Abstract

Stecci are hand carved medieval Bosnian gravestones considered as valuable cultural heritage objects. One of the best ways for their preservation is digitization. In this paper we compare digitization results and procedures for one of the most famous stecci - The Stecak from Donja Zgosca. The object was first digitized using a Minolta 910 laser scanner. Later we created the 3D model from photos using photogrammetry and improved it in 3ds max. We present advantages and drawbacks of these two procedures and characteristics of the obtained models. Results of this comparison will be used in future digitization projects.

Introduction

Cultural heritage within its set of materiality, traditions and knowledge helps us to better understand the past itself. Therefore it is very important to preserve these monuments in a way we see them now for next generations. Thus, new technologies can be very helpful. Today we are able to create virtual model of a real object using various techniques. We used two different techniques in our research, expensive technique of laser scanning and much cheaper, but also good technique for acquisition of 3D models from 2D images, photogrammetry. We also made a step forward and introduced some improvements of the model achieved using photogrammetry , what we will see in more details later in this paper.

Stecci are hand carved medieval Bosnian gravestones. We applied both techniques on The Stecak from Donja Zgosca, and got some interesting results. This stecak originates from the second half of 14th century. It has a great importance for Bosnian history because it is assumed that the Bosnian king Stjepan II, who died in 1353, was buried under this stecak. This monument is currently located in the botanical garden of the BH National Museum in Sarajevo.

In many cases, like with The Stecak from Donja Zgosca, the traditional modeling (for example manual modeling using 3ds Max or Maya) would require much more work and effort, and the final result would not be satisfactory. The model created using these methods would not be sufficiently accurate. It would not contain enough information about the real object. This is why we use methods such as 3D laser scanning, which produces a virtual model with high accuracy. In the first part of our research we used a Minolta 910 laser scanner, borrowed from our project partners, the University of Bristol, UK.

In the second part of our research, we used photogrammetry technique for virtual reconstruction of the stecak. In this project we used the Photomodeler software for implementation of photogrammetry technique with manual approach of creating 3D model. The model was later improved in 3ds max software.

The paper is organized as follows: the Section 2 gives a short overview of the related work in similar projects, in the Section 3 we describe the laser scanning approach, applied on the stecak. In the Section 4 we describe the process pipeline of photogrammetry combined with manual 3D post-modeling of the stecak model, and steps for its implementation. In the Section 5 we compare results achieved using these two approaches, and describe advantages and drawbacks of both techniques. In the Section 6 we present conclusions based on our experience from this project, and illustrate reasons why to use one technique instead of another in future projects.


The Stecak from Donja Zgosca

Figure 1: The Stecak from Donja Zgosca

Laser Scanning technique

Laser scanners provide a method of capturing accurate information about object's surfaces. The stecak was scanned with a Minolta 910 laser scanner. It is a scanner for close range and indoor applications. This scanner has accuracy of less than a millimeter. Individual scans, created by laser scanning method, were later connected together in a polygonal mesh by using the Stitcher software tool provided with the laser scanner. After mergin scanned mashes in one "watertight" mesh, model was imported in Maya software, where new material on object was applied.

The Stecak created using laser scanner - without textures The Stecak created using laser scanner - with material created in Maya
Figure 2: The model of the stecak before material application (left); Final model of the stecak after material application in Maya (right)


We used MeshLab tool for optimizing the stecak's model created by laser scanner. Figure 3 shows the unoptimized version of the model in MeshLab environment.

Unoptimized version of the stecak's model in MeshLab environment.

Figure 3: Unoptimized version of the stecak's model in MeshLab environment.


Photogrammetry technique

Photogrammetry technique was used for recovering 3D information from the stecak's photos, taken on site. The photos are imported in Photomodeler and we used manual photogrammetry approach for 3D model creation. Figure 4. shows the final model of stecak created in Photomodeler using manual photogrammetry technique. We can see that stecak's surfaces are mainly flat and we used post-modeling in 3ds Max, as next step to make relief on stecak flat surfaces.

The model of stecak created using Photomodeler software

Figure 4: The final model of stecak created using Photomodeler.


Photogrammetry technique in combination with post-modeling in 3ds Max

Images below presents the final model of The Stecak from Donja Zgosca created using photogrammetry method, and later improved in 3ds Max software. This is maximum achieved quality of model improved using post-modeling process.

Stecak improved in 3ds Max - view 1

Figure 5: The model of stecak created using photogrammetry
and later improved in 3ds Max - View 1

Stecak improved in 3ds Max - view 2

Figure 6: The model of stecak created using photogrammetry
and later improved in 3ds Max - View 2

Stecak improved in 3ds Max - view 3

Figure 7: The model of stecak created using photogrammetry
and later improved in 3ds Max - View 3

Stecak improved in 3ds Max - view 4

Figure 8: The model of stecak created using photogrammetry
and later improved in 3ds Max - View 4

Comparison of laser scanning and photogrammetry technique combined with manual post-modeling

We compared these two techniques by various factors. Some of the achieved results of this comparison are shown in the table below.

The Stecak created using laser scanner - with material created in Maya

Figure 9: Comparison of LS (laser scanning) and PP (photogrammetry in
combination with manual post-modeling) techniques by various factors

Author

Author: Goran Radosevic, email: goran.radosevic@hotmail.com

Supervisor: Selma Rizvic, email: srizvic@etf.unsa.ba

Digital catalog of Stecaks

We used the obtained model as a part of our project "Digital Catalogue of Stecaks". This project is a virtual museum of stecci from the collection of the National Museum of Bosnia and Herzegovina. The models created using photogrammetry combined with post-modeling work well considering all limitations of this online application.

You can visit Digital catalog of Stecaks website, for more information about this project and virtual models of stecci.