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Schema Implementation

The 3D Metadata Specification defines schemas as a description of the structure of metadata, consisting of classes with different properties, and enum types. The following is the description of a JSON-based representation of such a schema and its elements. It is used as the basis for defining the metadata structure in the following implementations:

  • 3D Tiles Metadata Implicit Tilesets - Assigns metadata to tilesets, tiles, groups, and contents in a 3D Tiles tileset. A property table is defined for subtrees of an implicit tile hierarchy, and stores metadata that is associated with the nodes of such a subtree.

  • EXT_structural_metadata — Assigns metadata to vertices, texels, and features in a glTF asset. A property table is defined in the top-level extension object. The property values are stored in standard glTF buffer views.

The full JSON schema definition for this implementation can be found in the Schema directory of the specification.

Schema

Defined in schema.schema.json.

A schema defines a set of classes and enums. Classes serve as templates for entities. They provide a list of properties and the type information for those properties. Enums define the allowable values for enum properties.

Note
Example

Schema with a tree class, and a speciesEnum enum that defines different species of trees. Later examples show how these structures in more detail.

{
  "schema": {
    "classes": {
      "tree": { ... },
    "enums": {
      "speciesEnum": { ... }
    }
  }
}

Class

Defined in class.schema.json.

A class is a template for metadata entities. Classes provide a list of property definitions. Every entity shall be associated with a class, and the entity’s properties shall conform to the class’s property definitions. Entities whose properties conform to a class are considered instances of that class.

Classes are defined as entries in the schema.classes dictionary, indexed by class ID. Class IDs shall be identifiers as defined in the 3D Metadata Specification.

Note
Example

A "Tree" class, which might describe a table of tree measurements taken in a park. Property definitions are abbreviated here, and introduced in the next section.

{
  "schema": {
    "classes": {
      "tree": {
        "name": "Tree",
        "description": "Woody, perennial plant.",
        "properties": {
          "species": { ... },
          "age": { ... },
          "height": { ... },
          "diameter": { ... }
        }
      }
    }
  }
}

Class Property

Class properties are defined abstractly in a class. The class is instantiated with specific values conforming to these properties. Class properties support a rich variety of data types. Details about the supported types can be found in the 3D Metadata Specification.

Class properties are defined as entries in the class.properties dictionary, indexed by property ID. Property IDs shall be identifiers as defined in the 3D Metadata Specification.

Note
Example

A "Tree" class, which might describe a table of tree measurements taken in a park. Properties include species, age, height, and diameter of each tree.

{
  "schema": {
    "classes": {
      "tree": {
        "name": "Tree",
        "description": "Woody, perennial plant.",
        "properties": {
          "species": {
            "description": "Type of tree.",
            "type": "ENUM",
            "enumType": "speciesEnum",
            "required": true
          },
          "age": {
            "description": "The age of the tree, in years",
            "type": "SCALAR",
            "componentType": "UINT8",
            "required": true
          },
          "height": {
            "description": "Height of tree measured from ground level, in meters.",
            "type": "SCALAR",
            "componentType": "FLOAT32"
          },
          "diameter": {
            "description": "Diameter at trunk base, in meters.",
            "type": "SCALAR",
            "componentType": "FLOAT32"
          }
        }
      }
    }
  }
}

Enum

Defined in enum.schema.json.

A set of categorical types, defined as (name, value) pairs. Enum properties use an enum as their type.

Enums are defined as entries in the schema.enums dictionary, indexed by an enum ID. Enum IDs shall be identifiers as defined in the 3D Metadata Specification.

Note
Example

A "Species" enum defining types of trees. An "Unspecified" enum value is optional, but when provided as the noData value for a property (see: 3D Metadata - No Data Values) may be helpful to identify missing data.

{
  "schema": {
    "enums": {
      "speciesEnum": {
        "name": "Species",
        "description": "An example enum for tree species.",
        "values": [
          {"name": "Unspecified", "value": 0},
          {"name": "Oak", "value": 1},
          {"name": "Pine", "value": 2},
          {"name": "Maple", "value": 3}
        ]
      }
    }
  }
}

Enum Value

Pairs of (name, value) entries representing possible values of an enum property.

Enum values are defined as entries in the enum.values array. Duplicate names or duplicate integer values are not allowed.