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ARTICLES > Technology Development1046
VARIM - A Useful System for Acquiring and Composing Images in Painting Analysis Techniques
by Juan Torres, Alberto Posse, José Manuel Menéndez, Araceli Gabaldón,
Carmen Vega, Tomás Antelo, Marián del Egido & Miriam Bueso
Introduction
Along the history, art has always been linked to technology development. As new techniques have arisen, the way artists show their ideas has changed. Nevertheless, technology progress has not only affected the creation of artistic compositions but has also improved their study and analysis, as well as their conservation and restoration. The different sort of analysis offer different information about works of art: advances in the chemistry field make possible, for instance, to carry out an exhaustive study of the art materials’ composition, new techniques of microscopic analysis provide with more data about the state of the paintings, etc. The increasing development of the computer systems and the advances in the image processing algorithms and computer vision not only provide new instruments of analysis but also permit to improve the ones already existing. In addition, this improvement allows the reduction of costs, making the technology accessible to a greater number of conservator-restorers, curators, art historians and researchers in general. Having this idea in mind, the VARIM project (Visión Artificial aplicada a la Reflectografía de Infrarrojos Mecanizada – Computer Vision applied to Mechanized Infrared Reflectography) was born. In this project, a complete system for acquiring and composing the infrared reflectography mosaic was designed and created. This system is able to create, in an automatic way, the entire image of the underdrawing layer of a painting using advanced computer vision techniques. But its main importance lies in offering study tools even to users having scant resources and knowledge due to a user friendly design. Moreover, it is an all-purpose tool which not only can create a reflectographic mosaic but its modular structure makes possible its use for other analysis techniques such as radiography, as further shown. VARIM history The infrared reflectography [1] is an analysis technique used since the 70’s and it consists of acquiring a set of contiguous images of a painting using a video camera sensitive exclusively to the infrared area. These images are later merged to obtain a unique image called mosaic. A large number of images of a painting are required so small details can be appreciated. The final mosaic offers information about the underdrawing layer which is not directly visible by the human eye. Although this technique has been used for decades, the traditional method entails several problems that make the task very slow and tedious. Moreover, as it was already stated, this kind of analysis has not been accessible to modest institutions. Thus, VARIM project tried to solve a set of problems which can be found in other systems offering a versatile tool for everyone. Among the difficulties, VARIM has brought improvements to old methods, such as: - Usually, the acquisition task is done in a manual way, placing the camera in front of the desired area of the painting for each capture. If a high resolution is required, the number of captured images must be increased. This implies a monotonous task, placing and controlling the camera in the right position. - The mosaicing is also a manual task. The user should select the sub-images one by one and mark at least a similar point in two of the consecutive sub-images. Depending on the number of sub-images, this task can involve several hours of tedious work. - In the traditional method, different computing applications are used for each step: acquisition, processing and mosaicing. To handle several programs for the same process can cause incompatibilities among them while integration in one only makes easier the user’s tasks. - The imperfections of the infrared camera, the mechanical elements and the variations in the light conditions make very difficult to obtain high quality mosaics. This way, VARIM provides different tools that solve the difficulties already enumerated, as well as it brings additional advantages to be mentioned further on. System Description The VARIM system is formed by two components: the physical devices composing the hardware architecture, and the software application which handles these devices and includes useful image processing tools. Although not all the physical devices are available to any user - for instance the infrared camera is an expensive device- the software application can be used independently to acquire colour images and to join any kind of images, i. e., colour, ultraviolet and radiography ones. In addition, the software is distributed for free and released under the terms of the GPL license. Any interested person can download the application from the web page of the VARIM project. Figure 2 shows VARIM’s physical diagram architecture, which is composed of the painting, a mechanical position system (handled through an Ethernet module), a near infrared camera, a lighting system, and a PC platform with a framegrabber. The mechanical position system is formed by: - A stable structure that allows camera controlled movements among an area of 1x1 meter (two degrees of freedom) operated by the PC using the Ethernet module. This structure can be transported to place it in front of the work of art at the laboratory or outside (left and center images in Figure 3).
- A mobile structure that makes the stable structure to achieve a height of 6 meters, allowing the system to reach paintings that can not be moved, such as those in a church (right image in Figure 3).
The system is able to move the structure in a smart way, acquiring each sub-image that will configure the mosaic and inserting them automatically in the application. This process is carried out without any other human intervention besides the input of some data: the initial and the final positions and the selected overlapping area between the acquired images. Human errors during the movement are thus avoided with this automatic acquisition. The lighting system is placed in a fixed way on both sides of the painting in order to obtain a uniform light condition all over the painting surface. Software application VARIM application is designed as modular software and it can be used separately. The different modules are as follows: - A Video and Image Acquisition Module (VIAM). This is the module in charge of the camera control, that is, the automatic acquisition of the subimages which compose the entire mosaic. - An Image Processing Module (IPM). Some generic image processing tools are implemented in this module besides the mosaicing algorithms (VIPS library is used). As well as the generic tools, three important methods have been designed and developed: 1. A noise pattern suppression method. This is an algorithm able correct the wear that some old infrared cameras have. This wear produces an undesirable pattern in all the captured images. 2. A geometrical distortion correction algorithm [2]. Some cameras show a geometrical distortion due to the imperfection of their lenses. Thus, a method for correcting this effect has been implemented. 3. An automatic luminosity control in acquisition [3]. This algorithm has been implemented in order to maintain a regular luminosity during the acquisition process, correcting the differences in the incident light and its variation in each capture. - A Mechanical Control Module (MCM). It is in charge of the manual and automatic movement and of positioning the camera in front of the painting. A main window interface, where all the acquired sub-images are listed and organized by rows, gives access to all the implemented functionalities. Figure 4 shows an example of the application. The background window lists the acquired sub-images; the foreground window shows the composed mosaic, together with a detail of a particular union and its position in the whole mosaic. In order to form the mosaic, every image registration in the mosaicing process requires one or two correspondence points in two consecutive (horizontal or vertical) images, points that are found by an automatic algorithm [4]. Then, VIPS library is used to merge the two images using those automatic calculated points. Figure 5 shows an example of this image registration. As earlier stated, some of the features of VARIM are available using specific physical devices. For instance, to acquire images in an automatic way, the mechanical position system is required. Nevertheless, the composition of any kind of images and the image processing tools can be used without any restriction.
Figure 1. VARIM application; Figure 2. VARIM’s physical structure set up for infrared reflectography mosaicing; Figure 3. Hardware sub-system; Figure 4. VARIM software, main and secondary windows; Figure 5 - Automatic image registration.
Applications and Results
Nowadays, the VARIM application produces very satisfactory results. Indeed, almost all the projects carried out by the Physics Studies area of the Instituto del Patrimonio Histórico Español (IPHE) use this software application. As an example, one of the most recent studies has been selected: "Estudio comparativo de tres pinturas sobre tabla". This study presents the fieldwork done due to the "Sumas y restas de las tablas de Arcenillas" exhibition in the Zamora Museum. In this exhibition, three wooden paintings belonging to an alterpiece ascribed to Fernando Gallego were shown: "Adoración de los Magos" (nowadays in the Museo de Bellas Artes of Asturias), "Noli me tangere" and "Pentecostés" (both in the Zamora’s Cathedral Museum). It is believed these paintings were made in the 15th century for the altarpiece of the Zamora’s Cathedral. In the 18th century, this altarpiece was replaced and purchased by a nearly village called Arcenillas. In 2007, a comparative analysis of the three paintings was requested. The applied intervention protocol was the regular one used by the IPHE’s laboratory of Physics Studies and the stages where VARIM was used were divided into two steps: the acquisition and mosaicing of the infrared reflectography and the mosaicing of the digitalized X-ray images.
Figure 6. “Adoración de los Magos” (visible); Figure 7. “Noli me tangere” (visible); Figure 8. “Pentecostés” (visible).
I.P.H.E. Sección de Estudios Físicos The infrared reflectography The first stage in a study on the response of a painting to infrared radiation is to decide between a better spectral or spatial resolution. Thus, initially a double sampling of the painting is done because the type of sensor to be used depends on the pigments’ properties: infrared reflectograph (mounted in an analog camera) or CCD (mounted in a conventional digital camera, with no infrared filter). In this case, the spatial resolution was the decisive feature, thus a CCD was chosen. This kind of sensor is low cost (compared to the infrared one) and more accessible to researchers. All the images acquired and composed were obtained using VARIM application and the study was divided in four stages: 1. Decision of the size of the mosaic. As this depends on the motif to be studied, its size was decided using a previous tracking of the painting. 2. Location and assembly of the mechanical position system. The mechanical support is usually taken in pieces because of the movement. Because of this, it is important to check if all devices are perfectly adjusted before acquisition. Moreover, this system must be placed parallel to the painting from a distance previously established in order to obtain a correct automatic composition. Later, the camera was placed in its support using a B+W 093 filter to avoid visible radiation inside the camera lens. 3. Acquisition and mosaicing. Using contiguous images, horizontal and vertical, the partial displacements were decided in order to obtain a correct overlapping area between images. This parameter depends on several factors but the main one is the type of drawing. Once the images were acquired, they were integrated in the workspace and the final mosaics were obtained. 4. Analysis and exhibition of the obtained results [5]. As a conclusion, some similarities in the preparatory drawing of the three paintings have been found. In fact, this also occurs in other studied paintings from the same author (Fernando Gallego), as for instance, in the altarpiece of Trujillo (Cáceres) and in "La Virgen de la Rosa" (Salamanca’s Cathedral Museum). Next, some observations can be drawn from the pervious statements. Firstly, the drawing is of high quality and the lines are confident, expressing a vigorous image. Moreover, in general, the visible composition fits with the preparatory drawing. A first approach to the pieces could give the sensation of great differences but similar areas demonstrate that the creation process is the same. Lastly, only one label1 (regarding the green colour, "verde") has been found in "La Adoración de los Magos" (Figure 9). The X-ray analysis First of all, it must be mentioned that this is a comparative study which will comprise other 11 wooded paintings from Arcenillas’ Church (Zamora). Thus, all the X-ray belonging to the wooden boards were made2 and developed3 under the same conditions. The used film4 has usually a format of roll of 30 cm width and 61 meters long. The film was cut into strips of the same height as the boards. Altogether more than 7 meters of film was needed. Once the X-ray was obtained, it was digitalized. The dissemination of the obtained digital document is important for conservation and the digitalized X-ray makes easier the study of the painting for a great number of researchers because no additional and expensive devices are needed. On the other hand, the preservation of this digital document is very important and its manipulation must be avoided. The document is of high importance as a witness of the conservation status of the painting. VARIM was also used in the joint of digitalized plates. Previously, other image processing applications were used but a clear and artificial line appeared in the joint area due to the lack of an overlapping area in the images. Using the union algorithms provided by VARIM, the joint process is automatic and instantaneous. On the other hand, the exhaustive analysis of these documents provides valuable data, invisible to the human eye. In this case study (Figures 12-14), some metallic staples were found in order to reinforce some cracks in "La Adoración de los Magos" (Figure 12).
The other paintings do not have any staples in spite of some cracks of the wooden boards. In addition, some differences were found regarding the fabric placed between the preparatory layer and the painting. This fabric is complete in the case of "La Adoración de los Magos" while the other two painting present fabric in the panels joints and in the cracks. Finally, some similarities were found between "La Adoración de los Magos" and "Pentecostés", where an incisive drawing and a clear vanishing point appears in both X-ray (Figures 12 and 14).
Figure 12. “Adoración de los Magos” (X-ray); Figure 13. “Noli me tangere” (X-ray); Figure 14. “Pentecostés” (X-ray).
I.P.H.E. Sección de Estudios Físicos Conclusions The VARIM system has demonstrated that it is of inestimable help and has brought improvement compared to the traditional method. VARIM is an application that integrates in only one work environment all acquiring and mosaicing tasks. This makes the entire process more efficient. Thus, the use of workspaces and the storage of all the information is done in an easy and fast way which is especially useful due to the great number of images that are handled in each session. Another interesting feature is the use of temporal and work directories. Using the application preferences, the user can specify the work directory and a different one where the partial results are stored. Furthermore, in a real environment, the application offers a tool where the acquisition and mosaicing tasks are user-friendly and easy. Moreover, thanks to the automatic methods previously mentioned, the system allows to save a considerable amount of time. Even users without deep computer knowledge can use the application in an easy way, thanks to the accessibility criteria applied in the design stage. Finally, thanks to the fact that the software is distributed for free, its extensive use and the obtained feedback made possible to correct some bugs and to improve its functionalities. One of the main objectives established at the beginning of the project was achieved: to create a functional tool, easy to use and accessible to a great number of people.
References
1. J. R. Van Asperen de Boer, “A contribution to the examination of earlier European paintings”, PhD thesis, University of
Amsterdam, 1970 2. J. Torres and J. M. Menéndez, “A practical algorithm to correct geometrical distortion of image acquisition cameras”, IEEE International Conference on Image Processing, vol. III, pp. 2451-2454, October 2004 3. J. Torres and J.M. Menéndez, “An adaptive real-time method for controlling the luminosity in digital video acquisition”, IAESTED International Conference on Visualization, Imaging and Image Processing, pp. 133-137, September, 2005 4. A. Posse, J. Torres and J.M. Menéndez, "Matching points in low contrast images”, International Conference on Image Processing (ICIP), San Diego, USA, October 2008 (Under revision) 5. T. Antelo, A. Gabaldón and C. Vega, "Sumas o restas: incógnitas en torno al retablo de Arcenillas”, in Sumas y restas de las tablas de Arcenillas. Fernado Gallego y el antiguo retablo de la Catedral de Zamora, Zamora, 2007
About VARIM project
VARIM - A Useful System for Acquiring and Composing Images in Painting Analysis Techniques
Universidad Politécnica de Madrid (Polytechnic University of Madrid) Grupo de Aplicación de Telecomunicaciones Visuales (G@TV) (Visual Telecommunication Application Group) G@TV is a research group that belongs to the Universidad Politécnica de Madrid (UPM), one of the most important universities in Spain. This group has experience in image/video processing, in the development of Intelligent Transport Systems and surveillance systems. The experience on image and video extends throughout computer vision applied to art conservation and restoration, coding, computer graphics, image restoration and analysis, object and movement location and tracking, etc.
IPHE operates within the Dirección General de Bellas Artes y Bienes Culturales of the Spanish Ministry of Culture and takes care of the development and implementation of plans for the conservation and restoration of historical heritage. Movable heritage and historical buildings are included. The IPHE also co-operates with other public administrations and public or private institutions for the development of these plans. The IPHE works on the available documentation, organization of work produced and archival on the historic heritage of the country, as well as research and study of criteria, methods and updated techniques in conservation and restoration of cultural heritage.
Notes: 1. The artists usually draw some texts in the underdrawing giving instructions to their pupils. In this case some labels regarding colours were drawn. 2. Each X-ray was obtained in one shot using a Philips MCN 165 device under the parameters 40 kV and 184 mAxs/m2. 3. In an automatic process during 8 minutes and 30 ºC. 4. Type II norm ASTM (D-7 by AGFA)
About the authors
Juan Torres contact: jta@gatv.ssr.upm.es He received the Telecommunications Engineer degree (Hons.) in 2004 from E.T.S. Ingenieros de Telecomunicación of the Universidad Politécnica de Madrid . Since 2002, he is a member of the Signals, Systems and Radio communications Department of the E.T.S. Ingenieros de Telecomunicación. In 2006, he obtained the Researcher Aptitude in the Ph.D. program called "Communications Technologies and Systems". Nowadays, he is a PhD candidate researching on the variations of the internal parameters of the video cameras digital acquisition. His master thesis was performed in the VARIM Project (Artificial Vision applied to Mechanized Infrared Reflectography) framework, funded by the Ministry of Industry. Moreover, he is author of several international and national papers and scientific contributions and has been invited to several national Congresses. In addition, he has taught a course belonged to a Master in Arqueometry of the Universidad Autónoma de Madrid. Alberto Posse He received the Telecommunications Engineer degree (Hons.) from E.T.S. Ingenieros de Telecomunicación of the Universidad Politécnica de Madrid in 2007. His master thesis was performed in the VARIM Project (Artificial Vision applied to Mechanized Infrared Reflectography) framework. His professional interests include image and digital video processing, image registration, remote sensing and computer vision. He is Ph.D. candidate and researcher assistant at the Visual Applications telecommunications Group being involved in several R&D National and European Projects in relation with Audiovisual and Remote Sensing technologies. José Manuel Menéndez He received the Telecommunications Engineer degree (Hons.) in 1988 and the Ph.D. degree in Communications (summa cum laude) in 1996, both by the E.T.S. Ingenieros de Telecomunicación of the Universidad Politécnica de Madrid. Since 1988 he is a member of the Signals, Systems and Radio communications Department of the E.T.S. Ingenieros de Telecomunicación, becoming associate professor in 1996. His professional interests include computer vision, image processing, digital video broadcasting and visual communications. He has been actively involved both in European (Eureka, Race, Esprit, ACTS, and IST, since the II FP) and in national projects since 1988. Dr. Menéndez has published about 40 international publications about computer vision and image processing, both in international journals and conferences, and he is co-author of a book (in Spanish) about Audio and Video Technology for undergraduate engineering level. Araceli Gabaldón Bachelor's Degree in Physical Science, X-rays installation supervisor at IPHE (Madrid) and the Museo Nacional Centro de Arte Reina Sofía (Madrid), she contributes to research and to develop technical studies with electromagnetic radiation on cultural beings. She is employed at IPHE Physical Studies Section since 1971. Carmen Vega Ph.D. in Biophysics, she has been collaborating at IPHE Physical Studies Section since 2001 as expert on infrared reflectography studies on cultural assets. She participates on publications and congresses about this specialised subject. Tomás Antelo Bachelor of Arts, X-rays installation operator, he develops projects on X-rays, UV, IR, visible studies on cultural assets at IPHE Physical Studies Section since 1970. Marían del Egido Bachelor's Degree in Physical Science, Curator of Museums and Chief of IPHE Scientific Department since 2000, she is involved in several projects on scientific studies of cultural heritage. Miriam Bueso Bachelor of Arts and Archaeology, Graduate in Conservation on the speciality of Archaeology and Assistant Curator of Museums, she is working at IPHE Physical Studies Section since 2005, contributing to develop research and publications about physical studies of cultural assets.
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