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A virtual reassembling project for the Assisi frescoes


An ambitious project of restoration of the destroyed frescoes of Giotto and Cimabue was undertaken by the ICR (Istituto Centrale del Restauro, Ministero per i Beni Culturali e Ambientali) after the earthquake of Sept. 97.

The problem was to place back in their original position tens of thousands of minute fragments produced by a tremendous impact caused by a 22 meter fall. It was immediately realized that an attempt to solve such an "impossible puzzle" should be faced resorting to adequate scientific and technological supports. The key idea behind the project was to perform a virtual assembly before physical restoration, i.e. to handle images of the fragments instead of the fragments itself, and place them in their proper position onto a "virtual ceiling" with the aid of computer. This idea was encouraged by the avalaibility of some pictures of the frescoes and, on the other hand, by some recent advancements in the field of computer vision, mainly developed for military and industrial applications, and considered today also for assisted search of wanted objects in image databases in multimedia applications.

Partially supported by the E.U., a collaboration of the ICR with the University of Rome "La Sapienza" has been established with the purpose of assessing the feasibility of the restoration project. In particular, the group of Signal and Image Processing of the Faculty of Engineering has been involved in a preliminary program aimed to perform the fragment image acquisition and to define techniques for computer assisted virtual restoration. This program is now in progress. Today, the acquisition of the whole set of available fragments of the "vela di S.Matteo" (Cimabue) has been carried out, and, on the other hand, experiments of computer assisted location techniques are being made.

The acquisition has produced a large high resolution image database of about 150 Gigabytes, containing more than 100.000 fragments (stored in 130 CDs). The acquisition was performed in a controlled environment, with systematic colorimetric and geometrical calibration measures necessary for assuring homogeneity of results.

The virtual restoration process includes algorithms aimed to perform automatic comparation of the fragment images with the available images of the unfragmented fresco, in order to individuate their position. The main difficulties of this operation are related to the details actually visible in the available pictures, and to their actual state of deterioration. Very often, their surface is severely damaged by the impact and by reciprocal abrasion.

Specific image processing techniques are under test. They are based on advanced mathematical representation methods which allow to obtain "likelihood maps" of the fragment position with reasonable computation times. Algorithms are implemented on PC workstations and their performance is tested using an experimental fresco prepared by ICR and subject to a process of segmentation and acquisition similar to the one undergone by original fragments. It is expected that such algorithms will provide substantial acceleration of the restoration process.



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