The Challenge of 3D Digitizing

By Terry Wohlers

Published in Vol. 18, No. 11, November 1995 issue of CGW
Copyright 1995 by Terry T. Wohlers

As an engineer, it's easy to get excited about 3D digitizing. After all, it's a technology that promises to do away with the need to laboriously re-create by hand the 3D data of an existing object. Instead, you just take the physical object, scan it, and voila, you instantly have a complete 3D model ready to be edited and modified in your CAD system.

Unfortunately, it's really not that simple, and before you let your plans for using 3D digitizing get too ambitious, it's important to understand the technology's limitations.

Of course, there are many different kinds of digitizing technologies, ranging from manual touch-probe devices and coordinate-measuring machines (CMMs) to laser scanning systems and industrial CT scanners, and each technology comes with its own set of strengths and limitations. However, almost all digitizing systems produce x, y, and z coordinate data. The challenge is to convert this data into a format acceptable to your CAD software. The difficulty of this task is often underestimated. Contrary to what you may believe, it's a process that is anything but easy and automatic.

Some digitizing systems come with translators that convert the point data into standard formats such as AutoCAD DXF, Wavefront OBJ, and STL (STL being the file format required by rapid prototyping systems such as stereolithography). These formats consist of a polygonal surface mesh, such as triangular facets, created from the points. These facets approximate the shape of the surface. The smaller the facets, the better the approximation. However, small facets come at the expense of file size and processing speed.

Most popular CAD systems can import DXF triangle data, but not OBJ and STL data. In AutoCAD, the DXF data imports as a polyface mesh. You can zoom, pan, rotate, and even render the data in AutoCAD as well as other CAD systems, but modifying the shape of the model can be difficult. To change the shape or size of the object, you must change the location of one or more vertices that make up the triangles. At best, it's a tedious job. People try, but they usually give up after a few minutes of fiddling. It's almost impossible to make smooth flowing changes in contoured areas and uniform changes at the sharp edges of holes, slots, grooves, ribs, and so on. Changes often produce large, elongated triangles which are not desirable. If possible, you should avoid trying to edit imported polygonal mesh data unless your CAD software is specifically designed to handle it. Most programs, unfortunately, are not.

The Crux of the Problem

One of the big reasons for the difficulty of working with 3D digitized data is that CAD systems and 3D digitizers do not define geometry in the same way. In most CAD systems, for example, a straight edge is defined by start and end points. A center point and radius or diameter define the edge of a cylinder or hole. Digitizers, on the other hand, place many points along these edges. An edge that's 3 inches in length, for example, would contain not two but 150 points if the resolution of the data is 0.020 inch (0.5mm). To make things more difficult, the points rarely run in a straight line, resulting in somewhat of a jagged edge. That's not what you want. The trick, then, is to snap the points into a straight line, but I don't know of any CAD software that enables you to do this. CAD systems simply were not designed to reposition clouds of point data.

Another problem that emerges when trying to use digitized point data stems from the fact that CAD systems treat both 2D and 3D graphics as geometric entities, such as cylinders, spheres, cubes or holes, ribs, fillets, and chamfers. Digitizers, in contrast, define graphics in terms of point data. To a digitizer, a cylinder or hole is represented by a group of points that roughly lie on the surface of the cylinder or hole. Converting this data to entity types that the CAD software expects is a problem.

To work around this problem, some users will use the CAD software's drawing tools to create new geometry over the top of the digitized data, essentially using the digitized data as a template. Another option is to buy one of the touch-probe digitizes that comes with CAD interfaces to specific products, enabling you to create 3D entities by picking points in response to command requests. With such digitizers, you could, for example, enter the Circle command and then pick three points on the object to define the circle. The circle's center point and radius become immediately known to the CAD software. To complete a hole or cylinder, you'd pick three points and then define the length of the hole or cylinder by picking start and end points.

It's also possible to fit mathematical surfaces, such as NURBS, through flowing shapes represented by digitized point data. This can produce smooth surfaces that are easier to control. The problem with this, however, is that most CAD systems don't have the features needed to produce the mathematical surfaces from the point data. This means you must do the conversion before you import the data into your CAD software, a process that requires special and sometimes expensive software designed for this task. One of the few products available for doing this is Surfacer from Imageware (Ann Arbor, MI), which sells for $15,000 for the NT version and $17,500 for the Unix version. Large companies have purchased this software because they can justify its cost, but many small companies cannot afford it.

Home-Grown Solutions

It's also possible to develop your own software. General Motors, for example, uses a program called SurfSeg, which contains features similar to Surfacer. The software was developed by GM for in-house use. Paul Deyo, a supervisor at GM's Design, Development and Verification Group located at GM's North American Operation (Flint, MI), claims they spend most of the reverse engineering time using SurfSeg to fit curves and surfaces through the point data. The digitizing process itself accounts for only about 10% of the total project time. All of his work focuses on styling design, so the surface data ends up in Alias for visual-appearance studies.

Finally, one person who's taken an interesting approach to working with 3D digitized data is David Alciatore of Colorado State University (Fort Collins, CO). He's developed computer-aided sculpting software that runs on workstations from Silicon Graphics. The software enables you to import and export DXF, OBJ, and STL files of digitized shapes, and then make smooth-flowing changes to them in a manner similar to the way you might push or pull on the surface of a rubber membrane. The software also allows you to produce a wall thickness to give the object volume and then output an STL file that you can use to produce a plastic part using an RP system Although the software is not commercially available at this time, it's a project that proves the viability of such an approach.

The bottom line, though, is that if you're interested in importing 3D digitized data into your CAD system, don't set your expectations too high. If you expect the model to behave like a CAD model should, you may be in for a surprise. View the digitizing system as a tool that will help you reverse-engineer, but don't think for a moment that the process is automatic. CGW

CGW contributing editor Terry Wohlers of Wohlers Associates (Fort Collins, CO) is an independent consultant focusing on CAD/CAM/CAE, rapid prototyping, and 3D digitizing systems and applications.

Copyright 1995 by Terry T. Wohlers