Introduction

The growing interest among the graphics, vision, and manufacturing communities in transmitting 3D computer-aided design (CAD) image files directly to inexpensive fabrication machines promises on-demand physical prototype consumer products, industrial components, mechanical parts, biomedical parts, scientific visualizations, heritage reproductions, and even abstract artwork.

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The most direct, natural way we have for understanding virtual reality is to first transform it into a real, finished physical object. You can then run your hands over it, pick it up, turn it over, maybe toss it in the air or bounce it off a wall. You can look it over from all angles in all kinds of light. Someone completely lacking in technical expertise or sophisticated graphics software can still explore and appreciate its intricacies—and its possibilities. And it’s portable.

Scientific visualizations and many other virtual 3D shapes are so complex they defy complete understanding, even by trained scientists interacting with them through 3D stereographics displays. Images alone must often be viewed many times before their geometries and the relationships among their components are fully understood, if they ever can be. Physical models can deliver the necessary insight much quicker.

Three increasingly affordable rapid prototyping technologies, discussed throughout this special section, are: stereolithography, which uses an ultraviolet laser to harden a light-sensitive liquid resin photopolymer; powder-based 3D printing, which produces parts directly from CAD files without tooling or molding; and thermoplastic fused deposition modeling, which builds part up in layers formed by extruding a melted plastic. Emerging industrial applications include rapid prototyping, or layered manufacturing or freeform fabrication, of 3D CAD data files in consumer product design and development. The prototypes help guide the design process in, say, consumer electronics, toys, medical devices, and car parts. Ideation and focus groups get a feel for the final retail version long before a company must choose whether to take on the huge and high-risk cost of manufacturing.

In many consumer-oriented industries, creating physical prototypes and conceptual models is critical to survival of the fittest, especially as each annual holiday season rolls around. Paul K. Wright has found that rapid prototyping helps a design team generate perhaps dozens of progressively refined prototypes, each a step closer to products that “surprise and delight” the first wave of consumers to use them, in the words of Noriaki Kano of the University of Tokyo. For Wright, rapid prototyping has become a psychological, as well as an engineering, tool, in product development and later in supply chain management.

Focusing on layered manufacturing for scientific visualization, Mike Bailey writes that 3D physical models are helping geologists, astronomers, oceanographers, cartographers, biomedical researchers, chemists, and other scientists touch, hold, rub, poke, pinch, rotate, and zoom the shape to ultimately better understand their own data. Reviewing the 10-year history of the Center for Visualization Prototypes at Oregon State University (originally at the San Diego Supercomputer Center), he explores how 3D visualization hard copies have helped scientists in, for example, a drug-search project at Oxford University recognize how the nooks and crevices of a physical model of an anthrax molecule suggest what other structures might bind with it. And in a blood-flow experiment at the University of California, San Diego, they produced an aortic aneurysm model out of latex to help surgeons appreciate how the vessel’s changing shape influences a patient’s prognosis.

Compared to traditional prototyping, one key advantage of rapid prototyping technologies, writes Sara McMains, is the relative ease of making complicated shapes with relatively inaccessible interior chambers, including what she calls ship-in-a-bottle-type geometries. By incorporating easy-to-remove supports, even fully assembled mechanisms (such as interlocking gears) can be fabricated. Someday, instead of waiting to get a replacement for a broken part to be shipped to a remote location, technicians might download its CAD model and 3D print it, just like we download and print an instruction manual today.

Given that graphics hardware is getting faster and 3D scanning hardware cheaper, demand for and the supply of 3D models will likely continue to increase. The result, write Thomas Funkhouser et al., will be an online environment in which 3D models are as plentiful as images, videos, and audio files today. Their Princeton 3D Model Search Engine hunts for and analyzes 3D models online, retrieving images and using data mining algorithms to discover their geometric relationships. For an interactive demonstration, go to shape.cs.princeton.edu/search.html and sketch an image on the screen in 2D or 3D, along with some descriptive text, then click on search. The system picks and chooses from a database of images crawled from the Web to find the ones that most closely resemble the sketch. This and other such tools are proving especially useful in computer graphics, mechanical CAD, and molecular biology.

All this industrial and scientific interest has led Carlo Séquin to explore computer-generated sculpture, using rapid prototyping to transform even highly abstract designs and mathematical forms into aesthetically pleasing physical objects. Offering his own experience as a kind of self-case study, he writes that despite the virtual environment being so conducive to experimentation and to the exploration of many fleeting ideas, he’s found that in almost every case the first physical models he fabricates reveal aesthetic flaws he didn’t notice on the screen. Rapid prototyping, he writes, provides a powerful visualization tool one should not do without, if one is concerned with finding the optimal design.

To prevent the theft of 3D digital models, David Koller and Marc Levoy are developing methods that defend the high-resolution geometric detail of their physical shape and geometric representation, while still allowing for their interactive display and manipulation. For example, Italian archeological officials and other curators of heritage artifact collections worldwide are turning to 3D digitization as a way to preserve and widen the scholarly use of their holdings. One such collection is the Stanford Digital Forma Urbis Project (formaurbis.stanford.edu), which includes more than 1,000 marble fragments of an ancient Roman map researchers want to make publicly available through a Web-based database—provided the 3D models have adequate protection. Given that the coarse shape of visible objects is so easily reproduced regardless of protection efforts, they are concentrating on the high-resolution geometric detail of 3D models usually used for exhibiting fidelity in the original object.

As convincing as 3D images can be representing the appearance and behavior of physical reality, they remain (useful, beautiful, accurate) virtual illusions, especially when viewed on a 2D screen or printed on a piece of paper. 3D layered manufacturing systems make it possible to convert virtual models into their physical counterparts, transforming 3D CAD images into some of the most complex (and familiar) geometrical objects one can imagine.

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