CAD Users Embrace Hybrid Modeling

Subdivision modeling gaining ground in production phase surface modeling.

Subdivision modeling gaining ground in production phase surface modeling.

A Mercedes-Benz vehicle model rendered in Siemens NX. Image courtesy of Siemens.


Automotive surfaces define a vehicle’s aesthetics and its aerodynamics. Surface design is a departure from solid modeling and parametric modeling. It takes a specialist’s careful judgment and manipulation to produce smooth curvatures with an economy of control points on NURBS/Bezier surfaces and curves. Though CAD programs have evolved over time, surfacing modeling methods remain unchanged for the most part.

McNeel’s Rhino is a software program known for its surfacing-modeling tools.

“Class A surface modeling generally has not evolved for a very long time,” says Kyle Houchens, trainer at McNeel. “The models are generally built the same way they have been built since the inception of Class A modeling, and there is massive industry resistance to change.”

But in conceptual design and even in select production phases, the use of subdivision modeling is gaining ground, according to industry insiders. In this article, we examine this new trend and the hybrid modeling strategy it inspires.

Standalone Surfacing

In 2019, Autodesk added subdivision modeling in Autodesk Alias, a popular package in automotive design. The company points out that the newly integrated subdivision modeling in the software complements the existing NURBS modeling, offering a true hybrid workflow.

“These are not polygonal subdivision surfaces like in Blender or Maya. It means you can use these surfaces all the way downstream and into engineering and manufacturing,” says Phil Botley, product manager, Autodesk Alias.

In the concept phase where shape exploration is the goal, users may employ the more intuitive subdivision modeling by pushing and pulling on blocks of geometry to sculpt the vehicle’s shape. But in the later phases that demand Class A surfaces, traditional NURBS/Bezier editing with control points remains the standard, Botley says. He believes Autodesk Alias is one of the few packages that satisfies the needs of conceptual design as well as the subsequent detailed design.

Having the design in a single modeling package reduces data loss from import-export operations. The transition from subdivision objects to Class A surfaces is highly manual, adding time into the design cycle. However, Botley believes users are warming up to a new approach.

“We’re actually finding out that, by a quirk of fate, the subdivision surfaces Alias produces are, of course, Bezier surfaces, so ideally suited for use all the way down to production. We have customers who are looking into maturing these subdivision surfaces into Class A surfaces over time,” says Botley.

Autodesk Alias is known for its surfacing tools in the automotive sector. The screenshot here shows the design of Le Mans concept car in Alias. Image courtesy of Autodesk.

If what Botley describes becomes the new norm, then the new workflow could eliminate the time-consuming step of object conversion. In the future, AI-driven tools might also help automate the conversion from subdivision and computer-aided styling surfaces to Class A surfaces, Botley says.

“The thing about Class A surfaces is, it’s like sculpting. The lighter and simpler it is, the better it is. A designer might build a very heavy surface with lots of control vertices. But a Class A modeler would simplify it to make it smooth and light, with fewer control points. In the end, that makes it easier to transfer to [Dassault’s] CATIA or [Siemens’] NX as well as milling and tooling from the data,” says Botley.

For Class A surfacing modeling, Alias offers quality checks, such as curvature analysis and highlight analysis tools.

“These tools help designers make informed decisions about whether his or her surfaces are manufacturable,” says Botley.

Integrated Surfacing in Siemens NX

Siemens NX, viewed as one of the leading CAD software packages in the automotive sector, also includes surface modeling tools.

“In NX, we have a mix of surface modeling, solid modeling, mesh modeling, and subdivision modeling tools,” says Tod Parrella, senior manager, NX, Siemens. “Bezier or NURBS surfaces, solid models and mesh models are equal players in NX. In surface modeling, we support both parametric NURBS modeling as well as control-point modeling. For example, you can draw a spline, sweep it into a surface, and then edit it using the control points.”

Siemens NX offers integrated surfacing tools, as seen in this user interface. Image courtesy of Siemens.

In interior modeling, subdivision modeling is gaining ground, due to the less stringent requirements for surface quality, he points out. “There’s some leeway in these areas, so there’s quite a bit of acceptance [of subdivision modeling]. Usually, you only need near-Class-A quality, like at least G-2 or better smoothness,” says Parrella.

NX surfacing tools include quality checks, such as embedded curvature checking, continuity checking, minimum-radius checking and stamping analysis.

“In Shape Studio [a suite of advanced surfacing tools in NX], you can use slope analysis and part breakup to ensure they’re stampable. It also has sharpness analysis to warn you if your part is too sharp for vehicle safety standards,” he says.

NX offers semi-automated tools to fit high-quality surfaces onto the concept design topology, but “quite frankly, this process is not as automated as people want it to be,” Parrella says. Like many other CAD developers, Siemens is also adding AI-powered functions into NX. The current command prediction and voice assistance exemplify these developments. The company also plans to add efficiency in the Surface Studio user experience according to Parrella.

Class-A Modeling Remains Unchanged

In the last decade, 3D modeling methods have evolved dramatically, forcing practitioners to learn new skills every few years or so. Parameter-based generative design is a good example of this development. However, Class A surface modeling remains unchanged for the most part, many experts note.

“In the concept phase, packaging phase, and the pre-Class A phase, things are evolving,” says Parrella. The emergence of polygonal or subdivisional modeling itself is a relatively new phenomenon, he notes. “But in Class A surfacing, people still use traditional [NURBS/Bezier] methods, because they have to ensure they get the lightest weight math, the most accurate representation, and the highest quality of segmentations,” he says.

“The automotive world is like a friendly dinosaur. It plods along very slowly,” quips Botley. “That’s because you have 5-, 6-year car projects, and you have to maintain the data for 20 or 30 years. If I look back five to 10 years, the changes in Class A surface modeling have been absolutely minimal.”

Embedded Tools vs. Dedicated Packages

Mainstream solid modelers like Autodesk Fusion, Dassault Systèmes’ SOLIDWORKS and PTC Creo are rooted in parametric modeling or history-based modeling. On the other hand, surface modeling stems from NURBS/Bezier editing. While general-purpose CAD programs have integrated some surfacing tools, they won’t replace the dedicated surface modelers, for good reasons.

“NX began as a wireframe and NURBS surface modeler and evolved into supporting the rich set of NURBS/Bezier surface, mesh, solid, explicit (history-free), and history-based methods,” says. Parrella.

“The automotive industry has very specific requirements for their surface and solid models,” says Houchens. “They serve different purposes and are maintained by different development disciplines.”

“You have surfacing capability in Autodesk Fusion, also in NX and CATIA, and they do a great job, but there’s a limit due to their mathematics and modeling kernels,” Botley says. “If a company wants best in class Class A surfaces, then they’ll need a standalone solution.” Parrella, however, points out NX’s surfacing tools “can fully support the Class A workflow in the single, end-to-end integrated environment.”

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Kenneth Wong's avatar
Kenneth Wong

Kenneth Wong is Digital Engineering’s resident blogger and senior editor. Email him at [email protected] or share your thoughts on this article at digitaleng.news/facebook.

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