An approach using open source and free software

Journal of Cultural Heritage 11 (2010) 350–353

Case study

Cultural heritage interactive 3D models on the web: An approach using open source and free software Alberto Guarnieri 1 , Francesco Pirotti ∗ , Antonio Vettore 2 Interdepartment Research Center for Geomatics, University of Padova, Viale dell’Università 16, 35020 Legnaro (PD), Italy

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Article history: Received 30 April 2009 Accepted 25 November 2009 Available online 23 March 2010 Keywords: Open source 3D models Cultural Heritage Web documentation

a b s t r a c t Cultural heritage sites and artefacts get a significant added value from high-resolution 3D models. These models are increasingly available due to improvements in technology and to higher integration of survey techniques such as laser scanning and photogrammetry. In this paper we present a case study on the development of a web-based application for user access and interactive exploration of three-dimensional models by providing integrated geometrical and non-geometrical information into an intuitive interface. The main feature of this interactive system is to provide the user with a completely new visit experience based on a free interactive exploration interface of the object (i.e., not constrained by any predefined pathway) and on the opportunity to get more detailed information on specific parts of interest. A parallel aim achieved was to use, in data processing and in the architecture, open source tools and free software, thus providing full transparency on adopted methodology and data processing methods, and a cost effective solution both for server and client. Furthermore, the aspect of data size has been considered using a segmentation and simplification scheme and server-side data management to keep transmission size to a minimum, thus improving access speed. © 2010 Elsevier Masson SAS. All rights reserved.

1. Research aims The objective of this paper is to illustrate a case study of a workflow for preparing 3D cultural heritage models for interactive consultation via the web and for setting up a portal for publication of the models and of associated data using Open Source (OS) tools. Three models from different survey techniques (laser scan, photogrammetry and mixed techniques) have been considered for processing into segmented submodels to divide the payload in terms of file-size/download-time and for permitting semantic linking of interest subareas parts to corresponding data for the user to access. OS tools provide independency from the operating system used both for development and for usage. While walking through the virtual reconstruction, the user can get information about historical, artistic and architectural aspects, which are usually found on the tour guides or specific books. 2. Introduction New opportunities and challenges for the development of web-based Virtual Reality (VR) applications in the field of Cul-

tural Heritage have been the direct effect of advances in the field of surveying and Internet-related technology. High level of detail and accuracy can be obtained by integration between laser scanning technology and photogrammetry [1,2,3]. Several works published so far have demonstrated how Cultural Heritage (CH) can greatly benefit from 3D modelling applied to object or historical/ archaeological site analysis [4], documentation, preservation and restoration [5]. VR web information systems have been used for dissemination of data in the CH world as well as for integration with geographic data and cooperation systems [6,7]. Open source tools have heavily become part of developer and user scenario for the usage of such systems [4], providing full transparency on adopted methodology and on used data. Topics such as the “truth likeness” of virtual reality reconstructions and the efficient representation of virtual objects [8] are still important issues in the field of VR applied to CH as well as the choice of the digitizing technology to use [9,10]. Given these premises, it is therefore the authors’ opinion that the integration of VR worlds into web-based applications will be the next step towards the dissemination of CH related contents to a wider audience. 3. Materials and methods

∗ Corresponding author. Tel.: +39 049 827 2710; fax: +39 049 827 2686. E-mail addresses: [email protected] (A. Guarnieri), [email protected] (F. Pirotti), [email protected] (A. Vettore). 1 Tel.: +39 049 827 2711; fax: +39 049 827 2686. 2 Tel.: +39 049 827 2688; fax: +39 049 827 2686. 1296-2074/$ – see front matter © 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2009.11.011

Methodology and results have been tested on three models produced in different research projects which were undertaken in the past years by our research group for documentation and preservation of Cultural Heritage objects.

A. Guarnieri et al. / Journal of Cultural Heritage 11 (2010) 350–353

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Fig. 1. Employed models in project: A. Pozzoveggiani Church, B. Bas-relief, C. Epigraphy.

3.1. Employed 3D Models Three models have been used to assess the procedure and to test the portal (Fig. 1): (a) the small medieval church of Pozzoveggiani [2], (b) a bas-relief of holy figure, (c) an epigraphy [4]. The Pozzoveggiani church, located 5 km from Padova, dates back to the 12th century; it represents an example of interlace between Carolingian–Othinian artistic culture and styles typically found along the Venice lagoon. The church has a planar dimension of 7 × 16 m and was fully surveyed in 2003 both with a Time-ofFlight (TOF) terrestrial laser scanner (Leica HDS 2500) and with photogrammetric methods as described in [2]. The laser scanner has a nominal accuracy of 4 mm at 50 m distance, a scanning range of 1 to 150 m and a field of view (FOV) of 40◦ both vertically and horizontally. Twenty-three scanning stations were necessary as the instrument does not have a rotating head. Scanning resolution was set at ∼1 cm at 5 m distance. Global registration of the scans gave a global decimated point set of about 8 million points with a residual error of 4.3 mm. The photogrammetric approach provided with 22 images from Nikon® Coolpix 5700 with a size of 1024 × 768 and a pixel footprint of approximately 1 cm. The bas-relief was surveyed with a Vitana close-range laser scanner, which uses the optical triangulation principle to attain an accuracy of 50 ␮meters at 30 cm distance. Measurements were acquired at a distance of 50 cm with 350 profiles at 256 points per profile. The object is composed by a smooth rocky material (probably marble) and comes from the Mantova-Benavides collection hosted by the Museum of Archaeological and Art Sciences in Padua. The epigraphy model was obtained by surveying an ancient milestone set down by a roman consul named Spurio Postumio Albino and preserved at the Museo Lapidario Maffeiano in Verona. It has been scanned with the same instrument as the bas-relief. An average distance of 50 cm was used to make 12 scans each with 250 profiles at 256 points per profile. The porous surface of the artefact shows a set of inscriptions some of which are clearly visible, while others are less distinguishable, being eroded by time and weather.

the Pozzoveggiani church is a separate entity, which can be clicked and opened as model, and corresponding data in the PostgreSQL database queried using PHP and viewed in a form which permits a user with data-modification privileges to alter or add new information which is saved dynamically in the database. The X3D format is viewed with Octaga© plug-in for the browsers (Fig. 2). Octaga is third-party free software which is not OS. It was chosen because it is a highly popular viewer and was already installed on most clients where testing took place. A list of OS X3D viewers is available at the WEB3D consortium web page. 3.3. Model simplification and segmentation To provide the user with full web-access to complex 3D models simplification and segmentation procedures are applied to the original model. The former is aimed to decrease the size of the data being transmitted over the net, while the latter is used to define on the 3D model user-selectable areas of interest. Blender is the OS software which was chosen due to its full X3D import/export capabilities and python scripting support, which can prove useful for future automation of the processes. Another valid OS software which can be used is MeshLab® . The model is resampled at different scales in order to have a coarse resolution overview of the whole object and more detailed parts, which can be separately exported and viewed as “children” of the overview model. Blender does this by applying what are called “modifiers” to the model, which can be a series of elaborations done to the original object. There are two modifiers which can be used; the simple modifier is called “decimate”, whereas the other is called “Poly reducer”. The “decimate” module decreases the number of faces in the model without any specific criteria other than

3.2. System Architecture The portal accessed by the user is created with Apache as server and PHP as scripting language for dynamic pages. The data and metadata related to the models, as well as hierarchical organization of the model parts and the link between model parts and relative information, is stored in a full open-source object-relational DBMS (ORDBMS) supporting almost all SQL constructs called PostgreSQL. Another OS DBMS which could replace PostgreSQL is MySQL; in this test the former was used. The models and parts of models are stored in X3D format, which supports binary compression (useful for sending lighter data over the Internet) and hypertext mark-up language. It supports a specific “anchor” tag for linking model parts to triggers such as actions opening a pop-up window displaying information or the part itself as 3D model. For example, the door of

Fig. 2. Schematic architecture of the system.

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Fig. 3. Example of segmentation of model in semantic sub-objects: in this case the attribute of the model considered is the symbolic interpretation of its parts.

deleting vertices every nth vertex depending on the specific decimation ratio specified by the user – it is an automatic approach which can be applied to simple surfaces. The “Poly-reducer” module can be tweaked by specifying not only the decimation ratio, but also boundary weights and area weights. These criteria can help to keep high levels of detail where there is high complexity and removes detail in simpler surfaces. Each model is saved separately and stored with a name which keeps track of the level of simplification it belongs to, so that when loading the model on the page, the portal can load the most simplified (thus lighter) model and then pass to the more complex model as the user requests to load it.

In order to create small size parts for loading via web, the 3D model is manually subdivided into different entities corresponding to the areas for which attributes and/or metadata are available. Each entity is saved in a X3D file with a unique name used for identification, through which object interaction can be easily provided as described in the next subsection. Semantic logic can be used on attribute data. This means that classification of an attribute is used to divide the object according to that attribute. Fig. 3 is an example of the segmentation process based on the symbolic meaning of parts of the basrelief.

Fig. 4. The portal accessed via web using a common browser.

A. Guarnieri et al. / Journal of Cultural Heritage 11 (2010) 350–353

3.4. Connecting attributes and metadata The final step deals with the linking of segmented areas (or the whole model) to respective data. This is not an easy task as it must be clear beforehand which logic to adopt for the segmentation. In the example of the bas-relief shown in Fig. 3, information was assigned to separate areas which represented physical characteristics (face, sacred heart, hand position) because it was interesting to highlight the symbolic meaning of these parts. Alternatively, model segmentation could be aimed at documenting the features of the artistic technique exploited to create the different parts of the artefact. Semantic logic can therefore lead to multiple types of segmentation because significant associations can intersect one another. In our system, model interactivity relies on the combination of PHP, PostgreSQL and X3D data format. Basically attributes and metadata are connected to the corresponding entity by adding in the X3D data file an html-like tag which contains both the link (URL) to the web page where data will be displayed, and a reference ID stored in a record of the PostgreSQL database. When the user clicks with the mouse on a selectable area of the overview model, the web page is dynamically generated by PHP script running on the server and all information (high resolution 3D model of selected entity, metadata and attributes) related to such ID is shown to the user. 4. Results and conclusions Five persons have been asked to use the portal with the common objective to access information on three specific elements (the door of Pozzoveggiani, the inscription in the epigraphy and the heart symbol in the bas-relief). They were then interviewed to assess their opinion on the portal and get feedback on ease of access and navigation. Results showed that in all five cases the easiest task was to navigate models and access data from model parts. The task found to be the hardest was to access the 3D model or model part from a query; this is probably because each person has a different approach at finding a specific object. Navigation of models was found to be a very arduous task by two of the testers because of having absolutely no experience on viewing 3D models on the com-

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puter. This information helped new ideas which can improve user experience. For example, two quick video tutorials can be linked for new users to learn 3D navigation and object query. This would overcome the problem of having users without such experience use the portal. New technologies such as voice avatars could increase accessibility for people with impaired vision; instead of having to read small text a voice recording could describe the selected object (Fig. 4). The use of our interactive system could therefore be potentially extended to more complex virtual exploration such as an archaeological site. Despite the fact that bigger and more numerous objects would require an extensive 3D modelling work, constant advances in the field of surveying technology and computer science make such objective not unrealistic at all. References [1] D. Barber, J. Mills, P. G. Bryan, Laser Scanning and Photogrammetry-21th Century Metrology, in: Proceedings of CIPA 2001 International Symposium “Surveying and Documentation of Historic Buildings, Monument”, Potsdam, 18–21 September, Germany, 2001, pp. 425–428. [2] A. Guarnieri, F. Remondino, A. Vettore, Photogrammetry and Ground-based Laser Scanning: Assessment of Metric Accuracy of the 3D Model of Pozzoveggiani Church, in: Proceedings of FIG Working week 2004, May 23–27, Athens, Greece, 2004, p 15. [3] A. Guarnieri, A. Vettore, S. El-Hakim, L. Gonzo, Digital photogrammetry and laser scanning in cultural heritage survey, Int. Arch. Photogram. Remote Sensing Spatial Info. Sci. 35 (2004) 154–159. [4] P. Grossi, F. Pirotti, GFOSS and archeology: a Web-GIS example for cultural heritage in Montegrotto Terme (Padua), in: C. Cencetti (Eds.), IX Meeting GRASS-GFOSS Users, February 11–12, Perugia, Italy, 2008, pp. 66–69. [5] P. Grossi, A. Buonopane, A. Guarnieri, F. Pirotti, L’impiego del laser scanner nel rilievo delle iscrizioni sui miliari, Epigrafia ed Antichità 25 (2005) 374–388. [6] S. Pescarin, Spatial data integration in real-time cooperative systems, Int. Archives of Photogrammetry, Remote Sensing Spatial Info. Sci. 38 (2009) 146–152. [7] E. Meyer, P. Grussenmeyer, J.P. Perrin, A. Durand, P. Drap, A web information system for the management and the dissemination of Cultural Heritage data, J. Cult. Herit. 3 (2002) 325–331. [8] C.L. Ogleby, The “Truthlikeness” of Virtual Reality Reconstructions of Architectural Heritage: Concepts and Metadata, The Int. Archives of Photogrammetry, Remote Sensing Spatial Info. Sci. 34 (2005) 192–199. [9] N. Yastikli, Documentation of cultural heritage using digital photogrammetry and laser scanning, J. Cult. Herit. 8 (2007) 423–427. [10] M. Pieraccini, G. Guidi, C. Atzeni, 3D digitizing of cultural heritage, J. Cult. Herit. 2 (2001) 63–70.