X3D JSON Encoding

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

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

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?
• If so, it would be good to spell them out and understand them well.
• We want conversion rules to permit implementations that can achieve user goals.

Initial implementations

• Experimental X3dToJson.xslt stylesheet converts .x3d into .json encoding. In version control at sourceforge
  • Current work: lists of comments; field/fieldValue representations for Scripts and prototypes
• Embedded in X3D-Edit for testing
• Initial Hello World examples deployed (.x3d) (.html) (x3dom .xhtml) (.json)
• Conversion syntax discussions continue on x3dom-developers mail list (with "dump scene as json" test by Dr. Yvonne Jung)
• X3D Examples include build scripts to convert .x3d to .json, then perform JSLint validation

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 JSON can include comments, and so 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.

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 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. Wikipedia (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)

Probably lots more... What else?

References

• JSLint validation
• XML to JSON Converter (provides option to assign a prefix to JSON attributes, default is @ character) http://www.freeformatter.com/xml-to-json-converter.html
• Apache Camel, XML JSON Data Format (camel-xmljson) [http://camel.apache.org/xmljson.html
• JavaScript Object Notation (JSON) Definition http://json.org
• ECMA 404: JSON Data Interchange Format (Final Draft) http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-404.pdf
• JSON Markup Language (JsonML) http://www.jsonml.org/
• XSLTJSON: Transforming XML to JSON using XSLT http://www.bramstein.com/projects/xsltjson
• Converting Between XML and JSON http://www.xml.com/pub/a/2006/05/31/converting-between-xml-and-json.html
• XML/JSON Perl Converter http://search.cpan.org/~ken/XML-XML2JSON-0.06/lib/XML/XML2JSON.pm
• IBM's PHP converter http://www.ibm.com/developerworks/library/x-xml2jsonphp
• Java Converter http://www.json.org/javadoc/org/json/XML.html
• Google Code Library https://code.google.com/p/x2js

Mailing list discussions

• Initial message [1]
• July's thread listing [2]
• Ongoing on x3d-public mailing list, occasionally on x3dom mailing list