We finish scanning the David

Text by Marc Levoy
Photographs by Marc Levoy and Paul Debevec
March 28, 1999

Our scan of Michelangelo's David, begun on February 15, took 4 weeks. Here is an overhead shot of our work site. At extreme left is our Cyberware gantry, sitting on its curb-hopping platform. At lower-right is our Faro arm / 3D Scanners scanner, mounted on its tripod. At top center is our Cyra time-of-flight scanner, sitting on a black cart. The workstations are a mixture of SGIs and PCs. The people are, from right to left, Kari Pulli (running the scanner), Marc Levoy (with his hand ready on the emergency stop button), Lucas Pereira, and Domi Pitturo.
Our Cyberware gantry can be reconfigured to scan objects of any height from 2 feet to 24 feet. These reconfigurations, which involve removing one or more pieces of the tower, take several hours. In this photograph the gantry is at maximum height. The 2' truss section roughly level with David's foot was added at the last minute after we discovered, much to our horror, that the statue was taller than we thought. (We designed our gantry according the height given in Charles De Tolnay's 5-volume study of Michelangelo. This height was echoed in every other book we checked, so we assumed it was correct. It was wrong. The David is not 434cm without his pedestal; he is 517cm, an error of nearly 3 feet!)
The scanner head is also reconfigurable. It can be mount atop or below the horizontal arm, and it can be turned to face in any direction. To facilitate scanning horizontal crevices like David's lips, the scanner head can also be rolled 90 degrees, changing the laser line from horizontal (shown here) to vertical. These reconfigurations are performed while standing on a scaffolding.
Our hours on the scaffolding give us plenty of time to contemplate Michelangelo's masterpiece. Although most of the statue is smooth, chisel marks can be seen in several places, such as here to the right of the sideburns. Also visible is a complex brew of marble veining, dirt, patina, and the effects of weathering and corrosion. These tell a story about the history of the statue, one we hope to capture with our color camera.
All good things must come to an end, and on March 28, we disassembled our gantry, packed up our computers, and prepared to move out of the Galleria dell'Accademia. During our stay here, we scanned 6 statues, including the David, and one violin from the Medici family collection. All our datasets together occupy about 150 gigabytes. Our next stop is the Medici Chapel.

Some statistics

Although we had run many back-of-the-envelope calculations in preparation for our scan of the David, the actual difficulty of the task surprised us. In particular, the statue contained more recesses and partially occluded surfaces than we anticipated, and positioning the gantry to reach them required more time and effort than we imagined. Here are some statistics from our 30-day marathon scan of this statue:

480 individually aimed scans For each scan, the gantry was moved to a new position. In its maximum configuration, our gantry weighs 1800 pounds. It can be rolled, with difficulty, by two strong people.
2 billion polygons Each part of the statue was covered by at least two scans. Difficult sections were covered 5 or more times, for example when trying to reach the bottoms of cracks.
7,000 color images Each image is 1520 x 1144 pixels. For each position, one image was taken with our calibrated spotlight and one without. Subtracting the two eliminates ambient lighting.
32 gigabytes Data was stored on hard drives. Each morning, the night's scans were copied to a second hard drive, which was walked back to our laboratory and backed up on tape.
1,080 man-hours of scanning This doesn't include 1,000 hours of editing, aligning, and combining the scans, and 1,500 or so hours pumping up our software to handle the increasingly large datasets.
22 people 2 professors, 1 research associate, 1 technical staff, 6 PhD students, and 12 undergraduates. Of these 22, 15 were full-time and 7 were part-time.

As many hours as we have spent on this project, we have not yet assembled a high-resolution computer model of the David. To see our first steps in this direction, click here.

© 1999 Stanford Computer Graphics Laboratory