3D scanner allowing replication of Michelangelo's works

17 Jun 2002

In the Galleria dell'Accademia in Florence, Italy, a group of researchers from the Stanford Computer Graphics Laboratory are digitizing the sculptures of Michelangelo down to the submillimeter.

The group, led by professor Marc Levoy, wanted to capture the chisel marks of the statues as well as the exact configuration of the measured works.

The Stanford group not only has the cooperation of the museum's administration, but they also have a 3D scanner designed and manufactured by Cyberware (http://www.cyberware.com).

Stanford's researchers had specifications for the need to scan the sculptures at high resolution without touching the sculpture.

Cyberware built a prototype which tested the tilt and pan concepts as well as proved that a scanner could be designed which could build the optics at the necessary resolution.

Then Cyberware worked on a final delivery model to cover specific needs such as the capability to stand back farther from the target and a larger field of use. "The prototype needed refinement so pretty much everything changed on it to the final one," said David Addleman, the president of Cyberware.

Including the design process, Cyberware spent about a year working on the scanner. "For us the biggest challenge was building this anamorphic lens to image the region that's being scanned," said Addleman. "They're difficult to design properly to get a lot of resolution."

Since the scanner must remain stationary for the entire time of the scan for the points to be matched to each other properly, which averages 20 to 30 minutes, the tower on which the scanner is placed cannot move and thus the weight of the scanner must not shift during scanning.

The scanner itself which Cyberware provided consists of a laser, a range camera, a fiberoptic white light source, and a high-resolution color camera. The laser and range camera allow digitization of 3D points with a depth resolution of 100 microns, a typical sampling pitch of 250 microns, and a standoff of 112 cm. The calibration working volume of the scan head is 2 meters wide by 1.75 meters high by .8 meters deep, and the light source and color camera have the ability to measure surface color with a pixel size of 125 microns over the same working volume.

The entire system also includes a 2.44 meter (8 feet) motorized vertical truss, a .91 meter (3 feet) horizontal arm which translates vertically on the truss, two non-motorized truss extensions, and a pan-tilt head which translates horizontally on the arm. The assembly rests on a rolling base which is fitted with weights to provide stability. The entire scanner head and pan-tilt assembly are encased in foam rubber and an elaborate system of automatic motions shutoffs and interlocks protects statues from contact with the gantry. Any translation of the pan-tilt assembly in one direction is counterbalanced by translation in the opposite arm of a lead counterweight sliding inside the horizontal arm, reducing deflection of the gantry during horizontal motion. The system also has a steady calibration, since a tower which must be portable to be brought in and out of a museum cannot be stiff enough to keep the scanner steady during transport.

Cyberware's scanner sweeps a fine line of laser light across the marble surface of the sculpture. The shape of the line as it curves across the features and chisel marks of the sculpture is captured by the scanner, which has a SCSI interface to store the data in computers as 3D coordinates for later processing into a surface model. The laser line of the scanner is 150 mm long, and operators program the scanner to sweep it across the sculpture hundreds of times, allowing each part of the sculpture to be captured from many angles to assure a complete and accurate model. The light source and color camera permit measurement of surface color with a pixel size of 0.125 mm.

"When you spread that resolution over the surface resolution of the David, it's hundreds of millions of points," noted Addleman.

The scanner's motion platform allows the viewing of objects from a vantage point over 7 meters high, and the pan, tilt, and translate motions for the scan head allow operators to scan in areas which are difficult to access.

For portions of the statue which can not feasibly be measured by the Cyberware scanner, Stanford is also using a jointed digitizing arm and small triangulation laser scanner made by Faro Technologies and 3D Scanners Ltd. and a time-of-flight laser scanner made by Cyra Technologies.

In November the Cyberware scanner arrived at the Accademia, where it was tested upon a resin copy of an statue called the Angel of Light.

During the actual scanning, high-tech challenges were augmented by logistics issues such as removing glass barriers, getting behind the sculpture, and coordinating the research with the desires of museum patrons to see prominent works.

On January 26 the researchers started the scanning process of Michelangelo's St. Matthew. With a depth resolution of 0.1 mm, spacing between sample points of 0.29 mm, and a scan with of about 30 cm, the process took about six days. The researchers obtained 104 laser scans from different viewpoints, 400,000,000 three-dimensional points, 800,000,000 triangles, and 4,000 color pictures. The storage of the date required 15 gigabytes.

Stanford's method to build complex models from range images converts the scan to a distance function, which is combined with data already acquired using a simple additive scheme. A run-length encoding of the volume achieves space efficiency, while time efficiency is achieved by resampling the range image to align with the voxel grid and traverse the range and voxel scanlines synchronously. The final manifold is generated by extracting an isosurface which is optimal in the least squares sense, and any gaps are filled in the model by tessellating over the boundaries between regions seen to be empty and regions never observed.

The measurements of the David are expected to last until Spring. As one of the most popular pieces of art in the Accademia, measurements can only be taken when the museum is closed. The David is 23 feet tall on its pedestal, and since the scanner will also be looking at David's head, the system must operate reliably at heights up to 25 feet (7.6 meters).

In addition to recovering chisel marks and other artifacts to study chisel manipulation and other sculpting techniques, the researchers are also obtaining accurate calibrated color measurements.

The Michelangelo project, which includes some works of art not produced by Michelangelo, will cover measurements of six sculptures and a Stradivarius violin in the Accademia, two tombs and the Madonna in the Medici Chapel in Florence, the Moses in St. Peter's in Chains in Rome, the Pieta in St. Peter's Basilica in Vatican City, the Laocoon in the Vatican Museum, and Forma Urbis Romae in Rome.