What’s the Difference Between Parametric and Direct Modeling?

A key milestone in the history of computer-aided design (CAD) came with the 1987 release of Pro/ENGINEER, now PTC Creo, which introduced the CAD industry to history-based parametric modeling. Since then, the parametric paradigm has been employed in almost every mainstream CAD program, including SOLIDWORKS, Autodesk Inventor, Creo Parametric, CATIA, NX, and Onshape. If you’ve done any CAD modeling in the past two decades, chances are you’re familiar with history-based parametric design.

In a nutshell, parametric design involves engineers building up a 3D geometry piece by piece. 2D sketches turn into 3D features, with constraints and relations duly applied to fit the designer’s intent. However, since each step follows from preceding steps, parametric design can require careful planning.

Despite the power and popularity of parametric modeling, another CAD paradigm has its own share of proponents: direct modeling. In direct modeling, geometry is king. Users no longer need to worry about the history of their part and can instead act directly on the 3D geometry as is. Some modern CAD systems use direct modeling instead of history-based parametric modeling, while others provide a blend of parametric and direct tools.

To better understand the differences between parametric and direct modeling, what use cases they might be best for, and how they might continue to evolve, we spoke to experienced CAD users and industry insiders for their take on the two paradigms.

Parametric Modeling

First of all, let’s clarify the terminology we’re using. The design paradigm pioneered by Pro/ENGINEER is perhaps most properly called “history-based,” as it’s the linear step-by-step nature of the model that really differentiates it from direct modeling, which might also be called “history-free.”

“History-based CAD software relies on the user to build an individual part with a series of features,” wrote veteran CAD user Matt Lombard in an eBook about Siemens’ CAD software Solid Edge. “The software remembers the features in order, and the model must resolve these features in order perfectly at every step to move forward. You can think of this method of model building like a computer program. You give the computer instructions for every step, and it executes those steps for every time the model must be rebuilt.”

Nonetheless, history-based modeling is often referred to as “parametric,” and so we’ll treat the two terms as similar. Just keep in mind this caveat from Lombard: “People frequently have the mistaken idea that only history-based software is parametric. Parametric means that the model is driven by parameters, which can include dimensions, pattern instances, wall thicknesses, hole diameters and depths, and so on.”

Lombard’s analogy of a computer program is an apt one for parametric modeling. Just as when writing a program, the more an engineer thinks ahead about what is trying to be accomplished, the better the end result. Even though a model, like a program, can be put together on the fly, it might be sloppy and difficult for other engineers to edit.

Roland Schwarz, another experienced CAD user, pointed out that this fact of parametric modeling can be a double-edged sword.

“If you work with somebody that's really good at parametrics and structures their project in a good, thought-out, rigorous way, it’s fantastic,” Schwarz said. “But you can have somebody that just gets in there and chops and changes and has stuff copied in the wrong places, and there’s not a logical system to it. That’s when it gets messy, and I just want to start from scratch with an export and import.” (Exporting and importing the model removes its feature history and turns it into what’s called a “dumb geometry,” which is just the geometry without design intent).

The best parametric CAD designers are so adept at structuring their projects that they can sometimes be identified just by looking at one of their models, as if they left behind digital fingerprints on the history tree. Even though the end model is exactly the same, there are many ways to get there—and some are definitely better than others.

“And my way’s the best,” joked Schwarz. “That’s what every good CAD modeler will say.”

Parametric modeling is extremely useful for products that are driven by dimensions—think standard office fare like desks and shelving or tools that can vary in size. With the parametric approach, engineers can quickly and easily create different configurations of their designs.

But from where the not-so-rigorous modelers live, cracks have started to appear in the parametric modeling paradigm.

“Sometimes I think you can get bogged down in the history,” said Bruce Bartlett, another CAD veteran. “As much as the history gives you the ability to change things quickly, the disadvantage is if you don’t understand how it’s built or you have somebody that’s building stuff that gets a bit out of control, you can spend more time wading through the history tree than just grabbing some faces and moving them.”

Another weakness of parametric modeling is that as the number of features increases, so does the computation required to update your model.

“If you have a part with 1,000 features in the history tree, when you edit the last feature in that tree it goes very quickly,” explained Dan Staples, Siemens PLM vice president of mainstream engineering. “But when you edit the first feature in that tree, it might take a very long time because it has to compute all 1,000 in between before it gets to the 1,001^st feature.”

With its balance of strengths and weaknesses, is parametric modeling the best option out there for CAD design? Staples doesn’t think so.

“The notion of editing a dimension to achieve a very predictable result that respects the intent of the designer is inherently valuable,” he said. “The implementation known as history-based design, done by Pro/ENGINEER back in 1987 and copied by virtually everyone else now for the last 25 years, is not the most efficient way to edit that dimension.”

Staples, naturally enough, advocates Siemens’ synchronous technology, a paradigm we’ll discuss later.

Direct Modeling

As its name indicates, direct modeling offers designers a what-you-see-is-what-you-get approach to building and editing their models.

“There seems to be no end of experts arguing over finely nuanced definitions and variations of this term, but in general, direct edit is a CAD method where you directly manipulate faces of the model, rather than indirectly edit feature definitions or sketches…how the geometry is made really doesn’t matter and, frankly, isn’t as interesting as how the geometry is changed,” Lombard wrote.

In direct modeling, designers can push and pull on their model to change it. One benefit of this capability is the ease with which designs can be altered, enabling rapid iteration and prototyping. For that reason, direct modeling has found a niche in the world of industrial design, where fluid curves and bold aesthetics play a big role.

“There are use cases, such as highly organic design, sculpting, concept design, generative design and de-featuring for analysis, where a totally non-parametric toolset can be an excellent and sometimes the best choice,” said Dave Corcoran, Onshape co-founder and vice president of research and development.

Furthermore, without the dependencies and history of parametric modeling, direct modeling can eliminate some of the problems of poor parametric design.

“I think the real test of a CAD modeler isn’t what they can make, it’s what they can change. Direct modeling definitely opens up the field vastly about what can they change without causing a complete disruption,” Schwarz said.

The ability to change a model without worrying about breaking it can mean a lot, especially for companies looking to extend the utility of their CAD model. Schwarz described CAD models as assets, and like any other asset, they require an investment to secure.

“A lot of time and money goes into making a CAD model, and you want to protect those, and you want them to remain useful. Direct modeling will protect that investment,” Schwarz explained.

But direct modeling comes with its own share of drawbacks.

“Most history free systems do not deal well with editing of dimensions and getting a predictable result,” Staples said. “Sometimes you’ll hear people talk about a push-me-pull-you kind of system, and they look very sexy, but when push comes to shove at the end of the day, you really want to be able to edit dimensions and create dimensional places and so on.”

Another problem with direct modeling is that, for most CAD modelers, it’s simply not what they’re used to. After years of experience with parametric design—and years of getting used to its shortcomings—there may just be a lack of motivation to consider an alternative.

“I don’t think parametric modeling’s going away anytime soon,” Schwarz said. “There’s a very large population of users out there that are well staked out in it and like it, or at least they’re comfortable with it and not going anywhere. I think direct modeling is a huge step forward. At the same time I think there’s a culture that’s got to evolve around it—where it becomes more accepted and more understood about the best ways to use it—especially in terms of managing a modeling project over the life of a product or even the life of its initial development without having it become a bigger problem than a solution.”

Blurring the Line Between Design Paradigms

Most mainstream CAD systems offer some mixture of both parametric and direct tools, although the capability of these tools can vary. 

“Onshape's a parametric modeler, but it does have a bit of direct modeling built over the top of it,” explained Bruce Bartlett, an Onshape user. “It's direct modeling, but it’s still parametric. Sometimes I wish it could be just a pure direct modeler, and we could just move faces around without any history."

What Bartlett is referring to is that even though these tools act like direct editing tools, they still create features in the history tree. Since true direct modeling is history-free, this approach is only quasi-direct. Like Onshape, SOLIDWORKS also offers direct editing tools that can be handy to users for certain tasks, but ultimately leave features in the history tree, maintaining the potential problems of parametric design.

Other CAD programs implement the hybrid tools in different ways. Autodesk’s Fusion 360, for instance, allows users to operate in either parametric or direct modeling mode. In direct mode, your design history will not be recorded at all. Users can also convert any feature to a direct modeling feature while leaving the rest of the parametric history tree intact.

“With synchronous technology I can either move faces by hand or I can move faces by editing dimensions,” Staples explained. “And we’ve got the unique technology to figure all this out and preserve all that design intent while doing so, but without the baggage of a history tree or any recompute.”

Staples believes that synchronous technology goes a step further than the other CAD programs’ hybrid blend of tools, sidestepping some problems that can arise in systems that implement direct modeling as a feature in the history tree.

“It seems like you’ve solved your problem. But what you’ve really done is added complexity to the history tree and invalidated the design intent. Now when someone comes back to that part a year later, they’re going to go edit that first feature in the tree, and it’s going to recompute and get overwritten later in the tree. So this notion that adding more features to the tree is solving some problem… it’s solving a very near-term problem but adding many more problems to your engineering process further down the line.”

While some CAD users may have reservations about the claims of synchronous technology, those who use it practically sing its praises. Does this mean the question of parametric vs direct modeling is moot, and synchronous technology is the way forward? That depends who you ask.

“There is no one modeling product approach optimal for all situations. Tools that are over-generalized and try to solve too many use cases in one toolset tend to be hard to use and over-complicated,” said Onshape’s Dave Corcoran.

Some Solid Edge users have reported difficulties using synchronous technology in certain scenarios, like complex surface modeling, a limitation which Staples admits to. “If you’re doing very aesthetically pleasing parts that require a lot of surface based design, synchronous is not well suited to that in its current state,” he said.

One might also wonder why if synchronous technology is as beneficial as its proponents claim, it’s not a more widespread paradigm. Staples believe the reason is that Siemens is just that far ahead of the technological curve, in large part because of its ownership of the Parasolid kernel and D-Cubed constraint solving SDK. But perhaps there’s a simpler reason, which he summarized nicely: “engineers are not people who like to change a lot.” History-based parametric modeling is still the dominant paradigm in the CAD world and, despite its drawbacks, it works just fine.

“At this point in history SOLIDWORKS works for 98 percent or better of what anybody’s doing out there,” Schwarz said.

Over time, however, as new generations of engineers enter the world of CAD modeling, perhaps the old history-based methods will give way to something more like synchronous technology, and the distinction between parametric and direct modeling may no longer be relevant.

“I think over time the line will get blurred, and it will just be different sides of the tool box,” Schwarz said.

In the meantime, here’s a non-exhaustive list of some of the most prominent CAD programs and what design paradigm they adhere to: