## 3 Iconostasis of the Serbian Orthodox Cathedral Church of Saint Nicholas in Sremski Karlovci
### Photogrammetric Surveying  
The 3D model of the iconostasis is created using photogrammetry approach. In order to obtain highly accurate 3D reconstruction results in terms of the relationship between the texture resolution and ground sample distance (GSD), it is necessary to design the on-site surveying plan. Ground sample distance is the size of the image pixel expressed in an object scale. Taking into account an object scale, a suitable level of detail of the 3D reconstruction can be defined by the GSD. The GSD has to be smaller than the smallest details the model should hold. Historic England [29] defined the maximum permissible GSD values for the different output scales of an ortho-image derived from photogrammetry. Figure 1: Block diagram. Designing the surveying plan for the data acquisition implies reconnaissance of site conditions and constraints, in order to determine preferable camera locations. By determining an appropriate GSD value, the shooting distance (distance from the camera to the object), the baseline (distance between two camera locations), and geometrical camera parameters (focal length) can be calculated according to an object site conditions and camera sensor size. We used a camera with a large sensor to avoid diffraction and signal-to-noise artefacts which decrease the image quality. In site image exposure parameters, such as aperture, shutter speed and ISO settings should be set in order to achieve the highest image quality and sharpness. The surveying plan is designed taking into account the specific iconostasis shape, distinguished by significantly lower object depth in relation to its width, as well as the church interior conditions, characterized by a very low illumination. The Aperture priority mode was employed to determine the correct exposure time. The surveying was performed using a tripod and a D7000 camera (NIKON, Tokyo, Japan, pixel number: 4928 \(\times\) 3275; pixel size: 4.78 \(\mu\)m; sensor size: 23.6 \(\times\) 15.6 mm; focal length: 18-109 mm), equipped with a wide-angle lens of 24 mm focal length. By taking advantage of unrestricted shooting distance, the GSD value was set according to the desired level of detail of the 3D reconstruction. The GSD value was 1.5 mm at 13 m shooting distance and the baseline of 50 cm. After the 3D model reconstruction, the overall accuracy of the model was 2.6 mm. The data were processed using the photogrammetry software based on the depth-maps method. The mesh was generated on the basis of both the point cloud and depth maps. For the further application, the mesh generated from depth maps was used, since it provided a better detail representation, by means of the more detailed 3D model with closed holes, at the same time containing far fewer polygons. The mesh generated from the depth maps is illustrated in Figure 2.  
The data acquired by the surveying process are images of an observed object, output usually in JPEG, TIFF or RAW format. Recording both JPEG and RAW files is beneficial, since the RAW format preserves the whole dynamic range offered by the camera, and allows image post-capture alteration without affecting the original data [29]. When it comes to the 3D model texturing, retaining the RAW format is useful as it allows us to readjust an image exposure with no loss of quality. In terms of the data processing and 3D reconstruction workflow, the software follows the common pipeline which consists of the following main steps:  
Figure 2: The mesh generated from depth-maps. - Sparse Point Cloud and camera poses)
* Tie points optimization (optional)
* Dense Cloud reconstruction (and/or Depth Maps generation)
* Dense Cloud optimization (optional)
* Mesh build (from Dense cloud or Depth Maps)
* Texture generation  
The very first step is the multi-image alignment. It implies feature detection, matching point pairs, and triangulation. The object points in 3D space are derived by triangulation of the image rays projected from at least two, but ideally more images.