Rational surfaces with linear normals and their convolutions with rational surfaces Maria Lucia Sampoli, Martin Peternell, Bert J ü ttler Computer Aided Geometric Design 23 (2006) 179–192 Reporter: Wei Wang Thursday, Dec 21, 2006

About the authors Marai Lucia Sampoli, Italy Universit à degli Studi di Siena Dipartimento di Scienze Matematiche ed Informatiche /docente.php?id=32 /docente.php?id=32

About the authors Martin Peternell, Austria Vienna University of Technology Research Interests Classical Geometry Computer Aided Geometric Design Reconstruction of geometric objects from dense 3D data Geometric Modeling and Industrial Geometry

About the authors Bert J ü ttler, Austria J. Kepler Universit ä t Linz Research Interests: Computer Aided Geometric Design (CAGD) Applied Geometry Kinematics, Robotics Differential Geometry

Previous related work J ü ttler, B., Triangular B é zier surface patches with a linear normal vector field. In: The Mathematics of Surfaces VIII. Information Geometers, pp. 431 – 446. J ü ttler, B., Sampoli, M.L., Hermite interpolation by piecewise polynomial surfaces with rational offsets. CAGD 17, 361 – 385. Peternell, M., Manhart, F., The convolution of a paraboloid and a parametrized surface. J. Geometry Graph. 7, 157 – 171. Sampoli, M.L., Computing the convolution and the Minkowski sum of surfaces. In: Proceedings of the Spring Conference on Computer Graphics, Comenius University, Bratislava. ACM Siggraph, in press.

Introduction(1) LN surfaces Some geometric properties Its dual representation

Introduction(2) Convolution surfaces Computation of convolution surfaces Convolution of LN surfaces and rational surfaces

LN surface Linear normal vector field Model free-form surfaces [Juttler and Sampoli 2000] Main advantageous LN surfaces possess exact rational offsets.

Definition LN surface a polynomial surface p(u,v) with Linear Normal vector field certain constant coefficient vectors

Properties(1) Obviously Assume That is

Properties(2) Tangent plane of LN surface p(u, v) where

Computation given a system of tangent planes Then,the envelope surface is a LN surface. The normal vector

Geometric property Gaussian curvature of the envelope

Geometric property K > 0 elliptic points, K < 0 hyperbolic points, If the envelope possesses both, the corresponding domains are separated by the singular curve C.

The dual representation A polynomial or rational function f the LN-surfaces p (u,v) the associated graph surface q(u,v) is dual to LN-surface in the sense of projective geometry.

The dual representation Since det(H) of q(u,v) So, det(H)>0 elliptic points, det(H)=0 parabolic points, det(H)<0 hyperbolic points.

The dual representation Graph surface LN surface q(u,v) p(u,v) elliptic elliptic hyperbolic hyperbolic parabolic singular points dual to

Convolution surfaces and Minkowski sums Application Computer Graphics Image Processing Computational Geometry NC tool path generation Robot Motion Planning 何青, 仝明磊, 刘允才. 用卷积曲面生成脸部皱纹的方 法, Computer Applications, June 2006

Definition Given two objects P,Q in, then Minkowski sum

Definition Given two surfaces A,B in,then Convolution surface

Relations between them In general, In particular, if P and Q are convex sets Where, =

Kinematic generation(1)

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Kinematic generation(2)

Convolution surfaces of general rational surfaces Two surfaces A=a(u,v), B=b(s,t) parameter domains Ω A, Ω B. unit normal vectors,.

Reparameterization such that Where,. Convolution of general rational surfaces ∥

Then, is a parametric representation of the convolution surface of Convolution surfaces of general rational surfaces

Assumed LN-surface A rational surface B Convolution of LN surfaces and rational surfaces

If correspond, that is Then, Convolution of LN surfaces and rational surfaces

So, That is Where Convolution of LN surfaces and rational surfaces

The parametric representation c(s, t) of the convolution C = A ★ B Convolution of LN surfaces and rational surfaces

The convolution surface A ★ B of an LN-surface A and a parameterized surface B has an explicit parametric representation. If A and B are rational surfaces, their convolution A ★ B is rational, too. Convolution of LN surfaces and rational surfaces

Example

Conclusion and further work To our knowledge, this is the first result on rational convolution surfaces of surfaces which are capable of modeling general free-form geometries. This result may serve as the starting point for research on computing Minkowski sums of general free-form objects.

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