EVA London Conference ~ 11–13 July 2007 Eva Zányi, Carla Schroer, Mark Mudge, and Alan Chalmers _____________________________________________________________________ LIGHTING AND BYZANTINE GLASS TESSERAE Eva Zányi†, Carla Schroer‡, Mark Mudge‡, Alan Chalmers† † Warwick Digital Laboratory University of Warwick Coventry CV4 7AL United Kingdom [email protected], [email protected] http://www.warwickdigital.org ‡ Cultural Heritage Imaging San Francisco USA [email protected], [email protected] http://www.c-h-i.org Abstract – A key component of many Byzantine churches was the mosaics on the curved walls and ceilings, which included gold and silver glass tesserae. As the viewer or the light moved within the church, these tesserae sparkled. In this paper we describe how we captured a Polynomial Texture Map of the apse mosaic at the Angeloktisti Church at Kiti, Cyprus and used it to investigate how the position of the lighting may have affected the appearance of the mosaic. Our study showed that the appearance of the mosaics is indeed significantly different when lit from various directions. INTRODUCTION From the outside Byzantine churches look unimposing; without much decoration, no paint or precious materials. This is very different to the interior, which provided those inside the space with dramatic visual affects aiming at alleviating and engaging the viewer to approach God [14]. The architecture used light and shadow to symbolically represent different sacral hierarchies and direct the attention of the viewer. Therefore the upper parts of the churches, which represented heaven, were better lit than the lower parts. In early Byzantium this was achieved with the help of daylight through small xxxx Figure 1. Outside of the church of Panagia Angeloktisti at Kiti, Cyprus. 22.1 EVA London Conference ~ 11–13 July 2007 Eva Zányi, Carla Schroer, Mark Mudge, and Alan Chalmers _____________________________________________________________________ openings in the upper parts of the walls. From middle Byzantium on, the buildings had less openings letting in natural light and these were replaced by oil lamps and candles [18]. The positioning of these artificial lights was regulated in great detail in manuals, so called typicons, in order to underline the difference between divine light and profane darkness and to let their flickering make precious materials such as the gold and silver of the icons, mosaics and frescoes, sparkle and draw the viewer into contemplation [1]. BYZANTINE GLASS MOSAICS The Romans perfected techniques for the design and construction of intricate floor mosaics, using natural resistant materials. The Byzantines extended these methods to wall mosaics and were able to now include fragile materials and more precious one such as the glass gold and silver tesserae [19]. Since so few of the mosaics are left, it has previously been assumed that they were very expensive, especially those which contained glass tesserae. However, recently James questioned this assumption and suggested that the raw material, glass, was not expensive, since Byzantine was close to desert regions and suppliers [5]. She further suggested that manufacturers of glass tesserae were spread all over the Byzantine Empire and in fact the setting of the mosaics was the most expensive aspect since it was very labour intense. All this means that mosaics were far more widely spread than previously believed and would also explain why small and politically insignificant churches such as Kiti, which were not situated in any major centre, were so decorated. The glass tesserae were manufactured in a number of ways and often coloured. Metallic tesserae such as gold and silver ones were made by covering a ca 6mm thick glass plate with, for example gold leaf, and then coating it with a thin layer of transparent or coloured glass. The sandwiched glass plate was then heated up until the layers fused and subsequently cut into pieces. The surfaces of the tesserae were slightly uneven and different effects could be achieved depending on if the glossy or the rougher surface was exposed on the mosaic [6.10]. SELECTION OF THE SITE Only few wall mosaics are left from the early Byzantium, since most were destroyed during the 300 years of Arab expansion and invasions, the iconoclasm period of Byzantine history of the 8th and 9th centuries and also because of natural disasters such as earthquakes and fires. The three extensive apse mosaics on Cyprus dating from the 6th and 7th century are consequently very unique. They show the Virgin Mary and Child and were placed in quite small and remote churches, which did not belong to the Pope or any other financially strong ruler [17]. The three churches are: the church of Panagia Kyra at Livadia, the church of Panagia Kanakaria at Lythrankomi [8] and the church of Panagia Angeloktisti at Kiti. From these three, the ones in Livadia and Lithrankomi are, since 1974, in the occupied Turkish section of Cyprus and therefore difficult to access [16]. Apart from the accessibility problem, these two were very badly damaged after the occupation. How much is left of the mosaic in Livadia is in fact not known. Kiti, close to Larnaca on the Greek part of Cyprus, has, however, a well preserved, unrestored apse mosaic and was thus chosen for our study. The mosaic comprises the Virgin Mary holding the Child with the Archangel Gabriel on the right and the Archangel Michael on the left [3.16], Figure 2. The mosaic is lit today from slightly below by spot lights, and thus a large part of the mosaic is not well lit. The church is still in full use with regular orthodox masses taking place and several visits by large tourist groups and school children per day. 22.2 EVA London Conference ~ 11–13 July 2007 Eva Zányi, Carla Schroer, Mark Mudge, and Alan Chalmers _____________________________________________________________________ REFLECTION TRANSFORMATION IMAGING (RTI) RTI is a term coined by Malzbender and Gelb of Hewlett Packard Labs. RTI captures the “real world” reflectance characteristics of a subject. A simple, robust, and forgiving way to capture RTI information is the use of Polynomial Texture Mapping (PTM). PTMs store surface reflection information with each image pixel. Malzbender et al., inventors of PTM [9], presented a mathematical model describing luminance information for each pixel in an image in terms of a function representing the direction of incident illumination. The illumination direction function is approximated in the form of a biquadratic polynomial whose six coefficients are stored along with the colour information of each pixel. This surface reflection information describes the subject’s surface normals. This normal information indicates the directional vector’s perpendicular to the subject’s surface at each location recorded by the corresponding image pixel. Consequently, PTMs are 2D images containing true 3D information. PTMs are also able to record approximations of other reflection-related properties including surface inter-reflection, subsurface scattering, and self-shadowing. PTMs can communicate useful shape information using purely image based transformations without full photometric stereo or other reconstruction from the surface normals using 3D geometry in Cartesian space. The normals of a surface describe its shape and are used by computer graphics lighting models to determine surface reflection properties. In 3D virtual reality representations, normals are used by lighting models to calculate how light rays will reflect off the surface of virtual 3D geometry. The normal information present in RTIs allows them to use similar 3D lighting techniques. The software used to view RTIs employs these 3D lighting models. RTI images are interactive. Their dynamic interplay of light and shadow works with the human visual system to communicate a powerful perception of the object’s shape [2.7] While RTIs can be used to communicate the effects of different illumination directions on a surface, they can also transform surface normal information to enhance the perception of surface features. This enables RTIs to not only disclose surface characteristics not visible in any of its constituent source photographs, but also reveal information not readily discernable by direct physical examination. This characteristic of RTI has been dramatically demonstrated by its recent use in revealing the nature and use of the Antikythera Mechanism, a 2nd century geared astronomical computation device [4]. A key characteristic of RTIs captured with the PTM method is that complete surface normal information can be acquired from highly shiny, specular materials such as gold without data loss associated with clipping due to specular highlights. PTMs have been demonstrated to effectively capture highly reflective surfaces without data loss due to shadows or specular highlights during the documentation of gold and silver coins as well as highly reflective stone tools [13]. This property has been used to great advantage in our project with the Byzantine mosaic and gold leafed icons at Kiti. The apse mosaic contains numerous tesserae, essentially glass gold and silver leaf ‘sandwiches’. The icons contain gold leaf and tempera painting techniques intended to produce reflective effects. Gold, silver and glass are notoriously difficult to capture through either using photography or active 3D range scanning, due to their highly specular nature. Our successful capture of these subjects and relighting with the simulated illumination conditions for which they were designed underscore the usefulness of PTM based RTI techniques for these classes of cultural heritage subjects. 22.3 EVA London Conference ~ 11–13 July 2007 Eva Zányi, Carla Schroer, Mark Mudge, and Alan Chalmers _____________________________________________________________________ The preceding attributes of RTIs and PTMs as well as their use in cultural heritage documentation projects, the natural sciences, and law enforcement has been detailed extensively elsewhere, including [3.4.9.11.12.13.20]. Capturing RTIs using PTMs To capture single-viewpoint PTM images, the subject is photographed from a fixed camera position. Multiple photos are shot, each illuminated from a different light position.