## 4 Experiments and Results
### Chrysippus Head  
The _[PERSON]_ is a 1 st century AD bronze head (overall height of 14.5 cm) originally located in the library of the Forum of Peace in Rome and nowadays exposed permanently in the Museum of the Imperial Fora, in the near Trajan's Markets. This portrait of the well-known philosopher of antiquity has a largely oxidised surface, hence the characteristic green colour. This work of art, well known in antiquity, was digitised for research applications using photogrammetric techniques in 2012 (software: Photoscan 1.0 from Agisoft) as part of the European 3D-ICONS (3 dicons-project.eu) project and published in the European digital library (www.europeana.eu accessed on 22 November 2021). In 2021, the dataset was re-processed with the current photogrammetric tools (Metashape 1.7) and re-elaborated according to the PBR material surface representation paradigm (see Section 2). The model creation workflow then focused on the creation of albedo, roughness, metalness and normal maps with a resolution of \(4096\times 4096\) pixels (see Figure 16). The photogrammetric survey produced a model with a high geometric resolution (91 million polygons for an object of 14.5 cm in height, see Table 1). The optimisation process that was followed involved the creation of a simplified mesh (53 k polygons) on which the geometric information was reported through the use of a normal map. Through the use of reference images of the original object, taken under different light conditions (grazing light, direct light, etc.) and through chromatic filtering operated on the albedo map, the metalness maps (the green areas are equivalent to oxidised material, while the remaining areas are the visible intact bronze) and the roughness maps (extraction of the micro-relief from the colour information contained in the albedo) were obtained. The albedo was obtained by means of delighting algorithms (software: Metashape de-lighter 1.7) starting from the diffuse channel already obtained by transferring the photographs onto the geometry within the Metashape photogrammetry software. Results  
The PBR model made it possible to address a very hot topic in the field of virtual reconstructions of antiquity, namely the difference in perception of the artefacts by ancient versus contemporary audiences. As already mentioned, originally the Chrysippus was located inside the library of the _Forum Pacis_ while it is now inside a museum case in the nearby museum of [PERSON]'s markets. The two different locations dramatically influence the lighting conditions and, consequently, the chromatic perception of this object (see Figure 17). In other words, placing the same PBR model within two different scenarios (ancient and modern contexts) results in two different visualizations. The potential of an online dissemination capable of conveying these nuances opens up new communication perspectives by guaranteeing a more realistic and scientifically validated perception of an ancient artefact. In ATON, two different scenes were created referencing the same 3D asset \"[PERSON] head\" in gIITF format (see Section 3.2) including all PBR textures. The two scenes allowed to simulate different scenarios and lighting conditions, modern lighting setup (A) and original context (B)--see Figure 18. For the first scene (A), a black background and a directional light were used to simulate the spot used to illuminate the bronze head in the Museum. For the second scene (B), an HDR panorama of the _Forum Pacis_ was employed to simulate original lighting conditions. A different setup with tinted panels was also created to assess multiple light-probes (see Section 3.6.2) arranged in a 3D scene with 3 instances of the [PERSON] head. Four light-probes per instance (a total of 12) were placed in the 3D scene (see Figure 19, top) in order to simulate local reflections and illumination.