## 5 Conclusions  
In this paper, we presented a free-form data-driven modelling method that aims to model buildings with complex shapes that are difficult to model automatically. The geodata used in this experimentation was obtained using a hybrid sensor, which is able to generate a georeferenced point cloud and produce nadir and oblique images. The proposed method \begin{table}
\begin{tabular}{c c c} \hline \hline
**Method** & **Model** & **Mean C2M [m]** \\ \hline \multirow{4}{*}{Free-form modelling by [PERSON] et al., 2020} & I & 0.2749 \\  & II & 0.2777 \\  & III & 0.3312 \\  & IV & 0.3495 \\  & V & 0.1745 \\ \hline \multirow{4}{*}{Free-form modelling proposed} & Tri-mesh & 0.066 \\  & Quad-mesh & 0.058 \\ \cline{1-1}  & D.S.1 & 0.029 \\ \cline{1-1}  & D.S.2 & 0.098 \\ \cline{1-1}  & D.S.3 & 0.088 \\ \cline{1-1}  & D.S.4 & 0.008 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of free-form modelling methods. was applied to some complex buildings located in the Old Town Centre of the city of Bordeaux, France. The fundamental advantages of the proposed method are:  
Modelling buildings with any kind of shape using a parametric method;  
The possibility of choosing which building facades to model and which to reconstruct;  
To build photorealistic 3D models through the use of hybrid sensors;  
To obtain light and manageable 3D models without loss of accuracy and precision. In order to present as complete a framework for modelling methods as possible, an analysis on the impact of footprint segmentation methods for buildings with complex roofs in model-driven modelling was performed. The segmentation was performed based on DSM, point cloud, nadir images, and OSM footprints. The comparison between the automatically extracted models shows that the building footprint segmentation that produces the most accurate model is the interactive segmentation based on nadir images. As shown in model 7, the model-driven method allowed a high level of accuracy to be achieved through the use of interactive editing. By comparing the results obtained from the free-form data-driven models (tri- and quad-mesh) with automatically produced model-driven models (from one to six), it was shown that the best automatic method for reconstructing buildings with complex structures is the proposed free-form data-driven method. However, the 3D reconstruction of an entire city must take into account the geometry of the buildings, which can be either simple or complex. Therefore, it can be deduced that the best strategy for automatically modelling an entire city is to use the free-form data-driven method and the model-driven method together and sequentially. According to this strategy, in the first step, large portions of the city (e.g., newly built districts) consisting of prismatic buildings and simple polyhedral buildings are automatically recognised and modelled with the model-driven method. Once the part of the city featuring simple geometries is modelled, in the second step, the complex buildings can be modelled in the data-driven free-form. The proposed strategy allows the modelling of large urban areas (model-driven method) and the detailed reconstruction of complex shapes (free-form data-driven method). Conceptualization, M.P., G.V., D.C. and V.S.A. ; methodology, G.V., D.C., M.P. and V.S.A. ; software, G.V., D.C., M.P. and V.S.A. ; validation, G.V., D.C., M.P. and V.S.A. ; formal analysis, G.V., D.C., M.P. and V.S.A. ; investigation, G.V., D.C., M.P. and V.S.A. ; resources, M.P. ; data curation, G.V., D.C., M.P. and V.S.A. ; writing--G.V., D.C., M.P. and V.S.A. ; writing--review and editing, G.V., D.C., M.P. and V.S.A. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. \"We wish to thank the reviewers for their suggestions. This research was carried out in the project: PON \"Ricerca e Innovazione\" 2014-2020 A. I.2 \"Mobilita dei Ricercatori\" D.M. n. 407-27/02/2018 AIM--Attraction and International Mobility (AIM1895471--Line 1)\".