorm.

Theorem 5. *The distance between the point of the generalized offset and the curve satisfies one of the following conditions:
**
Each of the open intervals and . is delineated by the self-intersections.*

*Proof. *We have the following:(1);(2), , , are the self-intersections of .

Then one of the following propositions holds
For the generalized offset , we have
Considering , let such that
and , is the point with the same parameter of on ; then we have
Thus
Therefore we prove Theorem 5, which is shown in Figure 2.

According to Theorem 5, each of the segments of the offset curve among its self-intersections should either be retained or rejected in its entirety when forming the trimmed offset.

##### 2.4. Remark

The curves in three-dimensional space can also be discussed analogously. As we know, a curve is not planar if and only if the torsion of the curve is not zero. Therefore, different from a planar parametric curve, the Frenet equations for a spatial parametric curve are where is the torsion, and we have

Based on the local coordinate system in the space, a generalized curve offset with the variable offset distance and offset direction can be defined. The properties of generalized offset curves can be given similarly. Since a torsion item is added in the Frenet equations, the calculations may become more complicated and the conclusions may not be expressed simply.

#### 3. Generalized Offset Surfaces

##### 3.1. The Definition and Regularity of Generalized Offset Surfaces

Note that the symbols used in Section 3 are all redefined.

For a regular parameter surface , its two unit tangent vectors in the directions of and and its unit normal vector are given by [17] where and are the corresponding partial derivatives of about parameters and . forms a right-handed system. Based on the local natural coordinate system of surface , a generalized surface offset with the variable offset direction and the variable offset distance can be defined.

*Definition 6. *For a regular smooth parametric surface , the generalized offset surface with the variable offset distance and offset direction is defined by
where , , and are the functions of the variables and . The offset direction and the offset distance are determined by , , and .

For a regular smooth parametric surface , the well-known first and second fundamental quantities and the Gauss curvature [14] are given as follows: Let , and the angle between and is ; then Thus the related parametric partial derivatives of generalized offset surface can be obtained by where are the corresponding partial derivatives of with respect to and are the corresponding partial derivatives of with respect to . are the radii of principal curvature and is the Gauss curvature. We can get the following equations: Thus the unit tangent vectors and unit normal vector of surface offsets are given as follows: where , and are, respectively, the basic quantities and Gauss curvature of surface offset . Moreover, we can get the tangent plane and normal line at one particular point of surface .

Let be a nonregular point of ; then Note that Regarding the regularity of generalized offset surfaces, we have the following theorem.

Theorem 7. *If
**
then is a nonregular point of .*

From the above explanation, we can easily prove Theorem 7.

In most cases the local natural coordinate system at each point of a regular parameter surface is not the orthonormal coordinate system. In order to discuss the relationship between generalized and standard offset surfaces, we need to do some parameter transformation of surface firstly. According to the theorem [14], for every point at the regular parameter surface , we can find a neighbourhood and a new parameter system to make the new local natural coordinate system be the orthonormal coordinate system. This theorem guarantees the existence of orthonormal parameter curve net on the regular parameter surface. Therefore for any regular parameter surface, we can make the local natural coordinate system be orthonormal by this means. In the following two paragraphs we suppose that the local natural coordinate system of a regular surface is the orthonormal coordinate system.

##### 3.2. Relationship between Generalized and Standard Offset Surfaces

We will prove that the generalized offset surface can be represented as the standard offset surface.

Theorem 8. *The generalized offset can be represented as a standard offset: , where is a new regular smooth parametric surface, is constant, and is the unit normal vector of .*

*Proof. *Let
where , , and are the functions of and ,
The parametric partial derivatives of surface are
where are the corresponding partial derivatives.

In order to establish the above relationship, the following two conditions must be satisfied: (i) must be the unit normal vector of . That is,
(ii) is constant. That is,
It follows that
Our goal is to get the values of , and by solving the above differential equations. From (66) and (69), we get
From (67) and (70), we get
Let
From (71) and (72), we get
By solving (69), (70), and (74), we get
where is an arbitrary constant, , , and can not all be zero. Thus
When , , and only need to satisfy (69) and (70).

Therefore, in any case there are three functions , and to guarantee the generalized offset can be expressed as the standard offset . That is, we can find so that becomes the standard offset of .

So far we have proved that the generalized offset can be transformed to the standard offset. Based on the current results of standard offsets, we can continue the research on the properties of generalized offset surfaces. This theorem also helps us to obtain the simpler and conciser expressions. The following paragraph explains the details.

##### 3.3. Properties of Generalized Offset Surfaces

To study the properties of the generalized offset surfaces, we can use the similar approaches which have been introduced in offset curves. Here we only give the integral and topological properties of generalized offset surfaces.

(i)The area of an offset surfaceThe area of generalized offset surface is denoted by . We consider the area element , which is shown in Figure 3. Consider the following: Since then (ii)The volume between and

The volume between and its generalized offset is denoted by . We consider a volume element , which is shown in Figure 4. The volume element can be divided into five subvolumes: where and the symbol denotes the volume of tetrahedron with four vertices , and .

After several computations, we get Therefore, we have

(ii) Topological property

For a regular parameter surface the distance between a point and the surface is defined as follows: For the standard offset , we have the following theorem.

Theorem 9. *The distance between the point of the generalized offset and the surface satisfies one of the following conditions:
**
Each of the open fields is delineated by the self-intersections.*

*Proof. *We have the following:(1);(2), , , , are the self-intersections of .

Then one of the following propositions holds
For the generalized offset
we have
Considering , let such that
and is the point with the same parameter of on ; then we have
Thus
Therefore we prove the above theorem, which is illustrated in Figure 5.

According to Theorem 9, each of the segments of the offset surface among its self-intersections should either be retained or rejected in its entirety when forming the trimmed offset.

#### 4. Applications

Offset for curves and surfaces plays an important role in CAGD. It can be widely used in varieties of applications [18]. Generalized offsets are the extending of standard offsets, which have more flexible properties. By using programming language VC++, some simple examples are given in the following to show how to use generalized offset technique.

*Example 10. *Generalized offset curves.

In computer aided plane flower design, let a circle parameter curve be the original curve. We choose the union normal direction or deviate a certain angle from the normal direction as the offset direction, and some trigonometric functions as the offset distance. For example, the original circle parameter curve is Then the unit tangent vector at each point of the curve is , and the unit normal vector at each point of the curve