when surfaces meet solids

1 Jan, 2003 By: Don LaCourse Cadalyst

With little fanfare, EDS PLM Solutions and the folks down in Huntsville, Alabama, added basic surface modeling functionality to Solid Edge v12 earlier this year. Though meant only to provide users with basic tools needed to import and repair 3D models that are less than perfect solids, this addition is a significant development. When viewed from the broader MCAD industry perspective, it signals one more nail in the coffin of the solids-only midrange MCAD application. SolidWorks Corp. with its flagship product broke similar ground in recent releases.

These MCAD developers recognize the important role that surface modeling can play in building complex solid models. They are joining the ranks of veteran surface/solid applications such as VX CAD/CAM from VX Corp. and thinkdesign from think3. Surfaces and solids are finally converging in midrange MCAD applications, to the benefit of all users.

When solid modeling made its debut in the mid-1980s, MCAD vendors employed two distinct database structures (CSG and B-rep). CSG (constructive solid geometry) used mathematical Boolean operations (union, subtraction, and intersection) to implicitly combine simple solid shapes to form more complex shapes. B-rep (boundary representation) explicitly positioned and related surface elements to form an airtight boundary that enclosed the volume defined by the part.

Figure 1. Imported surface data displays problem areas on several surfaces. Solid Edge v12’s new interrogation tools highlight problem areas of the surface model, in this case, nonstitched edges. Using Solid Edge’s new tools for trimming, extending, and creating surfaces from bounded edges, we fixed the surfaces and added missing ones, then stitched the surfaces together to form a solid body.
Both methods had their uses and merits. B-rep was predominantly used by the 3D surface modelers of the day. Early solid modelers employed CSG.

Surface and solid modelers today share the same roots within their database structures. CSG was fine for the early solid modelers, but soon went by the wayside as users demanded the ability to model more complex shapes that CSG alone could not handle. In stepped hybrid modelers that took advantage of the strengths of both CSG and B-rep.

As MCAD developers migrated toward their second generation of modelers, programmers discovered how to employ CSG-like operations using B-rep. You still find Boolean operations in today's solid modelers, even though virtually all of today's MCAD applications (surface and solids) use B-rep as their sole database structure.


Figure 2. SolidWorks’ Surface Fill command quickly and easily fills in a complex area with a surface that is tangent to all adjacent existing faces.
In today's modelers, a solid is nothing more than a set of surfaces whose topology forms a closed volume. Topology refers to the relationship of a part's mating vertices, edges, and faces. Solid modeling operations are governed by internal rules that maintain valid topology. Valid topology means that all surfaces are present and do not intersect each other and that multiple volumes do not share vertices, edges, or faces, among other things.

Surface modeling is not governed by these rules. You are free to create any number of surfaces, in any order. They may connect cleanly, or they may not. It's up to you to employ clean modeling techniques. This freedom attracts many users and is what makes surface modeling so powerful. Once a surface model is complete, you can use a simple "thickness" command to convert it to a solid.


Figure 3. Pro/ENGINEER’s Interactive Surface Design Extension (ISDX) converts a surface model to a solid.
Hybrid modeling now refers to MCAD applications that employ both surface and solid modeling operations. They combine the ease of use of solids with powerful surface creation and editing tools-the best of both methods. In a hybrid modeler, you can temporarily suspend valid topology rules of solid modeling under controlled conditions.

You can apply traditional solid feature operations, such as holes, pockets, protrusions, fillets, and chamfers, to open sets of surfaces. At the same time, you can explode a solid to gain access to its underlying set of surfaces, then apply surface modeling operations. You can add, remove, or replace surfaces, and perform local surface modifications such as control point editing (tweaking). When the required surface modeling is complete, sew the surfaces back into a solid (provided they form a closed volume).

Developers of traditional solids-only applications add surface creation tools for a number of reasons, and some vendors are further along than others. Other than the obvious need for complex surface handling, the solids-only roots of many modelers cause problems for

Figure 4. VX CAD/CAM v7 lets you work with surface or solid models or both within the same part or assembly. Solid feature tools can be applied to open surface sets such as those used to create the handle in this design.
(top) VX CAD/CAM provides complex surface creation tools as well as control point editing (tweaking) and analysis and visualization tools.
(right) A new Design Optimizer automatically adjusts driving dimensions to achieve a target parameter such as the volume in this bottle design.
users trying to import and work with surface models or imperfect solid models. Neutral format import and export translators such as IGES can produce topological errors, even if the part began as a solid. Such errors can be caused by faulty translator code, mixed interpretation of the standard, support for only a subset of the standard, and the like.

To solve this problem, solids-only developers are starting to provide the surface creation and editing tools needed to repair those less-than-perfect models everyone seems to get once in a while. The hybrid solid/surface modelers never faced this problem, but did need to provide better-than-average import and export translators to handle the more complex surface models out there.

All users will benefit when the industry unites on support for both surfaces and solids. The expanded design envelope that surfaces bring to users of traditional solids-only applications is only one benefit. Import and export translators should improve because developers will have to export the new surface entities their programs can now create. With improved export, improved import must follow.

Even with the convergence of surfaces and solids well underway, MCAD applications differ in their strengths and weaknesses in each area. You can count on all applications to offer the basics of feature-based solid modeling. They differ in their integration, approach, and support for surfacing functionality. If you are wondering, "What should I buy?," you should ask some additional questions as well.

Figure 5. Table Lighter created by Alessi Giovannoni Design using thinkdesign. This is another good example of the application of solid features on a free-form shape—see the thickness, ribs, holes, and bosses in the image on the left.
What will I use the application for? What you plan to design plays a significant role in deciding what products to look at. If you work with complex free-form shapes that require attention to aesthetics and ergonomics, a solid modeling application strong in surface creation and editing with tangency, curvature, and visual controls will be very helpful. On the other hand, if you work with known prismatic shapes with standard features such as holes, fillets, and pockets, basic surface functions will be adequate.

Do I work with large assemblies? If so, an application with in-context assembly design (the ability to edit a part within the context of the assembly) and assembly management capabilities is a plus. Also look for an application that allows surface models to be interchangeable with solid models within the assembly. Find out what the restrictions are, if any.

Do I frequently import models from neutral formats (IGES and STEP)? If the answer is yes, then consider an application with strong support for surfaces. For example, tool and die designers need to repeatedly import multiple versions of customer parts that may come in many different formats. The CAD application should not only support surfaces but also provide extensive topology healing tools that help repair those occasional problematic parts.

Figure 6. Advanced part creation using the unified shape technology in Autodesk Inventor 6 lets you mix solids and surfaces to create stylized, complex, and sculpted parts.
What applications will I be required to interface with? Make a list of those applications you send and receive part models from. You may afford to buy the same applications. If those on the list are based on licensed geometry kernels such as Parasolid or ACIS, look at applications that support that kernel to see if they can handle your design needs. If those on the list are high-end systems such as CATIA or Unigraphics, consider an application that is strong in surface modeling capability. Chances are that you will receive complex surface models.

As surface and solid modeling functions continue to converge, expect the line between traditional midrange solids-only applications and hybrid modelers to blur considerably. Though hybrid modelers have a leg up when it comes to developing the functionality and integration between surfaces and solids, this gap will continue to close as well.

From the user's point of view, midrange applications can only get better. Developers of hybrid modelers can't afford to sit around and simply tout that they offer the best of both breeds. This convergence will spur them to develop more advanced surface functionality and possibly new areas to set their products apart. Developers of solids-only applications will finally break free of those bonds and look to expand their user bases into more complex applications. The future of midrange MCAD application development looks bright indeed!

Glossary of Solid and Surfacing Terms

B-rep. Boundary representation. A database structure used by early solid modeling applications. It explicitly positions and relates the topological elements of a solid. B-rep is widely used in all surface and solids applications today. CSG-like Boolean operations can now be defined with B-rep.
Clean modeling. User-implemented techniques that help reduce the complexity and improve the validity of 3D models.
CSG. Constructive solid geometry. A database structure used by early solid modeling applications. It uses mathematical Boolean operations to implicitly combine solid shapes. CSG is not in common use today.
Hybrid modeling. In the early days, MCAD applications that employed two distinct database methods, such as CSG and B-rep, to represent a solid. Today, it refers to those MCAD applications that support and integrate both surface and solid modeling techniques.
Topology. Definition and relationship of the elements that make up a solid model. The topology table within the B-rep database structure defines the relationship between all vertices, edges, and faces (that is, surfaces).
Tweaking. Process of editing one or more control points of a surface to modify it locally.

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