Three-dimensional acquisition of large and detailed

Machine Vision and Applications (2006) 17:349–360 DOI 10.1007/s00138-006-0029-z

SPECIAL ISSUE

Three-dimensional acquisition of large and detailed cultural heritage objects Gabriele Guidi · Bernard Frischer · Michele Russo · Alessandro Spinetti · Luca Carosso · Laura Loredana Micoli

Received: 10 October 2005 / Accepted: 15 March 2006 / Published online: 23 August 2006 © Springer-Verlag 2006

Abstract Cultural heritage digitization is becoming more common every day, but the applications discussed in the literature address mainly the digitization of objects at a resolution proportional to the object size, using low resolution for large artifacts such as buildings or large statues, and high resolution for small detailed objects. The case studied in this paper concerns a huge physical model of imperial Rome (16 × 17.5 m) whose extremely small details forced the use of high resolution and low noise scanning, in contrast with the long range needed. This paper gives an account of the procedures and the technologies used for solving this “contradiction”. Keywords 3D Acquisition · Resolution · Uncertainty · Laser radar · Virtual archaeology 1 Introduction Mapping an existing city is an activity nowadays moving from the realm of research to well-established G. Guidi (B) · M. Russo · L. L. Micoli Reverse Modeling and Virtual Prototyping Labs, INDACO Department, Politecnico di Milano, Milano, Italy e-mail: g.guidi@ieee.org B. Frischer Institute for Advanced Technology in the Humanities (IATH), University of Virginia, Charlottesville, VA, USA A. Spinetti · L. Carosso Technology for Cultural Heritage Lab, DET Department, University of Florence, Florence, Italy

commercial applications. Through photogrammetry, GPS, satellite survey and even aerial laser scanning, measuring and modeling an urban context is quickly becoming routine. But when the city to be represented is not available anymore because it no longer exists, the work to be done cannot follow the customary pipeline. Modeling of an ancient building may start from the historical documentation, archeological studies undertaken in the past and sometimes from a new survey of the area. These data are then combined in the creation of a digital three-dimensional (3D) synthesis that represents a reasonable hypothesis of how the artifact once appeared. The construction of an entire city can proceed by repeating this method as long as needed, but the process would of course be extremely time-consuming, assuming it would be at all possible since sometimes (as in the case discussed in this paper) all the archaeological data that would be needed are not known. The project discussed in this paper forms an important part of the Rome Reborn Project, an international effort to create a real-time digital model of ancient Rome. The spatial limits of the Rome Reborn model are represented by the area enclosed by the late-antique Aurelian Wall; its temporal limits span from the Iron Age (tenth century B.C.), when the city began to be settled, to the Gothic Wars (sixth century A.D.), when the city suffered severe physical damage and significant depopulation. For a variety of practical reasons, work on the model commenced in 1997 with modeling of the lateantique phase (ca. 400 A.D.), which represents the climax of the development of the ancient city in terms of its urban fabric and population. The approach to modeling has been to work out from the city center in the Roman Forum, a multi-purpose space dedicated to political, economic, religious and entertainment activities. But

350

creating by hand the digital model of any single building has not been considered feasible in the framework of the project owing to the huge number of edifices involved (>50,000) and the fact that we lack archaeological data for the vast majority of the buildings. The innovative idea behind the project was therefore to merge a digitized physical model with born-digital models of some of the main buildings for which we have good archaeological documentation. The first step was therefore to identify a reliable physical model of Rome in the period of interest, to be used for generating a digital mold. The so-called “Plastico di Roma antica”, a physical model housed in the Museum of Roman Civilization (Rome/EUR) and designed by the famous archeologist–architect Italo Gismondi, was identified as the best available resource for implementing this approach. The model was created in a scale of 1:250 starting from 1930. The work was completed in 1973, and it has not been changed since. For the Rome Reborn Project the advantages of using the Plastico are that it could: • Provide an almost instant computer model of the project’s first, late-antique phase. • Repurpose the Plastico and keep it constantly updated and therefore useful to students and scholars in the 21st century. • Offer a total urban context for the new born-digital models of individual sites and monuments created by the Rome Reborn Project. These new born-digital models, such as the Roman Forum, Colosseum, Temple of Venus and Rome and other key public buildings and monuments, were worth creating despite the availability of the digital Plastico because they could be made in a scale of 1:1, could be textured photorealistically, could reflect discoveries made since the 1970s, and could (when archaeological data sufficed) include the interior spaces as well as the exteriors. As a physical model created at a small scale and intended to be viewed from a high balcony, these were features that the Plastico di Roma antica could not offer and, indeed, did not need to offer. The present project thus entailed creating a hybrid model of late-antique Rome that would be based on the digitized Plastico and the new born-digital models of specific sites and monuments in the historic city center. The purpose of this paper is to describe the procedures for acquiring and generating the digital model of such huge maquette that presented several difficulties. In the literature there have been several examples of the digitization of works of art. Starting from the first pioneering attempts proposed by the National Research Council of Canada (NRCC) in the 1980s [2, 8, 30],

G. Guidi et al.

Fig. 1 The “Plastico di Roma antica”, housed in the Museum of Roman Civilization (Rome/EUR) and designed by the archeologist–architect Italo Gismondi. In its current location, the visitors’ line of sight resides at about 3 m from the model level

international researchers have reported several important applications. After the first laboratory-based systems [28], in the mid-1990s on-site scanning with portable range cameras started to appear [3], leading to the first projects involving 3D acquisition and modeling of large statues [6, 22]. These studies demonstrate that in general range cameras are somewhat focused on a particular range of volumes. Most 3D scanners based on the triangulation principle are suitable for small objects and may generally work at distances ranging from one-half meter to few meters [4]. Their measurement accuracy over the whole range image stays below one-tenth of 1 mm, and the uncertainty lies between 50 and 200 μm. On the other hand, laser scanners based on time of flight (TOF), used for architectural elements and large structures (bridges, dams, etc.), allow much larger distances to be covered, up to few kilometers [1, 12]. Although accuracy remains high, the major drawback of TOF scanners is the loss of precision since the measurement uncertainty goes down to several millimeters. This absolute value is not a problem for measurements involving large structures because the relative precision remains high, but if the structure is large, and if small features must be captured, this kind of system is not usable. The “Plastico di Roma antica” lies, unfortunately, in the latter category, being a wide object (16 × 17.4 m) with houses and public buildings only a few centimeters tall. Therefore in this case, the use of conventional techniques was not feasible. The solution was found in a system created for advanced metrology applications. At first glance, the approach taken resembles TOF laser scanning, but its main improvement is in the procedure

Three-dimensional acquisition of large and detailed cultural heritage objects

employed for detecting the laser time-of-flight. Instead of conventional pulsed techniques, the method used for the Plastico uses a principle well known in CW radars, based on transmission and reception of a coherent frequency modulated wave. For this reason the system is indicated as laser radar (LR).

2 Hardware equipment Various experiences of 3D scanning applied to cultural heritage [4, 6, 15, 22] demonstrated that several difficulties occur working in the field, in addition to those usually found in a lab while digitizing specimens of little or no value that can be freely touched and moved. The reasons lie mainly in the safety of the artwork, which must not be exposed to risk of damage, and in logistics, since often the object to be acquired has to be shown to the public while the researchers work on its digitization. Acquiring the plaster-of-Paris model of ancient Rome in a lab would have been a complicated enough job, but the addition of further constraints made it almost impossible. For example, the administration of the museum prohibited the placement of any measurement machine directly over the “Plastico” in order to eliminate the possibility that the machine, or one of its parts or accessories, might accidentally fall onto the monument and damage it. This prohibition prevented using any aerial photogrammetry technique that probably could have been successfully used [29], or using a common laser light stripe scanning device mounted on a rail for covering the whole surface in parts [22]. The sensor was therefore chosen in order to satisfy this primary requirement: avoid any sensor flying over the “Plastico”. The solution was found in a very high-quality (and high-cost) LR. It is capable of giving the same performance as a relatively low-cost and short-range laser light stripe triangulation scanner, with the important additional feature of giving reliable results up to 24 m from the measured surface. Since the only drawback of this extremely powerful system is its slow speed, a simpler triangulation-based laser sensor was also used for capturing the areas close to the external border of the “Plastico”, easily reachable with a conventional range camera. 2.1 Laser Radar The most commonly used systems for creating a digitized 3D image of an object within a limited range (about 1 m) are based on optical triangulation. A laser forms a

351

light stripe scanning the object by means of a rotating mirror or a cylindrical lens, and a CCD camera collects the image of the illuminated area. The range information is retrieved on the basis of the system geometry [25, 26]. An alternative triangulation technique is based on the projection of patterns of structured light, i.e., a light pattern coded as spots or stripes, according to different coding strategies [23, 24, 27]. Both techniques generate a cloud of points that, after suitable processing, allows the creation of a 3D model of the object. The systems based on optical triangulation are extremely precise, allowing measurement uncertainty of