An Information System to Analize CulturalHeritage Information

J.C. Torres1, L. Lopez1, C. Romo1, and F. Soler1

Virtual Reality Laboratory. University of Granada, Spainjctorres@ugr.es,

WWW home page: http://lrv.ugr.es/chis

Abstract. Managing information related to cultural heritage sites is animportant task and much work has been devoted to developing specialpurpose document management systems. These systems are able to storeand retrieve large amounts of documents; however, while this is adequatefor some purposes, it is not sufficient for research and conservation work.Researchers need to determine relationships between data, and the mostimportant relationships in cultural heritage information are spatial rela-tionships.A new kind of information system is therefore needed, in which the 3Drepresentation of an object is a blackboard on which all data is repre-sented. This paper proposes the concept of Cultural Heritage InformationSystems, and presents our implementation of the system. An exampleapplication illustrating the use of the system is also presented.

1 Introduction

Archaeologists and other professionals in the field of Cultural Heritage (scien-tists, curators, restorers, architects, and so on) manage a large amount of infor-mation related to heritage objects. This information includes a wide variety ofelements, for example: photographs, paintings, maps, descriptive texts, historicaldocuments, annotations, measurements or test results.

In our opinion, although traditional information systems are very useful formanaging large amounts of data, they have two important drawbacks when usedfor Cultural Heritage applications:

– For most information, position is a relevant attribute. Although positionattributes can be attached to any database record indicating from whichpart of the monument the piece of data was collected, this will not workwhen several names are used for the same place. Moreover, the data may berelated to any area, such as the part of a wall that has been treated duringa restoration process.

– Document management systems do not allow computations related to thelocation of information to be performed. All queries performed in the systemare textual, and consequently the user obtains a list of relevant documents.However, scientists and other cultural heritage professionalsmay be also in-terested on queries involving the spatial distribution of the information.

Fig. 1. Screenshot from the application showing the head of a lion sculpture from theCourt of Lions at the Alhambra Palace.

In order to make up for these deficiencies, the system should be able torelate the managed information with its spatial location as well as implement amechanism to perform queries based on the information location. Additionally,the query results should be shown as spatial information.

Geographical Information Systems essentially work following this approach.However GIS can not be used for movable heritage items nor element that arenot deployed horizontally (as walls or facades).

In this paper we present a new software tool (named a Cultural HeritageInformation System) which allows information layers to be attached to the sur-face of any cultural heritage artefact. Layers can be rendered, edited, copied,queried interactively or combined, in the same way as a GIS. Figure 1 shows ascreenshot of our implementation of the system (Chisel). Our system is not aGIS, and does not use a GIS, it performs like a GIS.

The rest of this paper is structured as follows: Section 2 presents previouswork, Section 3 introduces the basic ideas upon which our system has beenconstructed and explains its basic functionality, and Section 4 describes a simpleexample of its application.

2 Previous work

GIS has become an standard for archaeological research. A comprehensible re-view of its use and a case study can be found in [Kata09]. But, as have beenargued on the previous section they are not adequate to study artefacts.

Data management systems have been successfully used to organize culturalheritage repositories, on which the main goal is to be able to retrieve an artefactfrom some of their metadata. One remarkable work on this area has been done

by Felicetti and Niccolucci [Feli11], who presented a management system buildusing open source components. This approach can be used basically to browseand search element collections.

Many attempt have been made to add annotations and external informationto 3D model. Most of them require to segment the model, that is to decide towhich section of the model it is possible to link the information. Very frequentlythis process must be done as a previous process, preventing to create new as-sociation dynamically. This approach is appropriate for museum, or for systemswhose purpose is to show a previously processed information, but not for re-search. For instance, ViSMan is an open-source visualization framework thathas been used for virtual reconstructions and data management in archaeology.Its allows to link external documents to 3D landscapes enabling its conceptual-ization [Diam10].

Perhaps the most flexible proposal of a segmented labeling system has beendone by an international group including VCL from Italy. Their system allowsto do annotation on different multimedia objects, using an uniform way to defineareas on different multimedia objects [Pena11].

3 Structure of the Cultural Heritage Information System

The main difficulty when designing a system that associates information layerswith the surface of an object is to establish mapping applications between layersrepresentation and surface points. While GISs use projections to perform thismapping, this is not possible for our system as the geometry of every artefact willbe different and their boundaries may constitute complex surfaces. The mappingfor Chisel is obtained by partitioning the surface of the artefact into cells andassigning an unique integer identifier to each one. This allows information layersto be represented as sequence of attribute values. Each cell is assigned the valuethat is stored at its corresponding position in the sequence.

Figure 2 shows a 2D diagram of the partitioning process. The space inter-sected by the surface is divided into a set of non overlapping cells of equal size.The interior part of the object is not indexed. The cells are assigned a uniqueidentifier (figures inside the cells) and layers are stored as a sequence of values.Cell identifiers can be assigned in any order, but it is important that they arefixed for each object. This allows data to be assigned to the surface and makecorrespondences between different information layers. The diagram in figure 2 is2D. In a real 3D case the cells are 3D boxes, but anyway identifiers are integers.This means that layers are a one dimensional arrays of values.

It should be noted that although the cells are all the same size, the area of thesurface within each cell is different. That is, cell size determines the maximumarea of the surface that will be assigned a value.

This representation allows raster layers to be managed flexibly, in a similarway to a GIS raster map. The simplicity of the representation allows to imple-ment complex operations in a simple way.

Fig. 2. Assigning identifiers to cells allows property layers to be defined as a one-dimensional array of values.

Layers can hold null values, implemented as a bits array, with the same sizeof the layer array, indicating whether the stored value is valid. It is possible toassociate layers with a database table in order to manage non-numeric data. Inthis situation, the layer array contains the primary key for the record associatedwith the cell. The application can deal with text, dates, numbers, images, filmsand any other document as a database record field.

4 System functionality

This section describes the functionality of the system, providing a general overviewof its capabilities.

Creation of the models The system has been designed to create representa-tions of artefacts from digital models generated using a laser scanner. It can reada 3D model stored using the PLY format, an open format that was specificallydesigned to store three dimensional data of 3D scanned objects.

The application can automatically import the surface description of an arte-fact from a PLY file, computing its cell decomposition and its topological rela-tionship. The user must specify the size for the cells. The prototype can managerepresentations of objects with a cell size of 2 mm for an object whose dimensionsare over 1 km.

The representation of the Chisel models contains three connected compo-nents: geometry, data layers and database. When the model is created froman external file it contains only geometric information. The user can create in-formation layers in several ways: interactively, from geometric information orfrom previously created layers. Figure 3 shows an overview of the applicationstructure.

Fig. 3. Schematic overview of the system. The representation of the Chisel modelcontains three connected components: geometry, data layer and database.

Layer rendering Layers are rendered as colour applied to the geometric model.The user assigns a colour table to every layer, and is able to select the orderedsequence of layers that will be rendered at any moment as well as whether theoriginal model texture will be visible under the layer colour.

Queries Users can obtain the property value assigned to any surface point,on any given layer, by simply clicking on the point. The input information isa surface point, and the output information is the associated record on thedatabase. This kind of query operation is similar to that which is available onmost labelled 3D models.

Our system also allows SQL queries to be made, whose results are shown as3D visualizations of the set of points satisfying the query condition. In this case,the input is a condition and the output is a subset of the surface.

To illustrate this, Figure 4 shows the result of a query on the ceiling of the“Hall of the Kings” in the Alhambra Palace, where a search was made for areasrestored by ‘Ramon’.

Layer operations. Attribute layers can be combined and transformed gener-ating new layers. These operations are used to analyze the information. Chiselincludes the following operations:

Transformations: operations that apply a function to the attribute values (inother words performing a recodification of the attributes).

Mathematical functions: the powerful r.mapcalc operation defined in GRASSGIS has been implemented in Chisel [Nete08]. This allows new layers to begenerated by specifying cell values using arithmetic and logical operations onpreviously defined layers.

Null manipulation: null cells can be assigned a value, and null values can beassigned to cells with specified values.

Fig. 4. Interactive query on a model of the ceiling of the“Hall of the Kings” in theAlhambra Palace.

Interactive edition. The most obvious way to introduce information to thesystem is to create information layers. Layers can be created interactively de-scribing the structure of the associated database table and editing the layer val-ues on the 3D model. The editing process comprises an interactive “paint-like”operation: in other words, the user selects the areas of artefact with a specifiedvalue by simply “painting” them with a brush.

Geometric layers. Analyses can also be made involving geometry. To do this,Chisel includes operations to compute information layers from geometry:

Curvature. This function generates a layer by assigning a curvature value toeach cell. Curvature is computed as the inverse of the average radius of tangentspheres on the surface at the cell and the surrounding cells.

Roughness. The resulting layer contains the average deviation of neighboringcells to the tangent sphere for each cell.

Distance field. The layer assigns to each cell its distance (along the surface)to the non null cells of a given layer or to a point on the surface.

Orientation. The output layer represents the orientation angle between thenormal vector at the cell and a given reference vector.

Normal. The output is a set of layers containing the components of the normalvector at every cell.

Reports. Most of the operations that can be performed generate a new layercontaining a result which can be rendered on the 3D model. This is convenientfor showing distribution values, but sometimes quantitative results are needed.Report tool generates information about the surface that is covered by eachattribute value.

Although this operation accepts only one layer as input, complex analyses canbe performed which combine more than one layer using the previously describedoperations.

5 Case study

We present here a simple application example working with a fragment of anIberian vessel (see Figure 5). This piece has been restored from several frag-ments that was glued together. The example try to analyse the curvature of thefragments border in order to study its erosion. The 3D model was acquired byscanning the object using a Konica Minolta Vivid-910 laser scanner. Using thisdevice we were able to obtain a triangular mesh with an accuracy in the order oftenths of millimeters. Using this geometric information as input, the applicationgenerated a 3D object with the cells size of 0.05 mm (See figure 5 top left).

Fig. 5. Case study: Analysis of the edge curvature of an Iberian vessel. See section 5for explanation.

Once the piece has been loaded by Chisel we create a segmentation layer thatidentifies every fragment. That is the category for any cell is the identifier of itsfragment (Figure 5 top center).

A second layer is created interactively identifying cell that are on the frag-ments edge (Figure 5 top right). From this layer a buffer is created extendingthe edges to the adjacent areas. This has been done computing a distance fieldfrom the edges and selecting the cells whose distance is bellow 0,5 mm.

The buffer layer is combined with the previously created fragments layer, cre-ating a fragment edge layer. Its cells have the fragment identifier if they are on theedges, and null otherwise (Figure 5 bottom left). This combination is done usingan algebraic expression: EdgesID = if(isnull(buffer), null(), fragment).

Now we compute a curvature layer (Figure 5 bottom center), and them com-bine the curvature with the buffer layer, generating a new layer whose value is

the curvature on the buffer area (Figure 5 bottom right shows the result for onefragment).

Finally the report of these layers gives the surface of each edge with everycurvature value.

6 Conclusions

In this paper we have presented an innovative approach to Cultural HeritageInformation Systems. This new strategy is based on the direct association oflayered data with the surface of an object, allowing the management, visualiza-tion and analysis of any kind of information (spatial or non-spatial data) relatedto any cultural heritage artefact or archaeological site.

Following this approach, an application has been developed, which runs ona personal computer. The application is being tested on real work at two em-blematic cultural heritage sites in Andalusia: The Alhambra Palace in Granadaand the Roman city of Italica, close to Seville. The main goal of these tests is toascertain the effectiveness of the proposed system at a conceptual level.

Acknowledgements

This work has been founded by the Andalusian Science Ministry (Consejerıa deInnovacion Ciencia y Empresa de la Junta de Andalucıa) under grant PE09-TIC-5276. All models are property of the Patronato de la Alhambra y del Generalifeand Museo de la Puebla de Don Fadrique.

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