Documentation and metadata
The documentation of 3D content should primarily follow the principles of the London Charter, specifically Principle 4:
Sufficient information should be documented and disseminated to allow computer-based visualisation methods and outcomes to be understood and evaluated in relation to the contexts and purposes for which they are deployed.
As with many other digital datasets, this documentation can exist on a number of levels, particularly if the creation of 3D models is part of a larger fieldwork project involving other methods of data collection and analysis. For 3D models specifically, documentation and metadata can be used to record information relating to the model’s geometry, appearance, and scene information, as well as documenting derived objects such as image and video files.
At a minimum, all datasets – 3D or otherwise – will be part of a larger project and as such require top-level metadata regarding the focus, dates, and people and organisations involved. A generic Dublin Core project metadata set is described elsewhere in these guides (Project Metadata) and also within the context of 3D data in section 5.2 of the AHDS Virtual Reality guide (2002). This set of metadata should be applied once at a top-level to cover project information but could also be applied to discrete sub-sets of data where different collection activities have taken place. This is demonstrated elsewhere in the Guides to Good Practice where specific metadata sets are described for Laser Scanning, Photogrammetry, and CAD projects.
Workflow and processing documentation
Where 3D datasets are generated as part of a complex workflow incorporating multiple acquisition and processing stages, metadata and documentation should be broken into logical components according to the data chosen for preservation. Important information to record includes: the type of device used for producing the initial raw sampling; the software tool(s) used to process the sampled data; the type of processing applied to the reconstructed model (e.g. holes filling, surface smoothing, surface simplification, etc.). These elements are discussed in more detail in the guides on laser scanning, photogrammetry, CAD, and structured light scanning.
The 3D-ICONS Report on Metadata and Thesaurii (D’Andrea & Fernie 2013, 30, 49) describes in detail the application of the CARARE2 metadata schema to 3D datasets for workflow documentation and focusses on the relationship between various digital outputs in addition to the provenance and capture/digitisation process. The CRMdig specification (Doerr & Theodoridou n.d.) for provenance metadata was used by the 3D coform project to implement its repository.
The level of detail required to fully document complex workflows will vary depending upon the complexity and range of technique used. Part of this is inter alia the documentation of research resources that have been consulted in order to create the 3D model, the documentation of processes that have been passed through during its development, the documentation of applied methods and a description of relationships and dependencies between its different components.
In some cases, file formats will allow the storage of certain metadata within the file structure. Nevertheless, it is important that such metadata be recorded separate to the file and externally deposited so that elements can be checked against the file’s content. Boeykens and Bogani (2008) describe such sets of metadata in relation to the storage and searching of 3d models in online repositories.
The metadata listed below should be considered as the minimum required at the file level (in addition to that described in table 3 of the section on Project Metadata. These complement the metadata at project and process level discussed above.
Table 3: File-level metadata for 3D models (in addition to that described in table 3 of the Project Metadata section).
|Number of Vertices||The number of vertices (points) in the model|
|Number of Triangles or Polygons||The number of triangles or polygons in the model|
|Geometry Type||The type of geometry used within the model (wire frame, parametric, CSG, B-Rep etc. if applicable).|
|Scale||What scale is existent, resp. what is represented by 1 unit.|
|Coordinate System||Does the model use a real world or arbitrary coordinate system?|
|Master model or processed model||Is the model the master model produced just after raw data processing, or is it a derived model produced from the master (e.g. after hole filling, simplification, smoothing, etc) ?|
|Level of Detail (LOD); Resolution||How detailed is the model, what is the resolution of the scan.|
|Layers||Does the model use layers? How many?|
|Colour and Texture||Does the model contain colour or texture information? How is this stored? If raster texture files are used then these have to be archived separately.|
|Material||Information about the material properties of the model and whether they match the physical properties of the actual object.|
|Light Source(s)||Number and accuracy of light sources used in the model.|
|Shader||Have special or extended shaders been used?|
|Animation||Whether animation is used in the model along with description of type (keyframe, motion capture).|
|External Files||List of external files that are required in order to correctly open the 3D model (e.g. texture or material files and images for OBJ files).|
 See specification here: https://www.cidoc-crm.org/crmdig/fm_releases