NPOWER Software - Advanced Modeling Plugins for 3DSMAX and RHINO


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Power NURBS Surfacing

Power NURBS surfaces are much more than just a NURBS control point surfaces. Power NURBS surfaces are defined as a procedural objects that can be recomputed whenever inputs change. There are a variety of ways that the user can control the construction or re-construction of a surface by manipulating the Power Surface Parameters. DIfferent criteria of accuracy and knot vector creation are required for different uses. For example, automotive and aerospace design often require "Class A" surfaces to give the highest geometric quality in terms of smoothness and continuity between surfaces.

  • "Class A" Surface Construction
  • Extensive Surface Continuity Controls
  • G2 and G3 Blending and Filleting
  • Extreme Rail Sweeping Tool
  • Extensive Analysis Tools

"Class A" Surface Construction

A good definition of "Class A" surfacing can be found at the Design Engine web site and is given to the right. The motivating factor behind "Class A" surface construction is visual quality of the finished and manufactured object. For example, the dashboard and fender of a car are examples of surfaces which contribute greatly to the aesthetic qualities of an automobile. The bottom line is that the better a car looks, the more people like it and buy it. Power NURBS provides the user with sufficient tools to control the parameters necessary to construct Class A surfaces.

Continuity Controls

Power NURBS utilizes a "Derivative Surface" as its primary mechanism for specifying and controlling surface tangencies. The derivative surface is associated to a 3-D sketch curve of some type and can be manipulated in many different ways. A unique feature of Power NURBS is the "Tangent Control Handles" which can be placed at arbitrary positions along a curve to specify change in both rotation and strength of the derivative surface. The "Derivative Surface White Paper" demonstrates how the control handles can be used to manipulate the derivative surface. See the Exploring Tangent Control Handles video to see how we have constructed the interface of Power NURBS to to take advantage of tangent handles. Our experience has been that the utilization of Tangent Control Handles actually reduces the number of curves required to construct a surface. The shoe video tutorial is an excellent example of this type of construction used to solve a real world problem.

G2 and G3 Blending and Filleting

G2 and G3 are terms used to specify geometric continuity of two and three respectively. The three common types of continuity are as follows:

  • G1 Continuity - direction of first derivatives of two surfaces match where they join.
  • G2 Continuity - G1 plus the radius of curvature matches.
  • G3 Continuity - G2 plus the change in radius of curvature matches.

The higher continuity blending means that you can join surfaces in such a way as that they look like a single surface. In G2 and G3 continuity blending and filleting it is difficult to determine where one surface ends and the other begins. G1 continuity blends and fillets have a more well defined transition between surfaces. Exteriors of automobiles are typically G3 continuity which gives a very good quality reflection lines. The "Blending White Paper" talks in more detail higher order blending. See the "Understanding G2 & G3 Continuity Blending" video for additional explanation on the meaning of geometric continuity.

 

Extensive Analysis Tools

Power NURBS has a rich set of surface analysis tools that are designed for use in an interactive surface construction environment. Good quality surfaces typically require good quality curves. The Sketcher also has a set of analysis tools to enable the user to determine the quality of the curves during construction and editing.


Surface Analysis Using Iso-Curve Curvature
  
Mean Surface Curvature Display Gaussian Surface Curvature Display
 

Class A Surfacing (from design-engine.com)

1) The fillets - Generally for Class A, the requirement is curvature continuous and Uniform flow of flow lines from fillet to parent surface value of 0.005 or better (Position 0.001mm and tangency to about 0.016 degrees)

2) The flow of the highlight lines - The lines should form a uniform family of lines. Gradually widening or narrowing but in general never pinching in and out.

3) The control points should form a very ordered structure - again varying in Angle from one Row to the next in a gradual manner (this will yield the good Highlights required).

4) For a Class A model the fillet boundary should be edited and moved to form a Gentle line - and then re-matched into the base surface.

5) Matched iso-params in U & V direction are also a good representation of class A.

6) The degree (order) of the Bezier fillets should generally be about 6 (also for arc Radius direction) sometimes you may have to go higher.

7) We also take care of Draft angle, symmetry, gaps and matching of surfaces Created with parent or reference surfaces.

8) Curvature cross-section needles across the part - we make sure the rate of Change of curvature (or the flow of the capping line across the top of the part) is Very gentle and well behaved.

n-Rail Sweeps

Power NURBS has the ability to sweep an arbitrary number of profiles along an arbitrary number of rail curves. The Power NURBS Rail Sweeping tool is one of the most advanced and flexible tools for sweeping available anywhere. The great thing about this is that the user can construct with a single surface complex objects which require many surfaces in other systems. Our sweeps also allow both closed rails and closed profiles as well as the ability to end a sweep at a single point where rails converge. See the "Rail Sweeping White Paper" for insight on how sweeping works in Power NURBS. See the following videos on Rail Sweeping: