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KHR_xmp_json_ld

Contributors

Copyright 2018-2021 The Khronos Group Inc. All Rights Reserved. glTF is a trademark of The Khronos Group Inc. See Appendix for full Khronos Copyright Statement.

Status

Complete, Ratified by the Khronos Group

Supersedes KHR_xmp

Dependencies

Written against the glTF 2.0 spec.

Overview

This extension adds support for XMP (Extensible Metadata Platform) (ISO 16684-1) metadata to glTF. Metadata is used to transmit information (such as attribution, licensing, creation date) about the glTF asset. Metadata has no normative effect on the glTF asset appearance and rendering. XMP is a technology for embedding metadata into documents and has been an ISO standard since 2012. An instance of the XMP data model is called an XMP packet (ISO 16684-1$6.1). XMP metadata is embedded in a top-level glTF extension as an array of metadata packets. XMP metadata packets can then be referenced from glTF objects of type: asset, scene, node, mesh, material, image, animation. XMP metadata referenced by the glTF top-level object asset applies to the entire glTF asset. XMP metadata is organized in namespaces (ISO 16684-1$6.2). This extension enables any XMP metadata namespace to be embedded in a glTF asset. XMP metadata packets in glTF use a restricted subset of features from JSON-LD. This allows both JSON parsers and JSON-LD parsers to read the individual packets. Serializing XMP metadata using JSON-LD is outlined in JSON-LD serialization of XMP (ISO 16684-3). The JSON-LD specification outlines the detailed use of JSON-LD 1.1. There are additional restrictions for glTF outlined in JSON-LD Restrictions and Recommendations.

XMP data types

This section describes how XMP data types (ISO 16684-1$6.3) shall be encoded in glTF. XMP supports three classes of data types: simple, structure and array. XMP structures are encoded via JSON objects. XMP arrays are encoded via JSON arrays. XMP simple values are generally encoded via JSON strings with the exception of Boolean, Integer and Real that are encoded as described below.

XMP Core Properties

The following table describes how XMP Core Properties (ISO 16684-1$8.2) are encoded in the glTF manifest.

XMP Property Type JSON Type
Boolean (ISO 16684-1$8.2.1.1) Boolean
Date (ISO 16684-1$8.2.1.2) String formatted as described in the XMP documentation
Integer (ISO 16684-1$8.2.1.3) Number
Real (ISO 16684-1$8.2.1.4) Number
Text (ISO 16684-1$8.2.1.5 String
Agent Name (ISO 16684-1$8.2.2.1) String formatted as described in the XMP documentation
Choice (ISO 16684-1$8.2.2.2) A value chosen from a vocabulary of values of a specific type.
GUID (ISO 16684-1$8.2.2.3) String formatted as described in the XMP documentation
Language Alternative (ISO 16684-1$8.2.2.4) A language alternative is a dictionary that maps a language code, specified via a lower-case BCP-47 string, to text. See property dc:title in the example below
Locale (ISO 16684-1$8.2.2.5) String formatted as described in the XMP documentation
MIMEType (ISO 16684-1$8.2.2.6) String formatted as described in the XMP documentation
Rational (ISO 16684-1$8.2.2.7) String formatted as described in the XMP documentation
RenditionClass (ISO 16684-1$8.2.2.8) String formatted as described in the XMP documentation
ResourceRef (ISO 16684-1$8.2.2.9) Object, we refer to the XMP documentation for a description of its properties
URI (ISO 16684-1$8.2.2.10) String formatted as described in the XMP documentation
URL (ISO 16684-1$8.2.2.11) String formatted as described in the XMP documentation

Defining XMP Metadata

An indirection level is introduced to avoid replicating the same XMP metadata in multiple glTF objects. XMP metadata are defined within a dictionary property in the glTF file by adding an extensions property to the top-level glTF root object and defining a KHR_xmp_json_ld object. The KHR_xmp_json_ld object defines one property:

  • packets: an array of metadata packets. Each packet is JSON-LD compliant (with additional restrictions) and requires a @context dictionary to be defined which includes references for the namespaces.

The following example defines a glTF scene with a sample XMP metadata.

"extensions": {
  "KHR_xmp_json_ld": {
    "packets": [
      {
        "@context": {
          "dc": "http://purl.org/dc/elements/1.1/",
          "rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
        },
        "@id": "",
        "dc:contributor": {
          "@set": [
            "Creator1Name",
            "[email protected]",
            "Creator3Name<[email protected]>"
          ]
        },
        "dc:coverage": "Bay Area, California, United States",
        "dc:creator": {
          "@list": ["CreatorName", "[email protected]"]
        },
        "dc:date": {
          "@list": ["1997-07-16T19:20:30+01:00"]
        },
        "dc:description": {
          "@type": "rdf:Alt",
          "rdf:_1": {
            "@language": "en-us",
            "@value": "text"
          }
        },
        "dc:format": "model/gltf-binary",
        "dc:identifier": "urn:stock-id:292930",
        "dc:language": {
          "@set": ["en"]
        },
        "dc:publisher": {
          "@set": ["Company"]
        },
        "dc:relation": {
          "@set": ["https://www.khronos.org/"]
        },
        "dc:rights": {
          "@type": "rdf:Alt",
          "rdf:_1": {
            "@language": "en-us",
            "@value": "BSD"
          }
        },
        "dc:source": "http://related_resource.org/derived_from_this.gltf",
        "dc:subject": {
          "@set": ["architecture"]
        },
        "dc:title": {
          "@type": "rdf:Alt",
          "rdf:_1": {
            "@language": "en-us",
            "@value": "MyModel"
          },
          "rdf:_2": {
            "@language": "it-it",
            "@value": "Mio Modello"
          }
        },
        "dc:type": {
          "@set": ["Physical Object"]
        }
      }
    ]
  }
}

Instantiating XMP metadata

Metadata can be instantiated in a gltf object by defining the extensions.KHR_xmp_json_ld property and, within that, an index into the packets array using the packet property. The following example shows a glTF Mesh instantiating the XMP metadata at index 0.

{
  "meshes": [
    {
      "extensions": {
        "KHR_xmp_json_ld": {
          "packet": 0
        }
      },
      "primitives": [
        {
          "attributes": {
            "NORMAL": 23,
            "POSITION": 22,
            "TANGENT": 24,
            "TEXCOORD_0": 25
          },
          "indices": 21,
          "material": 3,
          "mode": 4
        }
      ]
    }
  ]
}

Notes:

Precedence

Metadata applied to JSON objects in the top level arrays ( scenes, nodes, meshes, materials, images, animations) has precedence over the metadata specified in the asset property of a glTF. A glTF might reference resources already containing XMP metadata. A relevant example would be an image object referencing a PNG or JPEG file with embedded XMP metadata. Note that glTF clients and viewers may ignore the metadata embedded in the referenced resources (for instance PNG or JPEG images).

Restrictions and Recommendations

In order to keep glTF files easily readable with either a JSON or JSON-LD parser and keep compatibility with ISO 16684-3, there are additional restrictions that are required for KHR_xmp_json_ld metadata packets. Failure to obey these restrictions may create issues for parsers.

  • Expanded term definitions are forbidden.
  • Compact IRIs (prefix:suffix) are required by ISO 16684-3. Due to this, @base and @vocab keywords are always ignored.
  • Namespace prefixes are recommended to use the latin alphabet A to Z uppercase or lowercase, roman numerals 0 to 9, or _.
  • Aliases and multiple prefixes for the same namespace are forbidden.
  • Value objects are forbidden unless otherwise specified, such as with Language Alternatives or when providing a content language for Text types.
  • XMP fields containing arrays must use the @list and @set keywords as a parent to the array, to indicate if the array is ordered or unordered. See the Lists and Sets section for details.
  • Local contexts are forbidden, except when providing a content language for Text types.
  • Language alternatives must use an rdf:Alt type. See the Language Alternatives section for details.
  • Non-string data types such as Boolean, Integer, and Real must use JSON types.

These restrictions ensure that KHR_xmp_json_ld metadata remains readable regardless if the parser supports JSON-LD.

Additionally, the following are recommended:

  • XMP data types are always preferred. Only use a non XMP data type if there is no other option.
  • For proper compatibility with ISO 16684-3, an AboutURI should be included at the root level of each packet in an @id. For most purposes the value can be left blank.

Lists and Sets

The XMP specification makes a distinction between ordered lists and unordered sets, but in the case of JSON all arrays are considered to be ordered lists. (See RFC-7159.) In order to handle the differences between ordered lists and unordered sets, all arrays within a packet must be children of a @list or @set property to differentiate between ordered lists and unordered sets respectively. Descriptions for the two fields and examples are provided below.

  • @list denotes that the child array is an ordered list.
  • @set denotes that the child array is an unordered set.

Choosing when to use an ordered list or an unordered set is typically decided by the schema in use. For example, the Dublin Core (dc prefix) schema states that dc:creator uses an ordered list, and dc:contributor uses an unordered set.

An example packet using @list for an ordered list of creators:

{
  "asset": {
    "extensions": {
      "KHR_xmp_json_ld": {
        "packet": 0
      }
    },
    ...
  },
  "extensions": {
    "KHR_xmp_json_ld": {
      "packets": [
        {
          "@context": {
            "dc": "http://purl.org/dc/elements/1.1/"
          },
          "@id": "",
          "dc:creator": {
            "@list": ["CreatorName", "[email protected]"]
          },
          ...
        }
      ]
    }
  }
}

An example packet using @set for an unordered list of contributors:

{
  "asset": {
    "extensions": {
      "KHR_xmp_json_ld": {
        "packet": 0
      }
    },
    ...
  },
  "extensions": {
    "KHR_xmp_json_ld": {
      "packets": [
        {
          "@context": {
            "dc": "http://purl.org/dc/elements/1.1/"
          },
          "@id": "",
          "dc:contributor": {
            "@set": [
              "Creator1Name",
              "[email protected]",
              "Creator3Name<[email protected]>"
            ]
          }
          ...
        }
      ]
    }
  }
}

Language Alternatives

Language Alternatives provide a powerful way of handling internationalization within XMP metadata. In order to remain compliant with the requirements outlined in the JSON-LD serialization of XMP (ISO 16684-3) specification KHR_xmp_json_ld relies on usage of the @language and @value keywords contained in an object of the rdf:Alt type. A field containing language alternatives must contain a @type definition for rdf:Alt, and each alternative must be contained in a rdf:_N property, where N is the index. The rdf namespace (http://www.w3.org/1999/02/22-rdf-syntax-ns#) must also be included in the packet's @context. The rdf:_N property must have an object containing two properties:

  • @language with a IETF BCP 47 language code as the value.
  • @value with actual value of the XMP property as a string.

An example glTF with only one language:

{
  "meshes": [
    {
      "extensions": { "KHR_xmp_json_ld": { "packet": 0 } },
      ...
    }
  ],
  "extensions": {
    "KHR_xmp_json_ld": {
      "packets": [
        {
          "@context": {
            "dc": "http://purl.org/dc/elements/1.1/",
            "rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
          },
          "@id": "",
          "dc:title": {
            "@type": "rdf:Alt",
            "rdf:_1": {
              "@language": "en-us",
              "@value": "My Model"
            }
          },
          ...
        }
      ]
    }
  }
}

An example glTF with more than one language:

{
  "meshes": [
    {
      "extensions": { "KHR_xmp_json_ld": { "packet": 0 } },
      ...
    }
  ],
  "extensions": {
    "KHR_xmp_json_ld": {
      "packets": [
        {
          "@context": {
            "dc": "http://purl.org/dc/elements/1.1/",
            "rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
          },
          "@id": "",
          "dc:title": {
            "@type": "rdf:Alt",
            "rdf:_1": {
              "@language": "en-us",
              "@value": "My Model"
            },
            "rdf:_2": {
              "@language": "it-it",
              "@value": "Mio Modello"
            }
          },
          ...
        }
      ]
    }
  }
}

Transferring and merging metadata

When a glTF object (for example a Mesh) is copied from a glTF file to another, the associated KHR_xmp_json_ld metadata should be copied as well. Copying metadata requires adding the source object's metadata packets to the destination glTF's KHR_xmp_json_ld.packets array. In the following example two glTFs containing a mesh with KHR_xmp_json_ld metadata are copied into a third glTF.

NOTE: When merging metadata between two glTF objects, namespaces should be normalized based on the namespace URI. Example: Object A uses dc for the Dublin Core, but Object B uses dc11 for the Dublin Core, so the implementer must normalize these based on the namespace URI and pick either dc or dc11 as the prefix for the unified packet.

glTF containing the first mesh:

{
  "meshes": [
    {
      "extensions": { "KHR_xmp_json_ld": { "packet": 0 } },
      ...
    }
  ],
  "extensions": {
    "KHR_xmp_json_ld": {
      "packets": [
        {
          "@context": {
            "dc": "http://purl.org/dc/elements/1.1/",
            "rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
          },
          "@id": "",
          "dc:title": {
            "@type": "rdf:Alt",
            "rdf:_1": {
              "@language": "en-us",
              "@value": "My first mesh"
            }
          },
          ...
        }
      ]
    }
  }
}

glTF containing the second mesh:

{
  "meshes": [
    {
      "extensions": { "KHR_xmp_json_ld": { "packet": 0 } }
    },
    ...
  ],
  "extensions": {
    "KHR_xmp_json_ld": {
      "packets": [
        {
          "@context": {
            "dc": "http://purl.org/dc/elements/1.1/",
            "rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
          },
          "@id": "",
          "dc:title": {
            "@type": "rdf:Alt",
            "rdf:_1": {
              "@language": "en-us",
              "@value": "My second mesh"
            }
          },
          ...
        }
      ]
    }
  }
}

glTF containing copies of both meshes:

{
  "meshes": [
    {
      "extensions": { "KHR_xmp_json_ld": { "packet": 0 } }
      ...
    },
    {
      "extensions": { "KHR_xmp_json_ld": { "packet": 1 } }
      ...
    }
  ],
  "extensions": {
    "KHR_xmp_json_ld": {
      "packets": [
        {
          "@context": {
            "dc": "http://purl.org/dc/elements/1.1/",
            "rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
          },
          "@id": "",
          "dc:title": {
            "@type": "rdf:Alt",
            "rdf:_1": {
              "@language": "en-us",
              "@value": "My first mesh"
            }
          },
          ...
        },
        {
          "@context": {
            "dc": "http://purl.org/dc/elements/1.1/",
            "rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
          },
          "@id": "",
          "dc:title": {
            "@type": "rdf:Alt",
            "rdf:_1": {
              "@language": "en-us",
              "@value": "My second mesh"
            }
          },
          ...
        }
      ]
    }
  }
}

The xmpMM (ISO 16684-1$8.6) namespaces introduces mechanisms such as Pantry-Ingredient to handle the case when a glTF file is generated via derivation or composition of one or more source documents. The following example illustrates how KHR_xmp_json_ld metadata can be computed in the case of two glTFs, both containing asset metadata, that are processed together to obtain a new composited glTF document.

First glTF document:

{
  "asset": {
    "extensions": {
      "KHR_xmp_json_ld": {
        "packet": 0
      }
    },
    ...
  },
  "extensions": {
    "KHR_xmp_json_ld": {
      "packets": [
        {
          "@context": {
            "dc": "http://purl.org/dc/elements/1.1/",
            "rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
          },
          "@id": "",
          "dc:title": {
            "@type": "rdf:Alt",
            "rdf:_1": {
              "@language": "en-us",
              "@value": "My first glTF"
            }
          },
          ...
        }
      ]
    }
  }
}

Second glTF document:

{
  "asset": {
    "extensions": {
      "KHR_xmp_json_ld": {
        "packet": 0
      }
    },
    ...
  },
  "extensions": {
    "KHR_xmp_json_ld": {
      "packets": [
        {
          "@context": {
            "dc": "http://purl.org/dc/elements/1.1/",
            "rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
          },
          "@id": "",
          "dc:title": {
            "@type": "rdf:Alt",
            "rdf:_1": {
              "@language": "en-us",
              "@value": "My second glTF"
            }
          },
          ...
        }
      ]
    }
  }
}

Derived glTF document metadata:

{
  "asset": {
    "extensions": {
      "KHR_xmp_json_ld": {
        "packet": 0
      }
    },
    ...
  },
  "extensions": {
    "KHR_xmp_json_ld": {
      "packets": [
        {
          "@context": {
            "dc": "http://purl.org/dc/elements/1.1/",
            "rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#",
            "xmpMM": "http://ns.adobe.com/xap/1.0/mm/"
          },
          "@id": "",
          "dc:title": {
            "@type": "rdf:Alt",
            "rdf:_1": {
              "@language": "en-us",
              "@value": "My composed glTF."
            }
          },
          "xmpMM:Pantry": [
            {
              "xmpMM:DocumentID": "62bc2623-968e-4b09-8174-02dc53d6b856",
              "xmpMM:InstanceID": "1a33a91f-7351-471e-9b6c-24bca3213d2a",
              "dc:title": {
                "@type": "rdf:Alt",
                "rdf:_1": {
                  "@language": "en-us",
                  "@value": "My first glTF"
                }
              },
              ...
            },
            {
              "xmpMM:DocumentID": "6d70a25e-129e-42d8-9d63-93bd9eb0298b",
              "xmpMM:InstanceID": "235b5571-e6a9-48fd-8d6b-b88276f37ee3",
              "dc:title": {
                "@type": "rdf:Alt",
                "rdf:_1": {
                  "@language": "en-us",
                  "@value": "My second glTF"
                }
              },
              ...
            }
          ],
          "xmpMM:Ingredients": [
            {
              "xmpMM:DocumentID": "62bc2623-968e-4b09-8174-02dc53d6b856",
              "xmpMM:InstanceID": "1a33a91f-7351-471e-9b6c-24bca3213d2a"
            },
            {
              "xmpMM:DocumentID": "6d70a25e-129e-42d8-9d63-93bd9eb0298b",
              "xmpMM:InstanceID": "235b5571-e6a9-48fd-8d6b-b88276f37ee3"
            }
          ]
        }
      ]
    }
  }
}

Implementation notes

This section is non-normative.

glTF viewers are free to chose which metadata in the glTF to display. Our recommendation for viewers that choose to display XMP metadata is to include the metadata in the glTF asset object.

XMP is an extensible metadata format by design. In order to achieve better readability and interoperability across the industry, we recommend glTF creators to use the following preferred list of namespace prefixes and URIs.

Preferred prefix Namespace URI Usage
dc http://purl.org/dc/elements/1.1/ Dublin Core
xmp http://ns.adobe.com/xap/1.0/ Basic Descriptive Information
xmpGImg http://ns.adobe.com/xap/1.0/g/img/ xmp:Thumbnails
xmpMM http://ns.adobe.com/xap/1.0/mm/ Media Management, used by Digital Asset Management systems
stRef http://ns.adobe.com/xap/1.0/sType/ResourceRef# xmpMM:DerivedFrom, xmpMM:Ingredients, xmpMM:ManagedForm
stEvt http://ns.adobe.com/xap/1.0/sType/ResourceEvent# xmpMM:History
stVer http://ns.adobe.com/xap/1.0/sType/Version# xmpMM:Versions
xmpRights http://ns.adobe.com/xap/1.0/rights/ Legal restrictions associated with a resource
rdf http://www.w3.org/1999/02/22-rdf-syntax-ns# Resource Description Framework; Primarily used for Language Alternatives in XMP JSON-LD

Appendix: Full Khronos Copyright Statement

Copyright 2018-2021 The Khronos Group Inc.

Some parts of this Specification are purely informative and do not define requirements necessary for compliance and so are outside the Scope of this Specification. These parts of the Specification are marked as being non-normative, or identified as Implementation Notes.

Where this Specification includes normative references to external documents, only the specifically identified sections and functionality of those external documents are in Scope. Requirements defined by external documents not created by Khronos may contain contributions from non-members of Khronos not covered by the Khronos Intellectual Property Rights Policy.

In particular, the referenced paragraphs of ISO 16684-1:2019 are normative and so are INCLUDED in the Scope of this Specification and may contain contributions from non-members of Khronos that are not covered by the Khronos Intellectual Property Rights Policy. The links to https://github.com/adobe/xmp-docs are purely informative and so are EXCLUDED from the Scope of this Specification, but are provided for convenience. The references to ISO 16684-1:2019 shall remain normative if there are differences to information provided at any informative links.

This specification is protected by copyright laws and contains material proprietary to Khronos. Except as described by these terms, it or any components may not be reproduced, republished, distributed, transmitted, displayed, broadcast or otherwise exploited in any manner without the express prior written permission of Khronos.

This specification has been created under the Khronos Intellectual Property Rights Policy, which is Attachment A of the Khronos Group Membership Agreement available at www.khronos.org/files/member_agreement.pdf. Khronos grants a conditional copyright license to use and reproduce the unmodified specification for any purpose, without fee or royalty, EXCEPT no licenses to any patent, trademark or other intellectual property rights are granted under these terms. Parties desiring to implement the specification and make use of Khronos trademarks in relation to that implementation, and receive reciprocal patent license protection under the Khronos IP Policy must become Adopters and confirm the implementation as conformant under the process defined by Khronos for this specification; see https://www.khronos.org/adopters.

Khronos makes no, and expressly disclaims any, representations or warranties, express or implied, regarding this specification, including, without limitation: merchantability, fitness for a particular purpose, non-infringement of any intellectual property, correctness, accuracy, completeness, timeliness, and reliability. Under no circumstances will Khronos, or any of its Promoters, Contributors or Members, or their respective partners, officers, directors, employees, agents or representatives be liable for any damages, whether direct, indirect, special or consequential damages for lost revenues, lost profits, or otherwise, arising from or in connection with these materials.

Khronos® and Vulkan® are registered trademarks, and ANARI™, WebGL™, glTF™, NNEF™, OpenVX™, SPIR™, SPIR-V™, SYCL™, OpenVG™ and 3D Commerce™ are trademarks of The Khronos Group Inc. OpenXR™ is a trademark owned by The Khronos Group Inc. and is registered as a trademark in China, the European Union, Japan and the United Kingdom. OpenCL™ is a trademark of Apple Inc. and OpenGL® is a registered trademark and the OpenGL ES™ and OpenGL SC™ logos are trademarks of Hewlett Packard Enterprise used under license by Khronos. ASTC is a trademark of ARM Holdings PLC. All other product names, trademarks, and/or company names are used solely for identification and belong to their respective owners.