The Changing Shape of 3D Modeling
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
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
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
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
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
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
Article edited by Leslie Gordon,
Sr Editor, Machine Design
||Article reprinted by permission of Penton Media,
publisher of Machine Design