## 2 Related Work
Particularly interesting is the support to the _PBR_ (Physically-Based Rendering) model, crucial within the Cultural Heritage [14] and other fields to simulate advanced materials at runtime by approximating the flow of light. Furthermore, Khronos also recently released PBR extensions ([[https://www.khronos.org/blog/using-the-new-gltf-extensions-volume-index-of-refraction-and-specular](https://www.khronos.org/blog/using-the-new-gltf-extensions-volume-index-of-refraction-and-specular)]([https://www.khronos.org/blog/using-the-new-gltf-extensions-volume-index-of-refraction-and-specular](https://www.khronos.org/blog/using-the-new-gltf-extensions-volume-index-of-refraction-and-specular)), accessed on 22 November 2021) for glTF to support volume-based absorption, refraction (see Figure 1, top right), and complex specular reflections to be used by diverse renderers, from real-time rasterization to production-class path-tracing. gITF data like geometry can be also compressed: _Draco_ ([[https://google.github.io/draco/](https://google.github.io/draco/)]([https://google.github.io/draco/](https://google.github.io/draco/)), accessed on 22 November 2021) is an open-source library developed by Google for compressing and decompressing 3D meshes and point clouds. It is intended to minimize the storage and improve the transmission of 3D models over network connections. There are several open-source tools to perform Draco compression, and built-in gITF exporters in 3D modeling software (like Blender) already provide compression options for content creators. The gITF format is rapidly spreading and is being largely adopted by many platforms due to its high interoperability and perspectives to address specific archiving challenges [15]. Several 3D modeling software tools (Blender, 3 DS Max, Maya, etc.) as well as game engines like _Unreal Engine 4_--can export directly in gITF (including PBR materials), thus boosting the web publishing workflow. Several institutions, including the Smithsonian, already published open-access 3D models using this standard ([[https://www.khronos.org/news/press/khronos-smithsonian-collaborate-to-diffuse-knowledge-for-education-research-and-creative-use](https://www.khronos.org/news/press/khronos-smithsonian-collaborate-to-diffuse-knowledge-for-education-research-and-creative-use)]([https://www.khronos.org/news/press/khronos-smithsonian-collaborate-to-diffuse-knowledge-for-education-research-and-creative-use](https://www.khronos.org/news/press/khronos-smithsonian-collaborate-to-diffuse-knowledge-for-education-research-and-creative-use)), accessed on 22 November 2021). Furthermore, with the latest specification, Khronos is also preparing gITF to be submitted for transposition to an international standard. _3D Tiles_ is an open specification built on gITF developed by Cesium [16] for sharing, visualizing and interacting with massive heterogeneous 3D geospatial content, high-resolution photogrammetry datasets or BIM [17] across desktop, web, and mobile applications [18]. The foundation of 3D Tiles is a spatial data structure that enables Hierarchical Level of Detail (H-LOD) so only visible tiles are streamed and rendered, thus suitable to maintain interactive performances. Single tiles can also adopt Draco compression to further improve transmission size. 3D tiles specification for tilesets, associated tile formats and the associated styling specification are open formats that do not depend on any vendor-specific solution, technology, or products. The open specification is being adopted by the community outside Cesium, including NASA that is developing an open-source 3D tiles renderer ([[https://ammons.nasa.gov/](https://ammons.nasa.gov/)]([https://ammons.nasa.gov/](https://ammons.nasa.gov/)), accessed on 22 November 2021) for AMMOS project (see Figure 1, bottom).