## 2 Materials and Methods  
Mixed reality techniques enable archaeologists to perceive a final composite scenario constituted by two settings. On the one hand, video cameras installed in mobile devices capture the real environment. On the other hand, 3D software modeller scripts automatically produce digital 3D models displaying the previously unearthed finds, excavation landmarks, sediment samples and geological layers.  
To showcase the applicability of our mixed reality application, we chose the site of Les Cottes, located in west-central France [24]. Les Cottes consists of an artificial indoor underground excavation: \(10\times 12\times 4\) m. Such compact dimensions facilitate the implementation of a mixed reality approach [25]. The Les Cottes cave preserves occupations dated to circa 50 to 35,000 years ago [26], divided into nine stratigraphic units spread over a sequence measuring up to 4 m in depth (see Figure 2).  
Figure 2: View of the Les Cottés cave (**a**). Plan view (**b**) and oblique view (**c**) of the excavation at the foot of the cave (a 3D model of the excavation area is available in [27]).  
To create a mixed environment that will enable the operator to move freely and interact therewith, we included two computational platforms into the architecture of the Les Cottes mixed reality system. The hardware platform comprises an Apple(tm) iPad 6 th generation tablet. The software platform consists of four components with their respective processes (see Figure 3):  
* A _database software component_ that extracts the relevant information from the excavation database (DB) to standardise it and produce external data compatible with the platforms.
* A _mesh-creator software component_ that automatically creates 3D models (meshes) by relying on the above-mentioned normalised data that reflects the positional characteristics of artefacts, landmarks, sediment samples and geological layers encountered during archaeological work.
* A _locator software component_ that receives such virtual models and combines them with real-world features to position them while respecting the volumetric profile of the physical context.
* A _visualiser software component_ that interactively shows the combined environment in real-time and provides display and analytical facilities to interact with it.  
Accordingly, the software platform organises these four software components into three distinct digital environments or layers:  
* The DB component in a _DB management layer_ that uses the C# language enclosed in the Microsoft(tm) Visual Studio for Mac framework (v. 7.5 build 1254 integrated development environment (IDE));
* The mesh-creator component in a _3D modeller layer_ that utilises a script in the Python environment (v. 3.5.3 integrated script interpreter) in Blender (v. 2.82);
* Both the locator and visualiser components in a _software development kit (SDK) layer_ that employs, on the one hand, Xcode (v. 11.4 11E146) and Swift (v. 4.2) for programming, and, on the other, an operating system (iOS 13) for its execution on the iPad.  
Both hardware and software platforms interact through a predefined software tool, namely SDK [28] (ARKit v. 2.0 for the current implementation), which represents the predefined instrument that supplies mixed reality functionalities. These comprise (a) the rendering facility that combines the real-world scenario with the virtual one, (b) the tracking facility that localises and anchors 3D models in the composed scene and, finally, (c) the displaying facility that visualises the composite environment.  
Figure 3: Les Cottes (CTS) mixed reality software platform.