Reverse Engineering an Antique Italian Treasure (Case Study)1 May, 2008 By: Gloria Goskie
An architecture student enlists reverse engineering to survey an historic synagogue and produce an accurate architectural model.
For the Jews of Modena, Italy, the Modena Synagogue stands not only as a place of worship, but also as an elegant mix of classical and Venetian architecture and — most significant — as a monument to freedom and equality. In 1859, Modena's Jewish citizens regained the rights that had been taken away from them and their ancestors over the previous several centuries. Once free from the restrictions of the ghetto to which they had been confined, they decided to build a synagogue where they could meet freely without harassment. Designed by Ludovico Maglietta, the new temple was officially opened in 1873 (figure 1).
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Figure 1. The Modena Synagogue displays examples of classical architecture as well as Hebraic symbols. (All images courtesy of CRP Technology.)
Owned by the Israeli Community of Modena, the building was named an historical, artistic, and cultural site in 1975 by the Emilia Romagna Superintendency for Environmental and Architectural Heritage (figure 2).
In fall 2007, under the guidance of the architecture faculty at the University of Parma, student Chiara Lodi undertook the challenge of surveying the synagogue and some of its architectural details. Her goal was to make accurate models of the structure for demonstration and educational purposes.
Figure 2. An interior view of the synagogue.
Lodi used reverse-engineering (RE) and rapid-prototyping technologies because they enable the creation of a virtual and also a real gallery where sculptures and architectural details can be viewed in 3D (allowing rotations, zooming, and panoramic views). This was an important consideration because the synagogue is not open to the public. Visitors can enter only with permission. Having accurate models is the only way some elements can be studied, as in the case of the column capitals, which are 2 meters high.
A virtual analysis of a scanned object showing its volume and dimensions allows multiple views and cross-sections to be created. Comparing different files of the same object obtained by measurements taken at different moments can help to verify the state of preservation of an asset and to monitor damage caused by external agents.
Documenting the architectural detail required a range of the most basic to the most advanced equipment available. First, the team measured each physically accessible part of the monument with a simple measuring tape. Next, they photographed friezes, capitals, and other architectural elements and used Adobe Photoshop to clean up the photos before saving them as JPEG images to be traced later with AutoCAD 2007.
Photographing the building presented challenges because of its location in the city's historical center. Wedged between other buildings, the synagogue was difficult to photograph, and the team was able to thoroughly document only one of its exterior facades.
After the large parts of the temple had been assessed with basic measuring tools, the columns and their very ornate capitals were surveyed by use of RE. CRP Technology, an established company based in Modena that offers machining, rapid prototyping, and RE services, stepped in to assist with that portion of the project.
To obtain a mathematical description of the capitals, CRP used a portable, seven-axis Faro CMM Platinum Arm equipped with a Faro Laser Line Probe optical system (figure 3). The system, which was attached to a scaffolding structure placed at the side of the capital, had a spherical measuring range of 3 meters and a scanning speed of 19,200 points per second. This system was chosen because it allows contact measurements and optical scanning to be performed simultaneously inside the same reference system while avoiding damage to the object being measured.
Figure 3. The light weight of the measuring arm plus its extensive degrees of freedom enabled measurements of complex, hard-to-reach parts.
Measuring the surfaces of the capital, which took approximately 1 hour, generated a 3D point cloud that reproduced the shape of the capital in a CAD environment with a measuring error of roughly .1 mm (figure 4). To avoid having a file that was too dense, the resolution (the distance between the points captured by the scanner) chosen for this measurement was 1 mm, although a resolution as low as .1 mm is possible with this equipment.
Figure 5. 3D image of the capital drawing, generated from the point cloud.
Figure 4. The point cloud produced by the 3D scanning device.
The file generated by the scanning system must be optimized using filters and tools inside the RE software. Different point clouds, which come
from different measuring sessions, are aligned
in the same reference system, and the duplicate
points are deleted.
Figure 6. Scaled model of the column capital made in CRP s Windform GF composite material.
The ultimate goal of this project was to measure an architectural detail of interest and to create an accurate 3D model of the original object. The system, which works by the optical triangulation principle, is able to measure only the visible areas of the object, which means that the undercuts and other small cavities are not measured. A high-resolution scanner is able to scan the surfaces by staying very close — about 100 mm — to the object it is measuring, adding to logistic problems when parts of the component being measured are difficult to access.
In the case of the capital, the measuring device had to be fixed to some movable scaffolding close to the object, creating a multitude of problems because of the instability of the supporting structure.
The other main disadvantage of using this technique is the high cost of the equipment, which is expected to decrease over time.