Tech Trends-Molding a Saint31 Jul, 2006 By: Kenneth Wong
3D scanning helps create Robert Graham's Virgin Mary bronze.
Felicia Hernandez once supported herself through school by working as a cake decorator. Years later, when she became an established mechanical engineer, she approached her surfacing tasks in Unigraphics and in other CAD packages with the same aesthetic sensibilities she acquired from her short-lived pastry career. Something else also distinguishes her: Her name is among those permanently inscribed on a plaque outside the Cathedral of Our Lady of the Angels (www.olacathedral.org) in downtown Los Angeles, California. The plaque is a gracious artist's acknowledgment of the special technicians who helped him create one of his masterpieces. Hernandez was partly responsible for shaping the eight-foot-tall Virgin Mary statue that hovers above the 25-ton bronze doors and welcomes visitors with outstretched arms (figure 1).
Figure 1. Robert Graham's Virgin Mary statue as it now stands above the cathedral doors.
The journey began with Robert Graham, a renowned sculptor born in Mexico City, who was commissioned to create the two great bronze doors and the Virgin Mary statue. Early in conception, the Virgin was a 30" model made of plasticene, a type of malleable clay made of oil and wax. The artist also created a mold and polyurethane cast, made of hardened resin materials that are easier for scanning, for the design of the Virgin's clothing. Once approved by the Archdiocese of Los Angeles, the artwork was sent to Scansite (www.scansite.com), a 3D scanning service bureau in Woodacre, California.
David Bassett from Scansite recalled, "On [the cathedral] project, we used a Cyberware laser scanner [www.cyberware.com] and an ATOS scanner [www.capture3d.com]. Certain types of scanners can capture finer, smaller details better than others. For example, the Virgin's fingernails were very tiny, and Robert Graham wanted them to be perfect. Also, her hair was quite complicated. The Virgin's dress, on the other hand, had almost no details on it."
Because more than one session was necessary to capture the details, Scansite had to consolidate multiple chunks of data. "The ATOS uses photogrammetry, which means we put reference points, or targets, on the sculpture," said Bassett. "[The ATOS software] uses these targets to align the scans. That's a superior way to align scanned data. The laser scanner uses best fit, which means it uses captured data from previous scans to compare common topologies and similarities to reconcile the data. It then asks [the user] to make some corrections."
For effective scanning, Bassett advises providing service bureaus with appropriate materials: "People sometimes send us wax, acrylics, rubber—all kinds of things. It has to be rigid—that's the key. It cannot move or change shape while we're scanning. A lot of times, we have to move the object as we're scanning. So if it changes shape, best-fit algorithms won't work well."
Graham's exacting standards required that every detail—even his original tool marks—emerge intact in the final version. The scan for the Virgin statue comprised about 3.4 million polygons, by Bassett's estimate. Using Geomagic Studio (www.geomagic.com), Scansite organized the polygonal mesh into 1,632 patches in preparation for NURBS surfacing (figure 2). It was up to Ctek, another service bureau, to enlarge the digital model to its intended eight-foot height—three times the original size.
Figure 2. After polygonal data was imported, the polygonized mesh was organized into 1,632 patches in preparation for NURBS surfacing. The NURBS surface then was interrogated for fidelity with the polygonized mesh.
Below the Surface
Over the years, Felicia Hernandez discovered a good metaphor to describe surfacing in Geomagic. "Think of a fishnet," she said. "You have some closed volumes and open volumes, and you're throwing a net over the polygonal data to create the surface." But the net is rarely perfect, so it has to be adjusted—the more complicated the model, the more extensive the adjustment. Hernandez was hired by Ctek while the Virgin Mary project was in progress.
"Work for the majority of the tiles for the great bronze doors had been completed. Bassett had created the initial surfaces for the Virgin statue. I just aided in the processes and modifications," said Hernandez. "The enlargement didn't make [the surfacing] more difficult, but the amount of detail did. You want to maintain the integrity of the scan data in the final surface." With attention to details that rivals Graham's own high standards, Hernandez demanded a level of surfacing perfection that exceeded what could be accomplished with Geomagic Studio's automatic surfacing feature.
"I can train you to use the software [Geomagic Studio]," Hernandez observed, "but unless you are me, you won't see [surfacing possibilities] exactly the way I see it, and your surfacing patterns may be similar, but they won't be exactly the same as mine. That is what makes Geomagic surfacing unique. The more you use it, the more ways you discover to manipulate patch layout for the surfaces."
The educational background of Ping Fu, Geomagic's cofounder responsible for the earliest incarnation of the software, explains why the technology fosters such artistic tendencies. "My major in China was Chinese literature," she said. "I think being trained in both liberal arts and science gives us an edge in the brand essence of Geomagic—the magic of making it simple. One can say that the Geomagic brand carries that literary influence of elegance, reflection and poetic expressions."
Her pride in her technical proficiency notwithstanding, Hernandez believes the impressive end result is a reflection of the source material: Graham's original model and the thorough scan data from Scansite's Bassett: "My surfaces can only be as good as the scan," she said. "If you start out with a bad scan, there's only so much you can do."
The refined NURBS surfaces were sent to milling machines at Ctek's headquarters, where the full-sized figure was milled from clay in four separate pieces. Milled parts were later delivered back to Graham's studio, where he resurfaced them by hand. According to Geomagic, the studio and several foundries completed the process as follows: "After the final clay pieces were completed, silicone rubber molds were made. Graham used the silicone molds to cast the bronze statue using the lost-wax method . . . The bronze pieces were assembled and welded, and the finished figure was sandblasted and patinated in silver nitrate."
Hernandez recalled that, while the statue was still at the foundry, those who'd worked on the project were invited to a special event hosted by Graham, complete with authentic barbecued cuisine and live music. Hernandez was in attendance, along with her parents and her grandmother. They took family photos, standing in front to the Virgin statue that was still on the ground. Later, after the public unveiling of the cathedral, looking at the statue hoisted 20' above the doors, Hernandez pondered: "Nobody will ever have the opportunity to take those photos like ours from that special day."
For more information about this project, see Robert Graham: The Great Bronze Doors for the Cathedral of Our Lady of the Angels by Robert Graham, Peggy Fogelman, Noriko Fujinami and Jack Miles (Wave, Venice, California, 2002).
Kenneth Wong is a former editor of Cadence magazine. As a freelance writer, he explores innovative use of technology and its implications. E-mail him at firstname.lastname@example.org.