# Digital 3D Technologies for Humanities Research and Education: An Overview
## 7 3D Modelling Technologies
### 3D Digitization  
#### 7.2.1 Surface  
Various large-scale projects on 3D modelling in cultural heritage primarily focused on 3D digitization from contemporary survey data (c.f. [374,403,404]). As mentioned in Section 3.3.3, a major distinction is between active systems such as laser scanning and passive systems like photogrammetry from photographs. Today, both approaches are frequently combined to obtain ever more accurate and multiresolution digitization [84,318]. Most approaches lead to a 3D point cloud, which has to be calculated during the meshing step into a polygonal mesh, corresponding normals (as information about the orientation) and--in the case of photography-based sensing--radiometric information.  
**Photo-based approaches:** Passive approaches to surface modelling are primarily based on the use of images. Due to progress in both the quality and availability of camera hardware and software for image analysis [403], most digitized models are now acquired via photogrammetry. A well-established method is 3D modelling of architectural heritage from current, native digital photographs or video content via photogrammetric algorithms. The development of different feature matching methods [405,406] and the integration of those into the structure-from-motion workflow has led to extensive use (e.g., [164,407,408,411]). Today, a large variety of image spatialization algorithms are available [82,412,413], and photogrammetry is used in various settings such as 3D underwater archaeology [414], landscape surveying [415], and monument recording [416]. While currently extant approaches achieve remarkable quality for natively digital imagery, they often fail with historical imagery due to the demand for high quality and numbers of input images [409,417,418]. Consequently, current approaches often reach low quality or fail to process historical, non-native digital imagery due to sparse samples, missing metadata, incompleteness, and radiometric quality issues [406]. These are not only technical but fundamental barriers, and despite much research, current photogrammetry dealing with historical images [419,420], e.g., re-photo approaches [421,422], still requires much manual processing. Initial attempts to advance photogrammetry from historical imagery use video [423] or--invented by my group--architectural feature-based photogrammetry [424], and features derived by neural networks [425] seem promising avenues for further automation, but still require a great deal of previous optimization and large samples.  
**Range-based approaches:** Similar to photogrammetry, range-based approaches such as airborne, terrestrial, or close-range _laser scanners_[81,426,427] are most frequently used to acquire tangible heritage. All systems use laser beams sent to the surface to retrieve 3D information, e.g., via triangulation, pulse (_time-of-flight; ToF_), or _phase-comparison_[367].  
Figure 14: Data sources for 3D modelling projects (Review of \(n=208\) project publications, 1998–2011).  
Robust and fast scanning systems have been available since the mid-2000s and enable time efficient and high-resolution 3D scans [428]. Laser scanning was the primary acquisition method until 2010, but its relevance declined with advances in photo-based techniques [429]. Despite this, laser-based approaches are still of high relevance, especially to document large-scale objects in a short time and for specific material properties such as high reflecting surfaces [359; 427; 428].