Stanford Digital Forma Urbis Romae Project

SCANNING THE FORMA URBIS ROMAE

By Marc Levoy

  • The Idea
  • Getting Started
  • Scanning the Fragments in Rome
  • 23 New Fragments Travel to Stanford


     


    The Idea
    Recent improvements in laser rangefinder technology, together with algorithms for combining multiple range and color images, allow scientists to digitize reliably and accurately the external shape and surface characteristics of many physical objects. Examples include machine parts, design models, toys, and artistic and cultural artifacts. As an application of this technology, a team of 30 faculty, staff, and students from Stanford University and the University of Washington spent the 1998-99 academic year in Italy scanning the sculptures and architecture of Michelangelo (refer to Levoy 2000 or The Michelangelo Project for more information). During this year abroad, we became involved in several side projects. One of these was the digitization of the Forma Urbis Romae.

    The idea of using laser scanners and computer graphics to visualize the fragments of the Forma Urbis was first suggested to us in January of 1997 by Susanna Le Pera of the Archeological Superintendency of Rome. The idea of using computer algorithms to piece the map together was our own. We initially thought that it would be a simple matter of searching among digital photographs of the fragments for matches between their border shapes or the incised designs on their top surfaces. Unfortunately, these top surfaces are often eroded, reducing the effectiveness of such an approach. Moreover, considering the fact that scholars have spent the last 500 years searching for matches among these incised designs, it seemed unlikely that we would find many more. On the other hand, the fragments are several inches thick, and fragments that do fit together usually mate intimately across at least a portion of the interface surface between them. The idea was to develop compact signatures for these border surfaces and to search among the signatures for matches.

    The proposed approach is a classic N2 search problem. Since features on the scale of 1 mm can make or break a potential match, and since there are over a thousand fragments, the search space is large. However, there are several features of the fragments that can be used to speed it up. For example, the slabs comprising the map vary in thickness over a range of several inches, so they can be sorted by thickness; then matches can be sought by looking at close neighbors in the sorted list. Other useful features include marble veining, which is usually highly directional and therefore constrains the space of possible matches, the presence of straight borders and clamp holes, which denote fragments lying at the edges of slabs, and of course the continuity between fragments of the incisions themselves.

    In order to test these ideas, we needed to scan and build 3D geometric and photographic models of every fragment of the map. This was not an easy task.

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    Getting Started
    In March of 1999, we returned to the Archeological Superintendency with a proposal to digitize the 1,163 fragments of the map (23 more fragments were discovered after the completion of the scanning project in Rome). Although the superintendent, Eugenio La Rocca, was supportive, there was understandable hesitation from the curators - the map is a priceless artifact, and the fragments are heavy, fragile, and numerous. Eventually, however, they acquiesced, and two of the curators, Anna Mura Somella and Laura Ferrea, became enthusiastic and valued collaborators of the Project. In the space of a few weeks in April, 1999, we obtained the necessary permissions, found funding (some of which was provided by the City of Rome), assembled a team (which included Laura Ferrea), rented an apartment in Rome, and moved the group and several tons of computers and scanners from Florence to Rome.

      Until recently, the fragments of the Forma Urbis were housed in the Museum of the City of Rome (Museo di Roma) in Palazzo Braschi, a few steps from Piazza Navona. When that museum was closed for restoration, the fragments were packed into crates and moved to the basement of the Museum of Roman Civilization, (Museo della CiviltÓ Romana), a vast edifice built by Benito Mussolini in the 1930's in the Roman suburb of EUR (Exposizione Universale di Roma).

      When we visited the museum in EUR to plan the scanning project, the museum director, with officials from the archaeological office in attendance, brought us to the basement and directed our attention to a vast mountain of crates. Sensing our confusion and terror, the guide from the archaeological office explained with some amusement that these crates represented the entire contents of the Antiquarium Comunale - the fragments of the Forma Urbis occupied "only" 20 of them.

      This basement gallery provided a spacious room in which to set up a temporary computer graphics laboratory. To scan the fragments we used two Cyberware laser-stripe triangulation scanners. One was a custom design, originally intended for scanning the statues of Michelangelo, which was adapted for use in this project. The other was a Model 15, a relatively inexpensive desktop scanner, which we had shipped from the United States so that we could scan two fragments at the same time, doubling the throughput. Both devices had an X-Y sample spacing of 0.25 mm and a Z-resolution of 50 microns. For acquiring color data we used a Sony DKC-ST5 programmable 3-CCD digital still camera. It had a nominal resolution of 1030 x 1300 pixels, which was set up to provide a resolution of 0.25 mm (100 dpi) on the fragment surfaces.

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    Scanning the Fragments in Rome
    So the scanning project began. Attrition had pared the team from 22 people down to only 6, and the budget (and people's spirits) were nearly exhausted. Moreover, time was short: we had only four weeks left before the customs carnets on the scanners and computers expired, at which time they would have to be shipped back to the United States. To scan the 1,163 fragments of the Forma Urbis in such a short time, we instituted round-the-clock scanning: 24 hours a day, 7 days a week, using the two laser scanners and the photographic camera stand. The team kept up this pace for 25 days, sleeping in shifts.

      Although most fragments of the Forma Urbis are small, some are large and weigh several hundred pounds, so the Rome team included two helpers whose job it was to move the fragments from the crates and lower them gently onto a motorized rotary table for scanning.

      One of the smaller, unidentified, fragments is here in the process of being scanned. A laser-stripe scanner digitizes an object by sweeping it with a plane of laser light, imaging the resulting stripe as it moves across the object surface, and analyzing the shape of the stripe 30 times per second. Regardless of the fragment shape, many scans were required to completely cover its surface. To permit these scans to be aligned, each surface point was scanned several times to create overlaps. This redundancy also allowed us to downweight oblique views, which yield poor data in all laser triangulation systems and particularly poor data when scanning marble, due to subsurface scattering.

      Sitting at the Silicon Graphics Octane, Stanford Ph.D. student David Koller examines the computer model of a newly scanned fragment. Although we attempted to automate the scanning process as much as possible, fragments had to be moved by hand several times during scanning, forcing the team members to perform interactive alignment of the resulting scans. This step was followed by automatic pairwise alignment of scans using a modified iterated-closest-points (ICP) algorithm (Besl and McKay 1992), and finally by a global relaxation procedure designed to minimize alignment errors across the entire fragment (Pulli 1999). Once aligned, the scans were combined using a volumetric algorithm. At some point (small) holes will be filled using space carving (Curless 1996). The final result will be a (nearly) watertight, high-detail (0.29mm), triangle mesh representing each fragment.

      At left is a rendering of the raw 3D data for a fragment. The fragment is 5 inches across, small enough to be scanned using a Cyberware Model 15. In order to see the entire surface, linear scans were taken from 18 different directions. Even so, at least one hole is evident (blue circle). It is less than a millimeter across, so it will be filled later by using an automatic hole-filling algorithm. The 18 scans have been aligned with each other using the ICP algorithm described above. The residual error, about 0.25mm, is visible as interpenetrating surfaces (yellow circle). The volumetric range image merging software (vrip) will replace these interpenetrating surfaces with a single surface at an average location.

      To reduce the time required to scan each fragment, we did not use the color camera built into the Cyberware scanner. Instead, we assembled the separate color digitization station seen here, which operated in parallel with the Cyberware scanner. The spotlights surrounding the table have been arranged to cast a raking light across the fragments, a lighting configuration frequently employed when photographing incised stone artefacts. The camera we used was a Sony DKC-ST5 programmable 3-CCD digital still camera with a nominal resolution of 1030 x 1300 pixels. The camera was set up to provide a resolution of 0.25 mm (100 dpi) on the fragment surface. This resolution is sufficient to create a 1:1 museum-quality, photographic reproduction of the entire map.

    After each photograph was taken, it was dewarped to correct for camera lens distortion. To calibrate a camera, we separately photographed a planar target, located a number of feature points on it, and used these to calculate the camera's intrinsic parameters. The model included two radial and two tangential distortion terms, off-center perspective projection, and a possibly non-uniform (in X and Y) scale (Heikkila 1997). Given the high resolution at which we were photographing, large fragments had to be photographed in sections and mosaiced together. For this purpose, we mostly used Image Assembler, a commercial image mosaicing package from Panavue, Inc.

    23 New Fragments Travel to Stanford
    In 1999, 30 more fragments from the Severan marble plan were discovered in the Italian excavations of the Templum Pacis. Some were joined during conservation, making a total of 23 fragments. In an unprecedented and very generous move, prompted by Laura Ferrea, the Sovraintendenza offered to bring these new fragments to Stanford to be scanned there. In May of 2001, Ferrea personally oversaw the transfer of the valuable marble pieces from Rome to Stanford and the scanning of each fragment by our team in the graphics lab of the Computer Science Department.

    TECHNICAL PAPERS CITED

  • Besl and McKay 1992
    Besl, P., McKay, N., A Method for Registration of 3-D Shapes, IEEE Trans. PAMI, Vol. 14, No. 2, February, 1992, pp. 239-256.
  • Curless 1996
    Curless, B., Levoy, M., A Volumetric Method for Building Complex Models from Range Images, Proc. SIGGRAPH '96, ACM, 1996, pp. 303-312.
  • Heikkila 1997
    Heikkila, J., Silven, O. A four-step camera calibration procedure with implicit image correction, Proc. CVPR '97, IEEE, 1997, pp. 1106-1112.
  • Levoy et al. 2000
    Levoy, M., et. al., The Digital Michelangelo Project: 3D Scanning of Large Statues, Proc. SIGGRAPH 2000, ACM, 2000, pp. 131-144.
  • Pulli 1996
    Pulli, K., Multiview registration for large data sets, Proc. 2nd Int'l Conf. on 3-D Digital Imaging and Modeling, IEEE, 1999, pp. 160-168.
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