The Changing Shape of 3D Modeling

06/18/2009 by Blake Courter of SpaceClaim Corp.

Direct modelers provide a hands-on way to create and edit solid models

An emerging technology called "direct modeling" provides a new way to interact with solid models. As the approach becomes increasingly diverse, it can contain several different technologies that depart from traditional history-based modeling to give users a hands-on modeling experience.

Solid-modeling software can be classified according to different quadrants. The “X-axis” represents whether a geometry model is generated from a set of instructions (history-based), or depicted with an explicit, “dumb” geometry (geometry-based). The other axis represents how designers interact with the model, either by driving it primarily using hands-on tools (tool-driven), or by editing constraints (constraint-driven). Software such as SolidWorks Instant3D provides tools that let users interact with a 3D model as part of a traditional history-based modeler. Other programs such as NX and Solid Edge provide both a history-based and a constraint-driven direct modeler. The pure direct modelers in the upper-right offer an unadulterated direct modeling experience. (ST stands for synchronous technology.)

Traditional history-based solid modelers, including Pro/Engineer, SolidWorks, and Catia v5, create solid models by storing a recipe of instructions, or features, that generate the model. Most history-based modelers cache the generated solid in their files, but the underlying instructions contain all of the information required to reconstruct the model.

That recipe is a highly interdependent sequence of instructions, which, like software code, must be executed to produce a result. Thus, changes to early instructions can cause later instructions to fail, which is known as rebuild or regeneration failure. For this reason, designers using history-based software must put significant thought into the intent they use to program modeling operations.

Direct modelers, on the other hand, forgo instructions and instead simply store the solid model itself. To edit models, designers work with what they have, regardless of model history. Although the direct approach was developed before history-based modeling, recent advances in solid-modeling technology and increases in computing power have fostered a new generation of direct-modeling tools.

Direct modeling doesn’t describe a new technology as much as a new hands-on way to create and edit 3D solid models. Although some direct modeling is now included in most CAD systems, it is also available in stand-alone tools that may be useful to engineers who haven’t previously used CAD. These tools help companies finally deploy 3D to augment monolithic CAD systems for detailed design and drafting. For different tasks, different tools are appropriate, and direct modeling has created a new class of tools.

As shown in "The flavors of solid modelers," solid modelers can be classified according to how the user interacts with the model, and how the solid model is stored.

The constraint-driven approach — employed by both history and direct modelers — uses underlying constraint solvers to manage a model’s varying degrees of freedom and make edits that conform to those constraints. Constraints can be contained in the model sketches, between different components, and on the solids themselves. Constraints form a set of governing rules that dictate how a model may and may not be changed. They bind the model together so a change to one part, for example, can move many other parts and features. To change a model in a way that’s inconsistent with a given constraint setup, a designer must first reconfigure the system of constraints.

Tool-driven modelers, on the other hand, provide hands-on tools for making changes. For example, one tool might reposition geometry while another offsets it. Typically, on-screen "hints" help users preserve high-level design intent such as offset and mirror relationships. Users merely select the geometrical element that needs editing and make the edit using the appropriate tool. Design intent is captured by the use of dimensions and annotations. Dimensions in advanced tool-driven modelers can even precisely drive tools.

The traditional approach

History-based software was developed in the 1980s as a way to run solid-modeling software on the hardware then available. At the time, history-based programs provided distinct advantages over previous 3D approaches. For example, designers could actually make changes to solid models. Companies such as PTC and SolidWorks developed the so-called "feature-based," "parametric," and "associative" concepts that continue to work in all solid modelers today — and in the new direct modelers.

It is therefore more accurate to describe older systems as being "history based" instead of "feature based," and "constraint driven" instead of "parametric." Modern direct modelers, for example, further the concept of a feature by letting users save both selection groups and editing modes. Tool-driven modelers also permit precise dimensiondriven, parametric editing.

Traditional history-based systems are powerful tools in the hands of experts. Their sequential nature suits design problems that are already well understood and documented by concept models or 2D drawings. History-based modelers make an excellent choice for knowledge-based engineering and for generating families of parts. Manufacturers that create highly configured designs can benefit from the use of a history-based modeler because it lets companies program business and engineering logic into the model recipe.

However, there are drawbacks to the history-based approach. First, designers must have a well-thought-out game plan before they start modeling. Otherwise models can become a nightmare to work with and different users cannot edit each others’ designs. In addition, users must be well trained to be successful. A week or two of training is mandatory for most designers, and once trained, designers must plan on joining user communities to continue their education.

Also, in the past, the user experience in history-based modelers was relatively noninteractive. Edits mainly involved typing in dimension values, reprogramming placement constraints, and seeing if the model regenerated. Some CAD developers have since created rejuvenated user interfaces that allow direct manipulation using interactive tools to modify underlying histories.

These interactive history-based modelers, such as SolidWorks Instant3D, make model creation and editing more hands-on than traditional systems. For example, previously, editing the position of a hole entailed rolling back the design history to the point where the sketch existed but the extrusion didn’t. The trouble was that users couldn’t see ramifications of edits until the model regenerated. Interactive history-based modelers let designers modify sketches by directly moving the faces of the features, helping designers better visualize what they are doing.

Also, interactive history-based modelers let users make changes through special "tweak" or "local-operation" capabilities, so designers can edit solids regardless of their history. This capability smooths CAD interoperability problems but, if used inappropriately, can ruin models.

Consider the task of moving a hole created early in the model history. In a traditional system, such a move could cause regeneration failure and be tedious to fix. An interactive history-based modeler would let designers move the hole by adding further history. However, this "hackand- stack" approach is typically derided by experts as a recipe for disaster because modeling intent becomes muddied and the models become unnecessarily complicated.

Article edited by Leslie Gordon, Sr Editor, Machine Design

Article reprinted by permission of Penton Media, publisher of Machine Design