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DIGITAL PROCESSING OF X-RAYS OF : A CASE STUDY OF ALEIJADINHOS WORK

A de A Aralijo, C A Davis Jr , A L V Daker, L A C Souza, and A S Leal

Universidade Federal de ,

INTRODUCTION considered part of the World Property by UNESCO.

The contribution of science to the process of The use of X-rays for analyzing his technology of conservation/restoration of art works is highly related construction of sculptures is very important for its to the scientific examination of the objects. The contribution to the history of technology and also to physico-chemical analysis is very important for the the history of art. characterization of the author's technique and also for the history of technology. The application of The main objective of thc work is to build up a photographic and radiographic techniques to the study database which comprises two parts. One stores image of works of art is also very useful to bring information data from the digitized X-rays. The other one stores on the "state of conservation" and the technology of conventional data related to the internal structural construction of the object (1.2). aspects of the technology of Aleijadinho's polychromatic sculptures, such as: Through the use of infrared photography of paintings it is possible to obtain information on the drawing done Cl number of wooden blocks and typology of the by the artist over the ground of the painting, before the joints; application of the paint layers. It is a kind of retrieval of information which is generally not perceived by U presence or absence of cavities and also their naked eye. The use of radiographic techniques on characteristics; paintings can be. very helpful to detect the presence of paintings under the painting which is actually 0 position of the eye-glasses and characteristics of perceived by the eye. The presence and distribution of the nails; heavy chemical elements such as lead can also be put in evidence through radiographies (3.4, 5). U presence or absence of fissures and/or insects like termites and wood borers. The radiographic examination of wooden polychromatic sculptures is also important in order to These different aspects to be observed will also be know the technology of construction of these objects. explored through the use of Digital Image Processing. Information on the internal structure, localization of The results will be very important for characterizing joints and the number of blocks used to build the the author's style and technique, being useful, and can be obtained. probably fundamental, for the authentication of the object. This contribution is part of a long term work in the area of conservation/restoration of art works of the OBTAINMENT OF RADIOGRAPHIES colonial period in Minas Gerais, Brazil. This is a joint project carried out by the Computer Science For the execution of the radiographies of sculptures Department (DCC) and the Center of Conservation and with bigger dimensions than a normal film (35x43 Restoration of Movable Cultural Properties (CECOR) mm), we have used two methods: of the Federal University of Minas Gerais. U making radiographies of different parts of the This work reports and illustrates the use of a digital sculpture, in order to have several radiographies, image processing system to study the work of Ant6nio which could then be summed into one image of the Francisco Lisboa, well known as "Alejadinho", the whole structure. most important sculptor from the colonial period (XVII Century) in Minas Gerais. Aleijadinhol. was a U modifying the distance of the source to the films in 'mulato', a natural son of a Portuguese architect and an order to perform just one radiography, which is African slave. He was born in , a town then composed of various films, packed in photographic black paper, attached one to another by the borders. Aleijadinho - from the Portuguese "handicapped". The sculptor suffered from leprosy, a disease that The second technique has showed to be more practical mutilated him. than the first one due to the number of steps. The only 618 problem we had was the positioning of the packed of a sculpture. Therefore, it is of interest to digitally films, which did not allow us to be sure about the real process the x-radiographies to compensate for lack of position of the film. This problem originated the contrast, geometric distortions, and to enhance features failure to register some parts of the sculptures in the of interest. regions which correspond to the junctions of the films. Figures 3 to 6 show the head of the statue of St. Simon Figures 1 and 2 show the composition of several X- Stock, located at the Carmo Church, in Sabari, Minas rays taken from the statue of St. Peter Martyr. These Gerais. The plates were taken with 70 kV, 5mA, 2.4 plates were taken with 50 kV, 5mA, 3.5 min., 1.6 m. min and 1 m. Figure 3 is the original digitized Notice the faulty division between the plates. radiography. Notice the presence of a half scissors inside the statue. Figure 4 shows the effect of the PROCESSING OF X-RAY IMAGES application of a compass-gradient operator, enhancing the edges. Figure 5 illustrates the combination of the Digital image processing may be classified into six application of the extremum sharpening transform with major areas: digitization, coding, enhancement, and edge-enhancing filter. Figure 6 presents the use of restoration, scgmcntation, and description. Image a relief operator, after the application of contrast enhancement is usually concerned with contrast stretching techniques. In figure 7, a composition of stretching, smoothing, sharpening and highlighting of two original pictures was made, to show the position specific features. The application of image- of the scissors. enhancement techniques, in general, improves human viewing ability and may increase the chance of success The digital image processing system used in the work, in image analysis (6). Pixelware (7), is built upon a graphical user interface, developed to be a framework for development of Radiographing sacred wood-sculptures is noL a trivial application-oriented image processing solutions. The task. Normally, it must be done in situ, in churchcs or baseline of the system includes a suite of fundamental in museums. Problems such as illumination, film digital image processing routines. useful as packing and transport of the subjects, subjects height background for any application module. These and weight, arc quite common. Because of the height routines implement algorithms in image statistics, of the sculptures (1.60 m) it was necessary to build up pseudocoloring, contrast manipulation, smoothing, a mosaic of the X-radiographies of the different parts edge sharpening, segmentation, arithmetic operations,

fig. 1 - Composition of X-rays fig. 2 - Enlarged composition of X-rays 61 9

fig. 3 - Original digitized side view of St. Simon Stock fig. 4 ~ Edge enhanccmcnL alter contrast stretching

Cig. 5 - Extremum sharpcriing plus gradient opcrator tig. h - Kcliel operator 620 and geometric transformations. Four application modules covering subjects in remote sensing, microscopical imagery, archiving of historical documents. and restoration of works of art are under development. The system is being implemented to work on 286/386/486 microcomputers, based on the MS-DOS operating system, with SuperVGA video cards. With this kind of hardware, plus a video capture board or scanner, the system turns out to be a very low cost and effective image processing solution.

The enhancement of X-ray images does not bring spectacular results mainly because of the inherent problems of the images. The images have a wide dynamic range and detail both in the light and dark areas. The biggest single problem in the attempt to apply image rcstoration techniques is the lack of an adequate and mathematically tractable image formation model. The penetrating nature of X-rays is such that there are no opaque edges. Lastly, the absorption and scatter phenomena associated with X- rays are energy dependent. This means that the effects of the objects are extremely difficult to predict or even measure experimentally. Because of the problem of finding an adequate model, the state of the art in radiographic imagery is limited to enhancement with few attempts at restoration. Enhancement is aimed at fig. 7 - Front and side views of St. Simon Stock statue producing a subjective improvement (8.9).

Preliminary results of the application of classical 3. Van Asperen de, B.A.J., 1986, "Examination by methods of digital image processing to non-destructive Infrared Radiation", PACT - Journal of analysis of the interior of Aleijadinho's polychromatic European Physical. Chemical. sculptures are encouraging. The main objective is the Biological & Mathematical Techniaues Aylied construction of a database to characterize Aleijadinho's -to Archeoloay, 13,109-130. style and technique. A prototype version is being implemented on a 386 microcomputer, and a final 4. Van Schoute, R., and Verougstraete-Marcq, 1986, version is to be implemented on a SUN workstation. "Radiography", ibid., 13, 131-154.

ACKNOWLEDGMENTS 5. Bridgman, C.F., 1966, "Radiography in Art and Science", Eastman Kodak Co. Chemistrv, 19-37. The first author would like to thank the Brazilian Conselho Nacional de Desenvolvimento Cientifico e 6. Rosenfeld, A. and Kak, A.C., 1982, "Digital Tecnoldgico - ChTq, for the financial support of Picture Processing", 2nd. Edition, Academic Press. Pixelware, grant 400190/90-7/CC/PQ. 7. Davis Jr., C.A., 1991, "Pixelware: Um Sistema de Part of the X-ray equipment and its films are Processamento Digital de Imagens", Master of sponsored by the PRPQ/UFMG (proc. Science Dissertation , UFMG, Brazil. 23072022248.91-01). We would also like to thank CNPq for the grant to A.S. Leal. 8. Trussell, H. J., 1981, "Processing of X- ray Images", Proceedines of the IEEE, 69,615-627. -.REFERENCES 9. Hall, E. L. et al., 1971, "A Survey of Preprocessing 1. Asmus, J.F., 1987, "Digital Image Processing in and Feature Extraction Techniques for Radiograph Art Conservation",EJy& 151-165. Images", IEEE Transactions E CornouterS. a, 1032-1044. 2. Art, A., 1987, "Investigation Scientifique des Oeuvrcs d'Art", Cahier d'Erude. 11.5-41.