3D scanning in Cultural Heritage: model creation and digital restoration of the tombstone of the bishop Vasilije Petrović (Negosh) in Saint Petersburg

3D laser and optical scanning techniques are widely employed in Cultural Heritage preservation for documenting pieces of different sizes and shapes. However, due to limitations of the methods available today, the results obtained vary according to different characteristics of the object under study. External surfaces showing very smooth features make the capturing process and the posterior processing a more elaborate task, requiring special attention and additional retouching to improve the final 3D model. Tombstones are a typical example of this type of objects, combining both flat external contours and refined outlines. In this work the procedure of creation of the digital model of the tombstone of the bishop Vasilije Petrović (Negosh) is described, a piece of art which combines both technical and historical importance. Post-processing steps required involved different software (Geomagic Design X, ZBrush, Adobe Photoshop) for surface correction and cleaning, details and photorealistic texture improvements and lost material restoration.


Introduction
3D laser and optical scanning devices are now more intensively used in everyday tasks when high precision and repeatability are required.Together with these two features, their non-destructive working principle make them invaluable tools in different fields in which to keep the conservation state of the object under study is the top priority, which is the case of Cultural Heritage (CH) field.Nowadays, the use of 3D laser and optical scanning devices in CH field is widespread in many countries around the world [1], being used in multiple tasks for pieces of different sizes, materials and shapes.The good results obtained and their relatively low cost allow to apply them in new and challenging applications in the area.However, the results obtained in the scanning procedure vary according to some factors including the technology used, ambient light, material and color of the object and others [2][3].In particular, the capture presents some issues when the surface under study is very flat, requiring special attention in both the capturing procedure and the post-processing tasks.These problems become more apparent when they are combined with very delicate details on the exterior, like the ones seen in carved stone, being the case of most of the tombstones where the effect of environmental factors [4] along the time made their inscriptions and drawings barely noticeable in extreme cases.In this article, we present a case study of 3D scanning procedure of the tombstone of the bishop Vasilije Petrović (Negosh), established in 1766 in Saint Petersburg, Russia.The 3D scanning procedure and the creation of a virtual model of it has two main features: one technical and the other historical.The first one, from the technical point of view, to evaluate the use of 3D optical and laser scanning techniques applied for documentation of CH objects can also be used in pieces with very smooth surfaces with good results and the challenges that the practitioners faced in their employment.The second characteristic of the work comes from its historical importance, as this tombstone is a landmark of the shared history between the city of Saint Petersburg and the country of Montenegro.

Object under analysis and brief post-processing tasks description
The 3D scan set obtained for this task corresponds to a procedure done at Alexander Nevsky Monastery (Saint Petersburg, Russia) on June 2021.The scanned object was the tombstone of bishop Vasilije Petrović (Negosh).It is made from limestone and its dimensions are about 220 x 108 cm.The front part of it is carved showing a coat of arms and inscriptions.The tombstone was established on 1766.The coat of arms has significant deterioration and some parts of the material are lost.The scanning procedure was done with a handheld structured-light based 3D optical scanning device developed by Artec 3D (Artec 3D, Luxemburg), model Artec Eva Lite.Light conditions where interior light without any special additional lighting.Two scanning procedures were done over the object, paying special attention to the coat of arms and the inscriptions.The first scanning procedure is composed by 22 shots (around 300.000 vertices and 650.000 faces each) and the second procedure contains 43 shots (around 200.000 vertices and 400.000 faces each).The format used to store the captures is proprietary, developed by the manufacturer of the equipment and was exported to STL format for further processing in different software programs for 3D models creation and editing.For the post-processing tasks different tools were used, among them Geomagic Design X (3D Systems corporation, USA), ZBrush (Maxon Computer GmbH, Germany) and Adobe Photoshop (Adobe Inc., USA).The first stage required a careful selection of the scans to be employed, cleaning of the surfaces for capture errors and noise, alignment of the different shots (rough alignment and automatic adjustment) and hole filling.After that, the coat of arms and inscriptions were digitally improved to increase the texture and obtain a clearer detail; some missing parts were restored.Finally, an image of the model was used as a texture to give a photorealistic effect to the work.

Historical importance of the object under analysis
Vasilije Petrović Negosh (Njeguši, 1709 -Saint Petersburg, 1766).He was the metropolitan bishop of Cetinje (Bishop of Montenegro) and co-ruled in Montenegro from 1750 until his death together with his cousin, Bishop of Montenegro Sava Petrović.Vasilije Negosh is a very important person in the history of both Russia and Montenegro [5].Under his ruling the foreign policy of Montenegro developed in a very dynamic way, and at that time the political reputation of the state was raised significantly.Because of the loyalty and alliance with Russia, in 1752 he was able to get affinity of that great empire, which results with the patronage of Russia over Montenegro, and since that event Montenegro is found under the protection of Russia.He is the author of the History of Montenegro, published in 1754, the first book dedicated to this matter.He traveled several times to Russia and in particular to Saint Petersburg, where he met with empresses Elizabeth Petrovna and Catharine II and other important political figures, looking to establish political and diplomatic alliances between the states.He died during one of his trips in 1766 in Saint Petersburg and was buried, with great honors and on the expense of the Russian Empire, at the Alexander Nevsky Monastery in St. Petersburg [6], and his remains still are there in the Annunciation Church.The tombstone of bishop Vasilije Petrović is the sacred object (its photo is presented in Figure 1).

Problems of the creation of 3D models of flat surfaces
As it was above mentioned, 3D optical and laser scanning devices have been very successful in the creation of digital models of real-world objects from very different sizes and shapes.However, nearly flat surfaces present special challenges both for the personnel in charge of the capturing process and the specialists that post-process the obtained raw data to create the final model.During every 3D data capture process, there are some important practical rules that must be followed to try to obtain good enough quality information that will be required for the next steps in the workflow.One of them is that the surface scanned in each shot should be overlapped with the ones surrounding it, to allow a successful alignment procedure.A rule of thumb says that the overlapped surface should be roughly 30% or more.However, in flat surfaces with not enough remarkable features, it is advisable to increase this overlap area that will allow the post-processing operator to better find reference geometry to join the different scans.The necessary added shots involve a new set of issues for the operator and the process itself: more data to be saved and processed, a more cautious capture process to avoid blurred shots specially when using handheld devices, different takes from different angles to try to better capture subtle details that can be used as reference points and, consequently, longer capturing sessions.Sometimes markers are added to the object's surface to help working with these problems, but this generates missing data in the zone that should be corrected by hand during the post-processing phase, a task that requires well-trained operators to avoid losing the original accuracy of the 3D optical scanning procedure.Post-processing a flat surface has also its complications that must be surpassed in order to create a 3D model.As was described before, the first problem that must be faced is the lack of reference geometry due to the shape characteristics of the object.This issue is not only difficult to solve for the expert doing what is usually called as rough alignment, in other words, arrange approximately the different parts using reference points in the surface, but also it presents complications for a successful fine alignment, usually done by algorithms like ICP [7], [8] which can be misguided by the lack of geometrically important features to use as reference for the calculations, leading to misplacements between the different scans.Also, noise reduction algorithms should be used very carefully in these objects, as they may remove important subtle details that are present in nearly flat surfaces, like inscriptions and carvings.

Creation of the model of the tombstone of the bishop Vasilije Petrović
In this section some of the most interesting aspects of the creation of the model are discussed.We will pay special attention to the digital restoration of the coat of arms and the reconstruction of the bottom part and borders, which was added in the post-processing procedure to give it a complete shape, necessary for 3D reproduction tasks and also for virtual collections.and the reconstruction of the bottom part and borders, which was added in the post-processing procedure to give it a complete shape, necessary for 3D reproduction tasks and also for virtual collections.According to the History of Montenegro by Marko Dragović: "this coat of arms was presented as a coat of arms of Montenegro to the Russian Government by Danilo Petrović, first Montenegrin bishop that visited the Russian capital in 1714-1715.The original was held in the main archive of the former Ministry of Foreign Affairs in Moscow" [9].During the first phase of the post-processing workflow Geomagic Design X software was used.In the beginning, a selection and sequentially rough alignment of the different scan shots taken was done.Using the common landmarks, the selected data was joined to create a unique mesh, which was cleaned from artifacts and noise.Finally, holes appearing on the surface were filled using different tools according to their size and position.The second phase was completed with ZBrush software.It involved the increase of the relief of the surface and correction of deformed zones, between them zones with lost material that was digitally added.Special attention was given to restore the original details of the coat of arms.Also, the texture of the material was improved using the same tools to obtain a clearer inscription and give a more realistic look.A final third post-processing phase was done using a combination of Geomagic Design X and Adobe Photoshop tools to add photorealistic details to the digital 3D model, where images of the created object and real photos of it were combined.This procedure helped to improve the general look of the finished piece, covering small imperfections from missing data of some inaccessible areas for the 3D scanning device.

Digital restoration of the coat of arms
This part of the object was especially important for the creation of the 3D object for two main reasons, the first one, its fine work made carved on the stone and second, for the same reason, it was one of the most affected areas of the whole piece, even showing missing material.A detailed view of the level of deterioration of it could be seen in Figure 2. To make clearer the original and now significantly deteriorated details of the coat of arms, it was decided to use a digital sculpturing software tool (ZBrush) that allows to subtle edit the geometry of the model, in order to keep the accuracy values of the final digital representation between the resolution values of the original capture.As the initial information for sculpting, we used ancient images of coat of arms of bishop Vasilije Petrović shown in Figure 3 [10].Figure 4 shows the difference between the original surface obtained from the already clean and denoised data and the improved final model is not higher than 1 mm in the worst case, keeping the quality of the scanning data and visually improving the results.

Bottom and edge creation of the model
In order to create a 3D model of an object that has some areas that are difficult to scan or inaccessible as in the case of the piece under study which is lying on the ground, post-processing tasks are necessary to complete its shape.For some tasks like state monitoring this information could be not useful, but as we said before, it is fundamental for 3D reproduction of the object (all objects to be reproduced must be watertight) and also to give it a complete look for virtual exhibitions.In the case under study, the information given by the bottom part and the sides of it is not important for the documentation task in terms of metrology, so this part was added digitally using a 3D model post-processing software (Geomagic Design X).In this way, the size of the faces and number of vertices of these parts were under control, keeping the file size low and reaching the completeness goal of the object.Figure 5 shows a comparison between the original scan information containing background information from the floor surrounding the tombstone and how this post-processing task was used to complete the missing information from the side and bottom parts to fully complete the volume.To complete the process, the irregular edges of the front part obtained by the scanner where first corrected and straighten and then the volume was extruded using the background information of the floor as a reference to know the height of the front part of the tombstone from the floor.

Conclusions
The process of creation of a 3D model of flat surfaces from 3D scanned data is a complex procedure which demands special attention in both the capture and the post-processing tasks and some of them require a manual editing of the geometry of the model.It should be noted that only well-trained experts can achieve the best results without affecting the original accuracy of the data obtained by the 3D optical scanning device.In this work we studied how these tasks can be solved and the case study of scanning the highly deteriorated tombstone of bishop Vasilije Petrović (Negosh) was a good object for such research works.The final result combines the high quality that characterizes the use of 3D scanning and the digital restoration of the missing details that were lost along the years, reviving its original fine handcrafting.It should be noted that outcomes of this project in addition to scientific result also have high practical importance since obtained 3D model of the tombstone can be used for its replication.

Figure 1 .
Figure 1.General view of tombstone of bishop Vasilije Petrović

Figure 2 . 3 .
Figure 2. Detail of the state of the coat of arms Figure.3. Coat of arms of bishop Vasilije Petrović

Figure 4 .Fig. 5 .
Figure 4. Comparison of the results of the post-processing tasks.Left: Detail of the inscription in the coat of arms before digital restoration.Center: Inscription in the coat of arms after restoration.Right: Before / after measured differences; maximum measured difference is around +/-1 mm, in red could be seen the zones in which material was added to complete the missing material from the original piece