## 3 State-of-the-Art  
The integration of digital photogrammetry, 3D modelling, HBIM and XR in one digital workflow is opening an entirely new line of research based on the development of reverse engineering processes oriented to the generation of complex buildings in XR environments. Unmanned aerial vehicles (UAVs) paired with structure-from-motion and photogrammetric workflows have become widely used in different application fields (e.g., mapping, forestry, precision farming, etc.). The possibility of collecting photos collected by different platforms, UAVs and terrestrial, is opening new perspectives to reconstruct Cultural Heritage (CH) sites. The chance of matching UAVs and terrestrial data is connected to the possibility of acquiring oblique images in which the camera axis is intentionally angled \(\geq\) with a 5.0\({}^{\circ}\) from nadir. Integration of oblique images into the photogrammetric workflow for mapping complex scenes is not a new issue and has been widely studied for urban applications to obtain complete coverage of both roofs and facade features and elevated elements (e.g., bell towers, etc.). The integration of oblique images and terrestrial images has been widely studied in recent works [28]. Two main approaches are usually employed to process images acquired with different platforms. Image datasets acquired with different platforms are independently processed in the first approach: oriented, georeferenced and dense point clouds are generated. The obtained point clouds are then aligned and merged either by using common ground control points or using cloud-to-cloud solutions like ICP [29]. The main drawbacks of this procedures are misalignment of the clouds, extracted point cloud with different accuracy in the overlapping area, colour and texture mismatch. The second approach processes the images acquired by the various platforms in a combined way. From a theoretical point of view, this solution presents significant advantages with respect to the previous one reducing misalignment issues. However, the final derived results may be quite noisy due to different data resolution and inaccurate matching due  
Figure 1: The proposed digital workflow applied to the research case study. to large viewpoint and colour changes. Due to this reason, the definition of optimal workflows for processing oblique UAVs and terrestrial images is still pending. The use of oblique images in UAV-terrestrial photogrammetric workflows and their processing into a unique step has been shown to improve resulting outputs. However, applications in literature are documenting quite large documentation in the optimal workflow both in data acquisition and specifically for oblique camera angles and orientation. This paper will analyse three different workflows using commercial software for oblique and terrestrial image orientation. On the other hand, the complexity of the structural elements of built heritage needs new advanced methods to simplify the model generation and orient them to new forms of human-computer interaction and XR. These innovative solutions enabled the management of a significant amount of information also related to HBIM and XR through respect for four holistic requirements:  
1. Parametricity,
2. Interoperability,
3. Orientation and
4. Virtual interactivity. As is well known, BIM for heritage buildings (HBIM), favoured the storage and the information sharing of different types of data through tables, graphs, schedules, computing and cost analysis of each part of the building. The main advantage of this modelling approach is the parametricity (first requirement) [3,30]. This requirement of BIM applications aimed at the generation of intelligent digital models with multidimensional connections (building time management) and bidirectional relationship between 3D objects (morphological aspect) and database (typological characteristic), favouring the preservation projects of heritage buildings [29].