**4. Results**

The problems that are described in the previous chapter, the authors encountered in the practical implementation of 3D models of existing GO. These are four sacral buildings in the Czech Republic (CR): the church of saints Johns of Brno-Bystrc (Figures 3 and 4), the Church of St. Paraskiva in Blansko (Figures 4 and 5), further the Strejc's Church in Židlochovice and Church of St. Peter in Alcantara in Karviná city. All of the buildings were geodetically surveyed by the terrestrial method while using total station. The 3D model was created in both cases in the program AUTOCAD v. 18 and MicroStation.

**Figure 3.** Photo of Church of St. Jonh the Baptist and John the Apostle in Brno-Bystrc district (CR)-zoom.

**Figure 4.** Three-dimensional (3D) model of Church of St. Jonh the Baptist and John the Apostle in Brno-Bystrc district (CR).

**Figure 5.** Photo of wooden Church of St. Paraskiva in Blansko (CR)–zoom.

Geometric inaccuracy became evident when transforming the measured points from the terrain into suitable software (AUTOCAD) in several details. Of all cases, we will show as a demonstration example in Figures 3 and 4 during the 3D model creation of church of St. Jonh the Baptist and John the Apostle in Brno-Bystrc district (CR). Figure 3 shows a detail of the rotunda with the upper and lower arches indicated. Both arcs actually have different radii of curvature. When creating a wireframe, it was not possible to connect these arcs with vertical edges in AUTOCAD. The connection of the same edge to the lower arc was disconnected and vice versa when joining an edge to the upper arc. It was necessary to generalize the model in order to connect both arcs with vertical edges. The result after generalization is evident in Figure 4. The disadvantage is that similar inaccuracies limit the further practical use of the 3D model, e.g., making sections for the purpose of object reconstruction etc.

Problems with visualization are demonstrated on the 3D model of the Church of St. Paraskiva in Blansko (CR). In general, it involves laying textures or patterns with the real appearance of the building material used (e.g., roof tiles or shingles) on 3D model surfaces. In existing programs, this is only possible when the surfaces are planar. However, for real objects, the surfaces of buildings rarely meet that requirement. Most of these are general areas in space—see Figure 5. In this case, it is very difficult to apply realistic textures to these surfaces. Usually, it is necessary to decompose the surface into a numerous of planar patterns and then cover them with textures or photographs. Usually, triangles or planar quadrilaterals are used. However, this process is very time-consuming and laborious and in many cases the result does not correspond to the exerted effort. Figures 5 and 6 show the difference.

**Figure 6.** Visualization of 3D model of wooden Church of St. Paraskiva in Blansko (CR).

A similar situation can be seen in the models Strejc Church in Židlochovice—Figures 7 and 8—and the Church of St. Peter of Alcantara in Karviná (CZE)—Figures 9 and 10. The visualization of the Strejc Church (Figure 8) was done while using standard tools in the AUTOCAD program. The difference between the actual situation (Figure 7) and the display in AutoCAD (Figure 8) of the two images is obvious. For another 3D model of the Church of St. Peter of Alcantara in Karviná—Figures 9 and 10—visualization was performed while using own set of textures. The result is evident from the comparison of Figures 9 and 10. The visualization quality of the 3D model is many times higher than that of the Strejc's Church (Figures 7 and 8). However, the bases of both visualizations are the same—the textures in both models were placed on generalized (planar) surfaces. If we left the model without generalization, one of the methods described in [24] or [25] would have to be used for visualization.

**Figure 7.** Real view of Strejc's Church in Židlochovice (CR).

**Figure 8.** Visualization of 3D model of Strejc's Church in Židlochovice (CR).

**Figure 9.** Photo of Church of St. Peter of Alcantara in Karviná (CR).

**Figure 10.** Visualization of 3D model Church of St. Peter of Alcantara in Karviná (CR).

The described problems occur in all of the 3D models of real objects that the authors have encountered. These examples are only selected typical demonstration examples.

#### **5. Discussion**

Practically, the above-mentioned problems manifested in two aspects:


The solution of the above problems is dealt with in several works, of which the most important ones are mentioned.

The Geometric accuracy of the model is further explained in publications [11,18,23]. In paper [11] the geometry of the 3D model is complemented by a knowledge database that was obtained from a real object. It includes, for example, a tree that captures the GO topology. The article [18] is dealing with the problem of the precision issue with hybrid modeling. The elements of the building are extracted and then formally saved to the library as an object for further use. Another solution is presented in [23]. It is a complement of the list of coordinates with topological codes directly when surveying the object in the field. From the list of point coordinates, the exact drawing in the CAD program is then automatically displayed while using a Python script.

Visualization quality is discussed in [16,21,24,25]. The article [16] presents a visualization of historical objects that were captured by photogrammetrically special fish-eye camera. The object is measured by laser scanning. An interesting approach to surface modeling is described in [21]. The object's surface is approximated by curves that are defined by randomly selected points from the point cloud from laser scanning. Curves are created by special VB.NET applications in the AUTOCAD program. The work [24] proposes a method of classification of the surface of a 3D object that is based on the skeleton metric of this object. The result of the classification is a set of classes of segments that can be used for the whole surface. The most appropriate display method is then selected for each class. The publication [25] presents a design of the CatSurf system for displaying 3D objects in CAD (Computer-Aided Design). It is the surface texture information system, which is a part of the integrated CAD surface texture platform. The disadvantage of the latter two applications is that they are highly specialized systems that are difficult for ordinary users available.

### **6. Conclusions and Future Work**

The main problems for creating 3D models of existing GOs were identified and described. The experimental results show that the currently used CAD programs are relatively outdated in its repertoire when compared to the quality and possibilities of geodetic surveying of real objects. Current methods of data acquisition in the field use modern technologies that allow for surveying the object with high accuracy. In addition, data collection devices have a number of built-in features, which allow for people with basic training to use them.

The authors proposed a solution concerning the first part of the problem mentioned in Section 4, namely the geometric accuracy of the 3D model. A Python application was created to produce a wireframe 3D model from a list of coordinate points with topological codes in suitable software. This procedure will significantly speed up the whole process-see Figure 1 and make the work easier for users. The script can be added as a plug-in to CAD software.

Further research in this area will be focused on solving the problem of quality visualization of the 3D model of GO.

**Author Contributions:** D.B. has provided support materials, elaborated literature review and create system model, M.B. conducted an overall editorial of the whole article and a professional translation. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Grant No. FAST-J-19-5994 of the Brno University of Technology, Czech Republic.

**Acknowledgments:** We greatly appreciate the careful reviews and thoughtful suggestions by reviewers.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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