The transfinite interpolation of an irregular network of curves is an effective modelling methodology for the design of free-form shapes. This methodology is referred to as hybrid representation or H-rep to indicate that it is based on the merger of wire-frame modelling and solid modelling. The term H-rep also implies the integration of a curve fitting/fairing algorithm. The H-rep concept of Koelman (Koelman, H.J., 'Computer support for design, engineering and prototyping of the shape of ship hulls', TU Delft, 1999, ISBN 90-901-2888-3), which restores the traditional way of lines plan draughting in a computer method as lokal, one-dimensional process, which makes design changes in late stages tedious and expensive. This paper proposes a simple and efficient method that is not strictly local and does not destroy the consistency and the geometric continuity of the network. The problem is stated as follows: 'Given a certain region on a surface that interpolates a network of curves, manipulate all curves in that region simultaneously, in a way that does not destroy the consistency of the network and does not introduce unwanted geometric discontinuities.' If all data points are persistent throughout the modelling process (actually, only intersection points are persistent, but there will likely enough intersections to record the shape of a curve), then the complete model can be regenerated from the data points and the B-rep (a boundary representation, whose data structure consists of topological elements of type node, edge and face) alone, with the help of fitting/fairing algorithm. Thus, the problem can be reformulated as: 'Given a point set belonging to a consistent H-rep, shift a selection of points to a new position, so that the distance and the direction of the shift of each individual point varies smoothly over the set.' The shift vector for a data point, i.e. the difference between the position after and before shape modification, is defined as the vector sum of the sample of one or more three-dimensional vector fields. These are defined by a 'selection field', which is concentrated around a 'base' (a point, a curve or a surface). At the base location, the field intensity is unity, whereas it decreases to zero with increasing distance from the base. Examples of the shape variation of the fore ship and the bow bulb of a frigate illustrates the method to be a powerful modelling tool.
Shape modification of ship hulls in H-rep
Modifikation von Schiffsrümpfen in Hybriddarstellung
Ship Technology Research / Schiffstechnik ; 51 , 4 ; 162-172
2004
11 Seiten, 8 Bilder, 13 Quellen
Aufsatz (Zeitschrift)
Englisch
Shape Modification of Ship Hulls in H-rep
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