Fitting a 23-foot statue on your computer screen

Lately, it's been increasingly difficult to judge what draws more attention at the famed Galleria della Accademia in Florence, Italy - Michelangelo's illustrious sculptures or the Stanford University scientists busy scanning them.

Stanford, and some of Italy's most eminent museums, have combined forces to create a 3-D computer archive of Michelangelo's most revered cultural treasures. The goal is to make accurate models right down to the chisel marks and to eventually display them on computer screens from local art museums to personal computers.

Kari Pulli and his team of Stanford researchers, headed by Marc Levoy, set up their laser rangefinder and half-ton gantry in Italy last January, hoping to capture three-dimensional images of "David," "The Pieta," and "Moses."

However, the team found that art lovers were often more interested in the Digital Michelangelo Project, with its laser technology, computer graphics, and fancy gadgets than the actual statues.

"The tourists are always excited," says Dr. Pulli, a computer engineer. "Some of them have even sat down at our computers, if no one was there, and started moving the mouse or trying to log into the Internet."

Tourists or no, Stanford's lasers are ever busy. To create a matching overlay, which contains the surface characteristics required to reproduce the statue's 3-D image accurately on a computer screen, the scientists must illumine the statue with a sheet of red laser light, painting a thin stripe onto it. This computes the shape of the object on a grid of points in three-dimensional space called a "range image."

By connecting points, a mesh of tiny triangles is produced. The triangles are about 0.25 mm per side - small enough to capture Michelangelo's individual chisel marks.

After the team has recorded the statue's shape, it is scanned again, this time illuminated with a white light. At the same time, an additional video camera is used to record the statue's color. Put this all together on the computer screen and, presto, visitors can look directly into David's eyes, or manipulate the image to make him sit, or even throw a stone.

While building the equipment to compute the top of David's head, the researchers discovered the statue was taller than expected. Since "David" actually stands 23 feet tall (17 feet of statue and six feet of pedestal), not a mere 20 feet tall as the history books commonly claim, the team lost time rebuilding equipment.

Once behind schedule, round-the-clock scanning had to be done, night guards hired, and new software and hardware learned - all while trying to decide where to scan next in Florence.

The $1.5 million educational project began in January 1997, when Stanford specialists designed, built, and tested a collection of 3-D scanners and a suite of software for archiving Michel-angelo's large statues. The group has also brought along a jointed digitalizing arm enabling them to scan hard-to-reach-places like the top of David's head.

Even though the largest and most difficult part of the project was left behind with "David," digitally storing Michelangelo's reclining sculptures and embellished architecture in the Medici Chapel in San Lorenzo, just five blocks southeast of the Accademia, has not been problem-free.

Because of falling chunks of marble from the dome, scaffolding was set up in the middle of the chapel to keep the ceiling from coming down.

The museums working with the Digital Michelangelo Project were forced to make some alterations as well. Spotlights had to be turned off, curtains drawn, and lighting dimmed. Tourists are also kept farther back from the statues while the work is in progress. Yet there have been advantages to these conditions.

"Amidst the general gloom, the highly polished figure of 'Night' appears supernaturally white, as if bathed in moonlight - another part of Michelangelo's design," says Professor Levoy. "It is a moving experience to stand in the chapel when it is lit this way."