X3D JSON Encoding

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These are X3D Working Group notes on the creation of an X3D JSON encoding and corresponding conversion processes.

Overall status: encoding is essentially complete, as demonstrated by X3D to JSON Stylesheet Converter.

Further work awaits completed formalization of JSON Schema specifications. Current assets include preliminary interoperability implementations at JSON schema (documentation), X3DJSONLD JavaScript conversion library and draft X3D JSON specification.

Design Requirements, Goals and Use Cases

Design requirement:

  • Round-trippable encoding supporting X3D abstract specification

Design goals and primary use cases:

  • Exchange format for a variety of 3D geometry and scene graphs
  • Loader for various JavaScript-controlled renderers
  • Manipulate a scene graph using JavaScript

Design considerations: are there any special use cases for having X3D JSON available in JavaScript?

  • Are there any use cases that might modify how X3D is represented in JSON? Yes.
  • If so, it would be good to spell them out and understand them well... Accomplished.
  • We want conversion rules to permit implementations that can achieve user goals... Optimization efforts successful.

Observation: the more that this work has progressed, the more the JSON output resembles VRML/ClassicVrml structure with slight differences in syntax. This approach has a valuable extra benefit in that results are broadly understandable by many object-oriented programmers.

Initial implementations

X3D to JSON Stylesheet Converter, an experimental effort to help create a JSON encoding for X3D Graphics.

Related work in progress: X3D JSON loader to test round-trippability works satisfactorily

This X3D JSON Encoding page is periodically refreshed based on implementation/evaluation results.

Conversion Considerations

Primary design criterion: round-trippable lossless representation of X3D scene.

Conversion approach of greatest practical interest: XML to/from JSON. Issues:

  1. How to convert attribute names to distinguish them from child elements. Resolution: "@attributeName".
  2. JSON handling of container elements to preserve parent/child relationships, distinguishing child elements from attributes Resolution: use SFNode/MFNode field names as unique keys.
  3. Creation of JSON elements with datatypes appropriate to content (e.g., integer, float, strings, etc.). Note special JSON rules for floats (not equivalent to IEEE floats).
  4. Both X3D and JavaScript can include comments, although JSON does not. Need an option for inclusion (by default) or removal (optional) of comments in order to ensure 100% round-trip conversion capabilities.
  5. jslint-validatable field/fieldValue representations within Scripts and prototypes
  6. Support for singleton (self-closing) XML tags also needs to be considered, without loss of generality.
  7. Inclusion and preservation of embedded XML namespace information in an XML (.x3d) document: might not be necessary or possible.

The X3D to JSON Stylesheet Converter considers these and other interoperability issues in great detail.

Standardization

Probably smartest to first start out defining an X3D best practice.

This capability likely needs to be defined as one of the X3D standards as shown in the X3D Specification Relationships diagram.

The most probable place to put it is as a new Part 5 to ISO/IEC 19776. In this manner, it would correspond to the XML, Classic VRML, and Compressed Binary encodings.

Discussion points

Here are suggested discussion points for X3D teleconferences and future followups.

  1. Is there a good/consistent way for X3DOM, Cobweb or other tools to utilize such capabilities?
  2. Is there a way for Three.js X3D loader, D3.js X3D loader, or other javascript libraries, to utilize such capabilities?
  3. Is there a single authoritative reference for JSON itself? and for JSON-XML conversions? See [6] for the JSON Data Interchange Format, need to confirm no others.
  4. What is the right file extension? .json is distinguishable from .js used in plain Javascript in Script code; .x3dj distinguishes X3D JSON
    1. IETF (referencing ECMA) states that .json is the standard extension
    2. Internet MIME-type is application/json
  5. Can a file reader distinguish the incoming encodings (.x3d .x3dv .x3db .x3de .json/.x3dj) independent of file extension or MIME media type?
  6. Compare compression size and decompression speed of a TestMesh.x3d.json.gz to TestMesh.x3db and TestMesh.x3d.exi (EXI will likely win because it includes data typing)
  7. Show .dot notation in JavaScript code accessing elements and attributes, see if it is sensible

Lots of progress continues.

References

Mailing list discussions

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