Aplicación de la fotogrametría automatizada y de técnicas de iluminación con herramientas SIG para la visualización y el análisis de una piedra con relieves antropomorfos

Abstract

Este trabajo tiene por objetivo presentar una aproximación metodológica para la representación, visualización y análisis de modelos tridimensiones (3D) de relieves y detalles significativos sobre piedra a partir de la aplicación de herramientas de documentación digital y su correspondiente procesado. Para ello se toma como objeto de estudio unas figuras antropomorfas que aparecen sobre una piedra caliza en forma de lasca plana y de espesor contante hallada en la Sierra de Fontcalent (Alicante). Sobre ella se aplican a una escala de objeto, técnicas de documentación y registro con fotogrametría mediante fotografía digital y técnicas SfM (Structure from Motion) para su digitalización y posterior representación en 3D. Del modelo resultante se genera un MDE (Modelo Digital de Elevación) para estudiar las microtopografías de la pieza, a través de visualizaciones topográficas obtenidas mediante herramientas de análisis geográfico que nos proporcionan los programas SIG (Sistemas de Información Geográfica) a partir de distintas condiciones de iluminación y reflejo de las superficies que proporcionan técnicas como el sombreado (hillshading) o el LRM (Local Relief Model) y que posibilitan la interpretación arqueológica del objeto bajo estudio. El estudio se completa mediante la aplicación de los métodos de análisis y visualización basados en el Modelo de Residuo Morfológico (MRM) y de RTI (Reflectance Transformation Imaging), respectivamente, generados ambos mediante el software Cloudcompare. De este modo, la metodología propuesta se presenta como una alternativa precisa y completa para el estudio y análisis del patrimonio cultural existente, y abre nuevas vías para la revisión, reinterpretación y revalorización del patrimonio evaluado con anterioridad mediante técnicas tradicionales.

We present a methodological approach for the representation, visualisation and analysis of three-dimensional (3D) models of meaningful details in stone reliefs provided by digital documentation tools and subsequent processing. For this aim, anthropomorphous shapes engraved on a flat stone slab found in Sierra de Fontcalent (Alicante) are studied. The object under consideration was located near two archaeological sites, Cova del Fum –a cave with presence of the Chalcolithic material (López, 2010)– and the archaeological site of Fontcalent, with remains from different phases of occupation spanning from 7th-6th BC to the 20th century (Ximénez, 2012). In the last few years, the use of digital tools provided by new technologies and software development has left traditional work methodology behind (De Reu et al., 2014) while enabling the development of new approaches to both minimise heritage alteration and provide objective and accurate information (Lopez-Menchero, Marchante, Vincent, Cárdenas, & Onrubia, 2017). 3D documentation allows recording of cultural heritage at a reasonable cost with precision and quality through digital photography and SfM (Structure from Motion) photogrammetry with specialised software (De Reu et al., 2013). In this project, recording and documentation with digital photography and automated photogrammetric techniques are applied to the Fontcalent stone slab for its digitisation and subsequent 3D representation. From the resulting model, a two-folded line of study is obtained. On the one hand, a Digital Elevation Model (DEM) is generated to study the microtopographies of the stone with geographic analysis techniques provided by Geographic Information Systems (GIS) from different lighting conditions and surface reflections, which are calculated by hillshading or LRM (Local Relief Model) for the interpretation of the object (Carrero-Pazos, Vilas, Romaní, & Rodríguez, 2014; Gawior, Rutkiewicz, Malik & Wistuba, 2017). On the other hand, from both the 3D model and the point cloud, the study is completed with the application of the methods of analysis and visualisation based on the Morphological Residue Model (MRM) which stands out every single detail of the surface morphology of the object (Caninas, Pires, Henriques, & Chambino, 2016; Correia, Pires, & Sousa, 2014). Further visualisations are based on Reflectance Transformation Imaging (RTI) which provides different shadows and reflections over the object from the application of a multidirectional illumination (Happa et al., 2010; Malzbender, Gelb, Wolters, & Zuckerman, 2000; Mudge et al., 2010). The results thus obtained of the Fontcalent stone slab allow us to visualise several characteristic elements. The anthropomorphous figure awaking interest is also combined with the figure resulting from different visualisations applied with GIS techniques which may resemble a zoomorph. The use of visualisation techniques shown in this study has been fundamental in order to recognise the latter element. The composition reveals a zigzag line already appreciated before the study so that it is interesting to check if visualisations based on GIS techniques are able to highlight it though being shallow incisions. In our experience regarding this study, visualisation by using the hillshading technique shows a greater level of 3D detail than that provided by the application of the sky-view factor technique which offers a flattering view. However, the former technique may occasionally show shadows which hide other details, unlike the latter technique which plots the entire slab surface illuminated while differentiating the associated microtopography on the basis of its marks. The use of shaders in combination with hillshading and particularly combined with high pass filtering, contributes to improving the visualisation and accuracy of shadowed areas. As a result, we conclude that the results obtained in this work by lighting techniques with GIS add a greater level of detail in comparison to those provided by the mesh or the point cloud. The study of the Fontcalent stone slab paves the way for two working hypotheses to be developed: on the one hand, its anthropological origin possibly related to the Chalcolithic, and on the other hand, its study as natural geological formations with ichnofossils. The digitisation of cultural heritage with available 3D technologies should be a mandatory requirement when facing any study, analysis or intervention. With the current development of such techniques, we have verified their contribution to fundamental characteristics in the corresponding stages of visualisation and study. Thus, the proposed methodology is presented as an accurate and complete alternative for the study and analysis of the existing cultural heritage, and opens new ways for the revision, reinterpretation and revaluation of the previously evaluated heritage through traditional techniques.