xrfragment/doc/RFC_XR_Fragments.txt

Jens & Leon Internet Engineering Task Force              L.R. van Kammen
Internet-Draft                                         19 September 2025
Intended status: Informational                                          



                              XR Fragments
                   draft-XRFRAGMENTS-leonvankammen-00

Abstract

   An open specification for hyperlinking & deeplinking 3D fileformats.
   This draft is a specification for interactive URI-controllable 3D
   files, enabling hypermediatic (https://github.com/coderofsalvation/
   hypermediatic) navigation, to enable a spatial web for hypermedia
   browsers with- or without a network-connection.
   XR Fragments allows us to better use implicit metadata inside 3D
   scene(files), by mapping it to proven technologies like URI Fragments
   (https://en.wikipedia.org/wiki/URI_fragment).
   XR Fragments views XR experiences thru the lens of 3D deeplinked
   URI's, rather than thru code(frameworks) or protocol-specific
   browsers (webbrowser e.g.).  The standard comprises of various
   (optional) support levels, which also include W3C Media Fragments
   (https://www.w3.org/TR/media-frags/) and URI Templates (RFC6570)
   (https://www.rfc-editor.org/rfc/rfc6570) to promote spatial
   addressibility, sharing, navigation, filtering and databinding
   objects for (XR) Browsers.

   XR Fragments is in a sense, a <b>heuristical 3D format</b> or meta-
   format, which leverages heuristic rules derived from any 3D scene or
   well-established 3D file formats, to extract meaningful features from
   scene hierarchies.
   These heuristics, enable features that are both meaningful and
   consistent across different scene representations, allowing <b>higher
   interop</b> between fileformats, 3D editors, viewers and game-
   engines.

   Almost every idea in this document is demonstrated at
   https://xrfragment.org (https://xrfragment.org)

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.







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Table of Contents

   1.  Quick reference . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  What is XR Fragments  . . . . . . . . . . . . . . . . . . . .   5
   4.  HFL (Hypermediatic Feedback Loop) for XR Browsers . . . . . .   5
   5.  Conventions and Definitions . . . . . . . . . . . . . . . . .   8
     5.1.  XR Fragment URL Grammar . . . . . . . . . . . . . . . . .   8
   6.  Spatial Referencing 3D  . . . . . . . . . . . . . . . . . . .   8
   7.  Level0: Files . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  via href metadata . . . . . . . . . . . . . . . . . . . .   9
     7.2.  via chained extension . . . . . . . . . . . . . . . . . .  10
     7.3.  via subdocuments/xattr  . . . . . . . . . . . . . . . . .  10
     7.4.  JSON sidecar-file . . . . . . . . . . . . . . . . . . . .  11
   8.  Level1: URI . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  List of URI Fragments . . . . . . . . . . . . . . . . . .  13
     8.2.  List of *_explicit_ metadata  . . . . . . . . . . . . . .  14
   9.  Level2: href links  . . . . . . . . . . . . . . . . . . . . .  14
     9.1.  Interaction behaviour . . . . . . . . . . . . . . . . . .  14
     9.2.  XR Viewer implementation  . . . . . . . . . . . . . . . .  14
   10. Level3: Media Fragments . . . . . . . . . . . . . . . . . . .  15
     10.1.  Animation(s) timeline  . . . . . . . . . . . . . . . . .  15



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     10.2.  Specify playback loopmode  . . . . . . . . . . . . . . .  15
     10.3.  Controlling embedded content . . . . . . . . . . . . . .  16
   11. Level4: prefix operators  . . . . . . . . . . . . . . . . . .  16
     11.1.  Object teleports (!) . . . . . . . . . . . . . . . . . .  16
     11.2.  Object multipliers (*) . . . . . . . . . . . . . . . . .  18
     11.3.  De/selectors (+ and -) . . . . . . . . . . . . . . . . .  18
     11.4.  Sharing object or file (#|)  . . . . . . . . . . . . . .  18
     11.5.  xrf:// URI scheme  . . . . . . . . . . . . . . . . . . .  19
   12. Level5: URI Templates (RFC6570) . . . . . . . . . . . . . . .  19
   13. Top-level URL processing  . . . . . . . . . . . . . . . . . .  19
     13.1.  UX . . . . . . . . . . . . . . . . . . . . . . . . . . .  20
   14. Example: Navigating content href portals  . . . . . . . . . .  20
     14.1.  Walking surfaces . . . . . . . . . . . . . . . . . . . .  21
   15. Example: Virtual world rings  . . . . . . . . . . . . . . . .  21
   16. Additional scene metadata . . . . . . . . . . . . . . . . . .  22
   17. Accessibility interface . . . . . . . . . . . . . . . . . . .  23
     17.1.  Two-button navigation  . . . . . . . . . . . . . . . . .  24
     17.2.  Overlap with fileformat-specific extensions  . . . . . .  25
   18. Vendor Prefixes . . . . . . . . . . . . . . . . . . . . . . .  25
   19. Security Considerations . . . . . . . . . . . . . . . . . . .  28
   20. FAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  28
   21. authors . . . . . . . . . . . . . . . . . . . . . . . . . . .  28
   22. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
   23. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  29
   24. Appendix: Definitions . . . . . . . . . . . . . . . . . . . .  29

1.  Quick reference

   1.   Abstract (#abstract)
   2.   Index (#index)
   3.   Introduction (#introduction)
   4.   What is XR Fragments (#what-is-xr-fragments)
   5.   HFL (Hypermediatic Feedback Loop) for XR Browsers (#hfl-
        hypermediatic-feedback-loop-for-xr-browsers)
   6.   Conventions and Definitions (#conventions-and-definitions)
        1.  XR Fragment URL Grammar (#xr-fragment-url-grammar)
   7.   Spatial Referencing 3D (#spatial-referencing-3d)
   8.   Level0: Files (#level0-files)
        1.  via href metadata (#via-href-metadata)
        2.  via chained extension (#via-chained-extension)
        3.  via subdocuments/xattr (#via-subdocuments-xattr)
        4.  JSON sidecar-file (#json-sidecar-file)
   9.   Level1: URI (#level1-uri)
        1.  List of URI Fragments (#list-of-uri-fragments)
        2.  List of explicit metadata (#list-of-explicit-metadata)
   10.  Level2: href links (#level2-href-links)
        1.  Interaction behaviour (#interaction-behaviour)
        2.  XR Viewer implementation (#xr-viewer-implementation)



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   11.  Level3: Media Fragments (#level3-media-fragments)
        1.  Animation(s) timeline (#animation-s-timeline)
        2.  Specify playback loopmode (#specify-playback-loopmode)
        3.  Controlling embedded content (#controlling-embedded-content)
   12.  Level4: prefix operators (#level4-prefix-operators)
        1.  Object teleports (#object-teleports)
        2.  Object multipliers (#object-multipliers)
        3.  De/selectors (+ and -) (#de-selectors-and)
        4.  Sharing object or file (#|) (#sharing-object-or-file)
        5.  xrf:// URI scheme (#xrf-uri-scheme)
   13.  Level5: URI Templates (RFC6570) (#level5-uri-templates-rfc6570)
   14.  Top-level URL processing (#top-level-url-processing)
        1.  UX (#ux)
   15.  Example: Navigating content href portals (#example-navigating-
        content-href-portals)
        1.  Walking surfaces (#walking-surfaces)
   16.  Example: Virtual world rings (#example-virtual-world-rings)
   17.  Additional scene metadata (#additional-scene-metadata)
   18.  Accessibility interface (#accessibility-interface)
        1.  Two-button navigation (#two-button-navigation)
        2.  Overlap with fileformat-specific extensions (#overlap-with-
            fileformat-specific-extensions)
   19.  Vendor Prefixes (#vendor-prefixes)
   20.  Security Considerations (#security-considerations)
   21.  FAQ (#faq)
   22.  Authors (#authors)
   23.  IANA Considerations (#iana-considerations)
   24.  Acknowledgments (#acknowledgments)
   25.  Appendix: Definitions (#appendix-definitions)

2.  Introduction

   How can we add more control to existing text and 3D scenes, without
   introducing new dataformats?
   Historically, there's many attempts to create the ultimate 3D
   fileformat.
   The lowest common denominator is: designers describing/tagging/naming
   things using *plain text*.
   XR Fragments exploits the fact that all 3D models already contain
   such metadata:

   *XR Fragments allows deeplinking of 3D objects by mapping objectnames
   to URI fragments*

   It solves:






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   1.  addressibility and hypermediatic
       (https://github.com/coderofsalvation/hypermediatic) navigation of
       3D scenes/objects: URI Fragments (https://en.wikipedia.org/wiki/
       URI_fragment) using src/href spatial metadata
   2.  Interlinking text & spatial objects by collapsing space into a
       Word Graph (XRWG) to show visible links (#visible-links)
   3.  unlocking spatial potential of the (originally 2D) hashtag (which
       jumps to a chapter) for navigating XR documents
   4.  refraining from introducing scripting-engines for mundane tasks
       (and preventing its inevitable security-headaches)
   5.  the gap between text an 3d objects: object-names directly map to
       hashtags (=fragments), which allows 3D to text transcription.

   |  NOTE: The chapters in this document are ordered from highlevel to
   |  lowlevel (technical) as much as possible

3.  What is XR Fragments

   XR Fragments utilizes URLs:

   1.  for 3D viewers/browser to manipulate the camera or objects (via
       URI fragments)
   2.  implicitly: by mapping 3D objectnames (of a 3D scene/file) to URI
       fragments (3D deeplinking)
   3.  explicitly: by scanning href metadata *inside* 3D scene-files to
       enable interactions
   4.  externally: progressively enhance a 3D (file) into an experience
       via sidecarfiles (https://en.wikipedia.org/wiki/Sidecar_file)

4.  HFL (Hypermediatic Feedback Loop) for XR Browsers

   for XR Browsers)

   href metadata traditionally implies *click* AND *navigate*, however
   XR Fragments adds stateless *click* (xrf://....) via the xrf://
   scheme, which does not change the top-level URL-adress (of the
   browser).  This allows for many extra interactions via URLs, which
   otherwise needs a scripting language.  These are called *hashbus*-
   only events/

   |  Being able to use the same URI Fragment DSL for navigation (href:
   |  #foo) as well as interactions (href: xrf://#foo) greatly
   |  simplifies implementation, increases HFL, and reduces need for
   |  scripting languages.

   This opens up the following benefits for traditional & future
   webbrowsers:




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   *  hypermediatic (https://github.com/coderofsalvation/hypermediatic)
      loading/clicking 3D assets (gltf/fbx e.g.) natively (with or
      without using HTML).
   *  potentially allowing 3D assets/nodes to publish XR Fragments to
      themselves/eachother using the xrf:// hashbus (xrf://#person=walk
      to trigger walk-animation for object person)
   *  potentially collapsing the 3D scene to an wordgraph (for essential
      navigation purposes) controllable thru a hash(tag)bus
   *  completely bypassing the security-trap of loading external scripts
      (by loading 3D model-files, not HTML-javascriptable resources)

   XR Fragments itself are hypermediatic
   (https://github.com/coderofsalvation/hypermediatic) and HTML-
   agnostic, though pseudo-XR Fragment browsers *can* be implemented on
   top of HTML/Javascript.




































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   +=========+======================+=====================================+
   |principle|3D URL                |HTML 2D URL                          |
   +=========+======================+=====================================+
   |the XRWG |wordgraph (collapses  |Ctrl-F (find)                        |
   |         |3D scene to tags)     |                                     |
   +---------+----------------------+-------------------------------------+
   |the      |hashtags alter        |hashtags alter document positions    |
   |hashbus  |camera/scene/object-  |                                     |
   |         |projections           |                                     |
   +---------+----------------------+-------------------------------------+
   |src      |renders content and   |renders content                      |
   |metadata |offers sourceportation|                                     |
   +---------+----------------------+-------------------------------------+
   |href     |teleports to other XR |jumps to other HTML document         |
   |metadata |document              |                                     |
   +---------+----------------------+-------------------------------------+
   |href     |triggers predefined   |Media fragments                      |
   |metadata |view                  |                                     |
   +---------+----------------------+-------------------------------------+
   |href     |triggers              |n/a                                  |
   |metadata |camera/scene/object/  |                                     |
   |         |projections           |                                     |
   +---------+----------------------+-------------------------------------+
   |href     |draws visible         |n/a                                  |
   |metadata |connection(s) for XRWG|                                     |
   |         |'tag'                 |                                     |
   +---------+----------------------+-------------------------------------+
   |href     |filters certain       |n/a                                  |
   |metadata |(in)visible objects   |                                     |
   +---------+----------------------+-------------------------------------+
   |href     |href="xrf://#-foo&bar"|href="javascript:hideFooAndShowBar()`|
   |metadata |                      |                                     |
   +---------+----------------------+-------------------------------------+
   |         |(this does not update |(this is non-standard, non-          |
   |         |topLevel URI)         |hypermediatic)                       |
   +---------+----------------------+-------------------------------------+

                                  Table 1

   |  An important aspect of HFL is that URI Fragments can be triggered
   |  without updating the top-level URI (default href-behaviour) thru
   |  their own 'bus' (xrf://#.....).  This decoupling between
   |  navigation and interaction prevents non-standard things like
   |  (href:javascript:dosomething()).







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5.  Conventions and Definitions

   See appendix below in case certain terms are not clear.

5.1.  XR Fragment URL Grammar

   For typical HTTP-like browsers/applications:

   reserved    = gen-delims / sub-delims
   gen-delims  = "#" / "&"
   sub-delims  = "," / "="

   |  Example: ://foo.com/my3d.gltf#room1&prio=-5&t=0,100

             +============+=================================+
             | Demo       | Explanation                     |
             +============+=================================+
             | room1      | vector/coordinate argument e.g. |
             +------------+---------------------------------+
             | room1&cam1 | combinators                     |
             +------------+---------------------------------+

                                 Table 2

   |  this is already implemented in all browsers

   Pseudo (non-native) browser-implementations (supporting XR Fragments
   using HTML+JS e.g.) can use the ? search-operator to address outbound
   content.
   In other words, the URL updates to: https://me.com?https://me.com/
   other.glb when navigating to https://me.com/other.glb from inside a
   https://me.com WebXR experience e.g.
   That way, if the link gets shared, the XR Fragments implementation at
   https://me.com can load the latter (and still indicates which XR
   Fragments entrypoint-experience/client was used).

6.  Spatial Referencing 3D

   3D files contain an hierarchy of objects.
   XR Fragments assumes the following objectname-to-URI-Fragment
   mapping, in order to deeplink 3D objects:










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  my.io/scene.fbx
  +─────────────────────────────+
  │ sky                         │  src: http://my.io/scene.fbx#sky          (includes building,mainobject,floor)
  │ +─────────────────────────+ │
  │ │ building                │ │  src: http://my.io/scene.fbx#building     (includes mainobject,floor)
  │ │ +─────────────────────+ │ │
  │ │ │ mainobject          │ │ │  src: http://my.io/scene.fbx#mainobject   (includes floor)
  │ │ │ +─────────────────+ │ │ │
  │ │ │ │ floor           │ │ │ │  src: http://my.io/scene.fbx#floor        (just floor object)
  │ │ │ │                 │ │ │ │
  │ │ │ +─────────────────+ │ │ │
  │ │ +─────────────────────+ │ │
  │ +─────────────────────────+ │
  +─────────────────────────────+

   |  Every 3D fileformat supports named 3D object, and this name allows
   |  URLs (fragments) to reference them (and their children objects).

   Clever nested design of 3D scenes allow great ways for re-using
   content, and/or previewing scenes.
   For example, to render a portal with a preview-version of the scene,
   create an 3D object with:

   *  href: https://scene.fbx

   |  It also allows *sourceportation*, which basically means the
   |  enduser can teleport to the original XR Document of an src
   |  embedded object, and see a visible connection to the particular
   |  embedded object.  Basically an embedded link becoming an outbound
   |  link by activating it.

7.  Level0: Files

   These are *optional* auto-loaded side-car files to enable hasslefree
   XR Movies (#XR%20Movies).
   they can accomodate developers or applications who (for whatever
   reason) must not modify the 3D scene-file (a .glb e.g.).

7.1.  via href metadata

   scene.glb  <--- 'href' extra [heuristic] detected inside!
   scene.png  (preview thumbnail)
   scene.ogg  (soundtrack to plays when global 3D animation starts)
   scene.vtt  (subtitles for accessibility or screenreaders)
   scene.json (sidecar JSON-file with explicit metadata)

   *heuristics*:




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   *  if at least one href custom property/extra is found in a 3D scene
   *  The viewer should poll for the above mentioned sidecar-file
      extensions (and present accordingly)

7.2.  via chained extension

   scene.xrf.glb  <--- '.xrf.' sidecar file heuristic detected!
   scene.xrf.png  (preview thumbnail)
   scene.xrf.ogg  (soundtrack to plays when global 3D animation starts)
   scene.xrf.vtt  (subtitles for accessibility or screenreaders)
   scene.xrf.json (sidecar JSON-file with explicit metadata)

   |  A fallback-mechanism to turn 3D files into XR Movies
   |  (#XR%20Movies) without editing them.

   *heuristics*:

   *  the chained-extension heuristic .xrf. should be present in the
      filename (scene.xrf.glb e.g.)

7.3.  via subdocuments/xattr

   More secure protocols (Nextgraph e.g.) don't allow for simply polling
   files.  In such case, subdocuments or extended attributes should be
   polled:

   |  NOTE: in the examples below we use the href-heuristic, but also
   |  the .xrf. chained-extension applies here.

myspreadsheet.ods
└── explainer.glb      <--- 'href' extra [heuristic] detected inside!
    ├── explainer.ogg  (soundtrack to play when global 3D animation starts)
    ├── explainer.png  (preview thumnbnail)
    ├── explainer.json (sidecar JSON-file with explicit metadata)
    └── explainer.vtt  (subtitles for accessibility or screenreaders)

   If only extended attributes (xattr) are available, the respective
   referenced file can be embedded:

   $ setfattr -n explainer.ogg -v "soundtrack.ogg" explainer.glb
   $ setfattr -n explainer.png -v "thumbnail.png" explainer.glb
   $ setfattr -n explainer.vtt -v "subtitles.vtt" explainer.glb

   |  NOTE: Linux's setfattr/getfattr is xattr on mac, and Set-Content/
   |  Get-content on Windows.  See pxattr
   |  (https://www.lesbonscomptes.com/pxattr/index.html) for lowlevel
   |  access.




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7.4.  JSON sidecar-file

   For developers, sidecar-file can allow for defining *explicit* XR
   Fragments links (>level1), outside of the 3D file.
   This can be done via (objectname/metadata) key/value-pairs in a JSON
   sidecar-file (https://en.wikipedia.org/wiki/Sidecar_file):

   *  experience.glb
   *  experience.json <----

   {
     "/":
       "aria-description": "description of scene",
     },
     "button": {
       "href": "#roomB"
     }
   }

   |  This will make object button clickable, and teleport the user to
   |  object roomB.

   So after loading experience.glb the existence of experience.json is
   detected, to apply the explicit metadata.
   The sidecar will define (or *override* already existing) extras,
   which can be handy for multi-user platforms (offer 3D scene
   customization/personalization to users).

   |  In THREE.js-code this would boil down to:

    scene.userData['aria-description'] = "description of scene"
    scene.getObjectByName("button").userData.href = "#roomB"

    // now the XR Fragments parser can process the XR Fragments userData 'extras' in the scene

8.  Level1: URI

   |  *XR Fragments allows deeplinking of 3D objects by mapping
   |  objectnames to URI fragments*

   XR Fragments tries to seek to connect the world of text (semantical
   web / RDF), and the world of pixels.
   Instead of forcing authors to combine 3D/2D objects programmatically
   (publishing thru a game-editor e.g.), XR Fragments *integrates all*
   which allows a universal viewing experience.






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  +───────────────────────────────────────────────────────────────────────────────────────────────+
  │                                                                                               │
  │                          U R N                                                                │
  │ U R L                      |                                                                  │
  │  |       |-----------------+--------|                                                         │
  │  +--------------------------------------------------|                                         │
  │  |                                                                                            │
  │  + https://foo.com/some/foo/scene.glb#someview             <-- http URI (=URL and has URN)    │
  │  |                                                                                            │
  │  + ipfs://cfe0987ec9r9098ecr/cats.fbx#someview             <-- an IPFS URI (=URL and has URN) │
  │                                                                                               │
  │  ec09f7e9cf8e7f09c8e7f98e79c09ef89e000efece8f7ecfe9fe      <-- an interpeer URI               │
  │                                                                                               │
  │                                                                                               │
  │  |------------------------+-------------------------|                                         │
  │                           |                                                                   │
  │                         U R I                                                                 │
  │                                                                                               │
  +───────────────────────────────────────────────────────────────────────────────────────────────+

   Fact: our typical browser URL's are just *a possible implementation*
   of URI's (for untapped humancentric potential of URI's see
   interpeer.io (https://interpeer.io) or NextGraph
   (https://nextgraph.org) )

   |  XR Fragments does not look at XR (or the web) thru the lens of
   |  HTML or URLs.
   |  But approaches things from a higherlevel local-first 3D hypermedia
   |  browser-perspective.

   Below you can see how this translates back into good-old URLs:




















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 +───────────────────────────────────────────────────────────────────────────────────────────────+
 │                                                                                               │
 │   the soul of any URL:       ://macro        /meso           ?micro      #nano                │
 │                                                                                               │
 │                2D URL:       ://library.com  /document       ?search     #chapter             │
 │                                                                                       xrf://  │
 │                4D URL:       ://park.com     /4Dscene.fbx ─> ?other.glb ─> #object ─> hashbus │
 │                                                │                           #filter     │      │
 │                                                │                           #tag        │      │
 │                                                │     (hypermediatic)       #material   │      │
 │                                                │     (  feedback   )       #animation  │      │
 │                                                │     (    loop     )       #texture    │      │
 │                                                │                           #variable   │      │
 │                                                │                                       │      │
 │                                               XRWG <─────────────────────<─────────────+      │
 │                                                │                                       │      │
 │                                                └─ objects  ──────────────>─────────────+      │
 │                                                                                               │
 │                                                                                               │
 +───────────────────────────────────────────────────────────────────────────────────────────────+

   |  ?-linked and #-linked navigation are JUST one possible way to
   |  implement XR Fragments: the essential goal is to allow a
   |  Hypermediatic FeedbackLoop (HFL) between external and internal 4D
   |  navigation.

8.1.  List of URI Fragments

   +=======================+======================================+===========+=============+
   |fragment               |type                                  |example    |info         |
   +=======================+======================================+===========+=============+
   |#......                |vector3                               |#room1     |positions/   |
   |                       |                                      |#room2     |parents      |
   |                       |                                      |#cam2      |camera(rig)  |
   |                       |                                      |           |(or XR floor)|
   |                       |                                      |           |to xyz-      |
   |                       |                                      |           |coord/object/|
   |                       |                                      |           |camera and   |
   |                       |                                      |           |upvector     |
   +-----------------------+--------------------------------------+-----------+-------------+
   |Media Fragments        |media fragment                        |#t=0,2&loop|play (and    |
   |(https://www.w3.org/TR/|(#media%20fragments%20and%20datatypes)|           |loop) 3D     |
   |media-frags/)          |                                      |           |animation    |
   |                       |                                      |           |from 0       |
   |                       |                                      |           |seconds till |
   |                       |                                      |           |2 seconds    |
   +-----------------------+--------------------------------------+-----------+-------------+




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                                  Table 3

8.2.  List of *_explicit_ metadata

   These are the possible 'extras' for 3D nodes and sidecar-files

     +======+========+================+==========+===================+
     | key  | type   | example (JSON) | function | existing          |
     |      |        |                |          | compatibility     |
     +======+========+================+==========+===================+
     | href | string | "href":        | XR       | custom property   |
     |      |        | "b.gltf"       | teleport | in 3D fileformats |
     +------+--------+----------------+----------+-------------------+

                                  Table 4

9.  Level2: href links

   Explicit href metadata ('extras') in a 3D object (of a 3D file), hint
   the viewer that the user ''can interact'' with that object :

   | fragment | type | example value | |href| string (uri or predefined
   view) | #pyramid
   #lastvisit
   xrf://#-someobject
   ://somefile.gltf#foo
   |

9.1.  Interaction behaviour

   When clicking an ''href''-value, the user(camera) is teleport to the
   referenced object.

   The imported/teleported destination can be another object in the same
   scene-file, or a different file.

9.2.  XR Viewer implementation

   | *spec* | *action* | *feature* | |-|-|-| | level0+1 | hover 3D file
   href (#via-href-metadata) | show the preview PNG thumbnail (if any).
    | | level0+1 | launch 3D file href (#via-href-metadata) | replace
   the current scene with a new 3D file (href: other.glb e.g.) | |
   level2 | click internal 3D file href (#via-href-metadata) (#roomB
   e.g.) | teleport the camera to the origin of object(name roomB).  See
   [[teleport camera]].| | level2 | click external 3D file href (#via-
   href-metadata) (foo.glb e.g.) | replace the current scene with a new
   3D file (href: other.glb e.g.) | | level2 | hover external 3D file
   href (#via-href-metadata) | show the preview PNG thumbnail (if any



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   sidecar, see level0) | | level2 | click href (#via-href-metadata) |
   hashbus: execute without changing the toplevel URL location (href:
   xrf://#someObjectName e.g.) | | level3 | click href (#via-href-
   metadata) | set the global 3D animation timeline to its Media
   Fragment value (#t=2,3 e.g.) |

   |  NOTE: hashbus links (xrf://#foo&bar) don't change the toplevel
   |  URL, which makes it ideal for interactions (in contrast to typical
   |  #roomC navigation, which benefit back/forward browser-buttons),
   |  see <a href="#hashbus">hashbus</a> for more info.

10.  Level3: Media Fragments

   |  these allow for XR Movies with a controllable timeline using href
   |  URI's with Media Fragments

   Just like with 2D media-files, W3C mediafragments (#t=1,2) can be
   used to control a timeline via the #t (##t) primitive.  XR Fragments
   Level3 makes the 3D timeline, as well as URL-referenced files
   *controllable* via Media Fragments like:

   *  level2 hrefs (href: #t=4 e.g. to control 3D timeline)
   *  level4: xrf: URI scheme:
      -  href: xrf:foo.wav#t=0 to play a wav
      -  href: xrf:news.glb?clone#t=0 to instance and play another
         experience

10.1.  Animation(s) timeline

   controls the animation(s) of the scene (or src resource which
   contains a timeline)

   | fragment | type | functionality | | <b>#t</b>=start,stop |
   [vector2] (default:`#t=0,0`) | start,stop (in seconds |

   | Example Value | Explanation | | #t=1 | play (3D) animations from 1
   seconds till end (and stop) | | #t=1,100 | play (3D) animations from
   1 till 100 seconds (and stop) | | #t=0,0 | stop (3D) animations at
   frame 0 |

   |  Use [[#s 🌱]] to control playback speed

10.2.  Specify playback loopmode

   This compensates a missing element from Media Fragments to enable/
   disable temporal looping. .





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   | fragment | type | functionality | | <b>#loop</b> | string | enables
   animation/video/audio loop | | <b>#-loop</b> | string | disables
   animation/video/audio loop |

10.3.  Controlling embedded content

   use [[URI Templates]] to control embedded media, for example a simple
   video-player:

 foo.usdz
    │
    ├── ◻ loopbutton_enable
    │      └ href: #loop           <-- enable global loop
    │
    ├── ◻ loopbutton_enable
    │      └ href: #-loop          <-- disable global loop
    │
    ├── ◻ playbutton
    │      └ href: #t=10&loop      <-- play global 3D timeline (all anims) (looped)
    │
    └── ◻ playbutton_external
           └ href: https://my.org/animation.glb#!&t=3,10   <-- import & play external anim

11.  Level4: prefix operators

   Prefixing objectnames with the following simple operators allow for
   *extremely powerful* XR interactions:

   *  #!
   *  #*
   *  #+ or #-
   *  #|
   *  xrf: URI scheme

   |  *Examples:* #+menu to show a object, #-menu to hide a menu, #!menu
   |  to teleport a menu, #*block to clone a grabbable block, #|object
   |  to share an object

11.1.  Object teleports (!)

   Prefixing an object with an exclamation-symbol, will teleport a
   (local or remote) referenced object from/to its original/usercamera
   location.

   [img[objecteleport.png]]






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   Usecases: * show/hide objects/buttons (menu e.g.) in front of user *
   embed remote (object within) 3D file via remote URL * instance an
   interactive object near the user regardless of location * instance
   HUD or semi-transparent-textured-sphere (LUT) around the user

   <div class="border padding" style="border:4px solid #888"> <span
   class="big hi1">#!menu</span> </div>

   Clicking the href (#via-href-metadata)-value above will:

   1.  *reposition the referenced object* (menu) to the
       usercamera's-coordinates.
   2.  *zoom* in case of (non-empty) mesh-object: rescale to 1 m³, and
       position 1m in front of the camera
   3.  toggle behaviour: revert values if 1/2 were already applied
   4.  #+ is always implied (objects are always made visible)

   This tiny but powerful symbol allows incredible interactive
   possibilities, by carefully positioning re-usable objects outside of
   a scene (below the usercamera's floor e.g.).

   *  href: #whiteroom&!explainer&!exitmenu

   |  This will teleport the user to whiteroom and moves object
   |  explainer and exitmenu in front of the user.

   *  href: `https://my.org/foo.glb#! (https://my.org/foo.glb#!)

   Clicking the href (#via-href-metadata)-value above will:

   1.  import foo.glb from my.org's webserver
   2.  show it in front of the user (because #! indicates object
       teleport)

   *  href: https://foo.glb#roomB&!bar

   Clicking the href (#via-href-metadata)-value above will:

   1.  replace the current scene with foo.glb
   2.  teleport the user to #roomB inside foo.glb
   3.  *instance the referenced object* (bar inside foo.glb) in front of
       the user.
   4.  it will update the top-Level URL (because xrf: was not used)
   5.  hide the *instanced object* when clicked again (toggle
       visibility)






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   |  *NOTE*: level2 teleportation links, as well as instancing
   |  mitigates the 'broken embedded image'-issue of HTML: *always*
   |  attaching the href-values to *a 3D (preview) object* (that way
   |  broken links will not break the design).

   *Example:* clicking a 3D button with title 'menu' and href (#href)-
   value xrf:menu.glb?instance#t=4,5 would instance a 3D menu (menu.glb)
   in front of the user, and loop its animation between from 4-5 seconds
   (t=4,5)

   |  *NOTE*: combining instance-operators allows dynamic construction
   |  of 3D scenes (#london&!welcomeMenu&!fadeBox e.g.)

11.2.  Object multipliers (*)

   The star-prefix will clone a (local or remote) referenced object to
   the usercamera's location, and make it grabbable.
   Usecases: * object-picker (build stuff with objects)

   |  *NOTE*: this is basically the #! operator (#%23%21) which
   |  infinitely *clones* the referenced object (instead of
   |  repositioning the object).

11.3.  De/selectors (+ and -)

   *  href: #-welcome (or #+welcome)

   Clicking href-value above will do:

   1.  show/hide the target object (and children)

   *  href: #https://my.org/foo.glb/#bar&-welcome

   |  *NOTE:* the latter shows that (de)selectors can also be with
   |  regular href (#href)-values

11.4.  Sharing object or file (#|)

   The pipe-symbol (|) sends a (targeted) object to the OS.  Clicking
   the href-value below will:

   1.  share the (targeted object in the) file to a another application

   |  This URL can be fed straight into Web Share API
   |  (https://developer.mozilla.org/en-US/docs/Web/API/Web_Share_API)
   |  or xdg-open (https://www.freedesktop.org/wiki/Software/xdg-utils/)

   *  href: xrf://#|bar



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   |  *NOTE*: sharing is limited to (internal objects) via xrf: scheme-
   |  only

11.5.  xrf:// URI scheme

   Prefixing the xrf: to href (#href)-values *will prevent* level2
   (#📜%20level2:%20explicit%20links) href (#href)-values from changing
   the top-Level URL.

   |  *Usecase*: for non-shareable URLs like href: xrf:#t=4,5, to
   |  display a stateful msg e.g.).

   *Reason:* XR Fragments is inspired by HTML's href-attribute
   (https://en.wikipedia.org/wiki/Hyperlink), which does various things:

   1.  it updates the browser-location
   2.  it makes something clickable
   3.  it jumps to another document / elsewhere in the same document
   4.  and more

   The xrf: scheme will just do 2 & 3 (so the URL-values will not leak
   into the top-level URL).

   |  *compliance with RFC 3986*: unimplemented/unknown URI schemes
   |  (xrf:... e.g.) will not update the top-level URL

12.  Level5: URI Templates (RFC6570)

   )

   XR Fragments adopts Level1 URI *Fragment* expansion to provide safe
   interactivity.
   This is non-normative, and the draft spec is available on request.

13.  Top-level URL processing

   |  Example URL: ://foo/world.gltf#room1&t=10&cam

   The URL-processing-flow for hypermedia browsers goes like this:

   1.  IF scene operators and/or animation operator (t) are present in
       the URL then (re)position the camera (to room1) and/or animation-
       range (10) accordingly.
   2.  IF no camera-position has been set in <b>step 1 or 2</b> assume
       0,0,0 as camera coordinate (XR: add user-height) (example (https:
       //github.com/coderofsalvation/xrfragment/blob/main/src/3rd/js/
       three/navigator.js#L31]]))




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   3.  IF a camera-object exists with name cam assume that user(camera)
       position

13.1.  UX

   End-users should always have read/write access to:

   1.  the current (toplevel) <b>URL</b> (an URLbar etc)
   2.  URL-history (a <b>back/forward</b> button e.g.)
   3.  Clicking/Touching an href navigates (and updates the URL) to
       another scene/file (and coordinate e.g. in case the URL contains
       XR Fragments).

14.  Example: Navigating content href portals

   navigation, portals & mutations

         +==========+==================+========================+
         | fragment | type             | example value          |
         +==========+==================+========================+
         | href     | string (uri or   | #room1                 |
         |          | predefined view) | #room1                 |
         |          |                  | ://somefile.gltf#room1 |
         +----------+------------------+------------------------+

                                 Table 5

   1.  clicking an outbound ''external''- or ''file URI'' fully replaces
       the current scene and assumes room2 by default (unless specified)

   2.  relocation/reorientation should happen locally for local URI's
       (#....)

   3.  navigation should not happen ''immediately'' when user is more
       than 5 meter away from the portal/object containing the href (to
       prevent accidental navigation e.g.)

   4.  URL navigation should always be reflected in the client URL-bar
       (in case of javascript: see [here
       (https://github.com/coderofsalvation/xrfragment/blob/dev/src/3rd/
       js/three/navigator.js) for an example navigator), and only update
       the URL-bar after the scene (default fragment #) has been loaded.

   5.  In immersive XR mode, the navigator back/forward-buttons should
       be always visible (using a wearable e.g., see [here
       (https://github.com/coderofsalvation/xrfragment/blob/dev/example/
       aframe/sandbox/index.html#L26-L29) for an example wearable)




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   6.  make sure that the ''back-button'' of the ''browser-history''
       always refers to the previous position (see [here (https://github
       .com/coderofsalvation/xrfragment/blob/main/src/3rd/js/three/xrf/
       href.js#L97))

   7.  ignore previous rule in special cases, like clicking an href
       using camera-portal collision (the back-button could cause a
       teleport-loop if the previous position is too close)

   8.  href-events should bubble upward the node-tree (from children to
       ancestors, so that ancestors can also conain an href), however
       only 1 href can be executed at the same time.

   9.  the end-user navigator back/forward buttons should repeat a back/
       forward action until a #... primitive is found (the stateless
       xrf:// href-values should not be pushed to the url-history)

   » example implementation
   (https://github.com/coderofsalvation/xrfragment/blob/main/src/3rd/js/
   three/xrf/href.js)
   » example 3D asset
   (https://github.com/coderofsalvation/xrfragment/blob/main/example/
   assets/href.gltf#L192)
   » discussion (https://github.com/coderofsalvation/xrfragment/
   issues/1)

14.1.  Walking surfaces

   |  By default position 0,0,0 of the 3D scene represents the walkable
   |  plane, however this is overridden when the following applies:

   XR Fragment-compatible viewers can infer this data based scanning the
   scene for:

   1.  materialless (nameless & textureless) mesh-objects (without href
       and >0 faces)

   |  optionally the viewer can offer thumbstick, mouse or joystick
   |  teleport-tools for non-roomscale VR/AR setups.

15.  Example: Virtual world rings

   Consider 3D scenes linking to eachother using these href values,
   attached to 3D button-objects:

   *  href: schoolA.edu/projects.gltf#math
   *  href: schoolB.edu/projects.gltf#math
   *  href: university.edu/projects.gltf#math



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   This would teleport users to the math-projects of those universities.
   Now consider adding a 'webring index'-button to each file, with this
   href-value:

   *  href: workgroup.edu/webrings.glb#!webringmenu

   This would allow displaying the (remote 3D file) webring menu with
   various href-buttons inside, all centrally curated by the workgroup.

16.  Additional scene metadata

   XR Fragments does not aim to redefine the metadata-space or
   accessibility-space by introducing its own cataloging-metadata
   fields.  Instead, it encourages browsers to scan nodes for the
   following custom properties:

   *  SPDX (https://spdx.dev/) license information
   *  ARIA (https://www.w3.org/WAI/standards-guidelines/aria/)
      attributes (aria-*: .....)
   *  datapackage.json (https://datapackage.org) findability,
      accessibility, interoperability, and reusability of data

   ARIA's aria-description-metadata is normative, to aid accessibility
   and scene transcripts

   |  *NOTE*: please always start aria-description with a verb to aid
   |  transcripts.

   The following metadata are non-normative but encouraged, since they
   are popular and cheap to parse:

   *  RDF/JSON-LD (https://json-ld.org) like this example
      (https://mvmd.org/standards/gltf/) or via glTF's KHR_xmp_json_ld
      extension
      (https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/
      Khronos/KHR_xmp_json_ld)
   *  Open Graph (https://ogp.me) attributes (og:*: .....)
   *  Dublin-Core (https://www.dublincore.org/specifications/dublin-
      core/application-profile-guidelines/) attributes(dc:*: .....)
   *  BibTex (https://bibtex.eu/fields) when known bibtex-keys exist
      with values enclosed in { and },

   |  Example: object 'tryceratops' with aria-description: is a huge
   |  dinosaurus standing on a #mountain generates transcript
   |  #tryceratops is a huge dinosaurus standing on a #mountain, where
   |  the hashtags are clickable XR Fragments (activating the visible-
   |  links in the XR browser).




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   Individual nodes can be enriched with such metadata, but most
   importantly the scene node:

       +================================+=========================+
       | metadata key                   | example value           |
       +================================+=========================+
       | aria-description,              | An immersive experience |
       | og:description, dc:description | about Triceratops (*)   |
       +--------------------------------+-------------------------+
       | SPDX                           | CC0-1.0                 |
       +--------------------------------+-------------------------+
       | dc:creator                     | John Doe                |
       +--------------------------------+-------------------------+
       | dc:title, og:title             | 'Triceratops` (*)       |
       +--------------------------------+-------------------------+
       | og:site_name                   | https://xrfragment.org  |
       +--------------------------------+-------------------------+
       | dc.publisher                   | NLNET                   |
       +--------------------------------+-------------------------+
       | dc.date                        | 2024-01-01              |
       +--------------------------------+-------------------------+
       | dc.identifier                  | XRFRAGMENT-001          |
       +--------------------------------+-------------------------+
       | journal (bibTeX)               | {Future Of Text Vol 3}, |
       +--------------------------------+-------------------------+

                                 Table 6

   |  * = these are interchangable (only one needs to be defined)

   There's no silver bullet when it comes to metadata, so XR Fragment-
   implementations should support where the metadata is/goes.

   |  These attributes can be scanned and presented during an href or
   |  src eye/mouse-over.

17.  Accessibility interface

   The addressibility of XR Fragments allows for unique 3D-to-text
   transcripts, as well as an textual interface to navigate 3D content.
   Spec:
   <Br>

   1.   The enduser must be able to enable an accessibility-mode (which
        persists across application/webpage restarts)
   2.   Accessibility-mode must contain a text-input for the user to
        enter text




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   3.   Accessibility-mode must contain a flexible textlog for the user
        to read (via screenreader, screen, or TTS e.g.)
   4.   the textlog contains aria-descriptions, and its narration
        (Screenreader e.g.) can be skipped (via 2-button navigation)
   5.   The back command should navigate back to the previous URL (alias
        for browser-backbutton)
   6.   The forward command should navigate back to the next URL (alias
        for browser-nextbutton)
   7.   A destination is a 3D node containing an href with a #... XR
        fragment (which matches a 3d object name)
   8.   The go command should list all possible destinations
   9.   The go left command should move the camera around 0.3 meters to
        the left
   10.  The go right command should move the camera around 0.3 meters to
        the right
   11.  The go forward command should move the camera 0.3 meters forward
        (direction of current rotation).
   12.  The rotate left command should rotate the camera 0.3 to the left
   13.  The rotate left command should rotate the camera 0.3 to the
        right
   14.  The (dynamic) go abc command should navigate to #scene2 in case
        there's a 3D node with name abc and href value #scene2
   15.  The look command should give an (contextual) 3D-to-text
        transcript, by scanning the aria-description values of the
        current #... (3D object) value (including its children)
   16.  The do command should list all possible href values which don't
        contain an #... XR Fragment
   17.  The (dynamic) do abc command should navigate/execute
        https://.../... in case a 3D node exist with name abc and href
        value https://.../...

17.1.  Two-button navigation

   For specific user-profiles, gyroscope/mouse/keyboard/audio/visuals
   will not be available.
   Therefore a 2-button navigation-interface is the bare minimum
   interface:

   1.  objects with href metadata can be cycled via a key (tab on a
       keyboard)
   2.  objects with href metadata can be activated via a key (enter on a
       keyboard)
   3.  the TTS reads the href-value (and/or aria-description if
       available)







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17.2.  Overlap with fileformat-specific extensions

   Some 3D scene-fileformats have support for extensions.  What if the
   functionality of those overlap?  For example, GLTF has the OMI_LINK
   extension which might overlap with XR Fragment's href:

   |  Priority Order and Precedence, otherwise fallback applies

   1.*Extensions Take Precedence*: Since glTF-specific extensions are
   designed with the format’s specific needs and optimizations in mind,
   they should take precedence over extras metadata in cases where both
   contain overlapping functionality.  This approach aligns with the
   idea that extensions are more likely to be interpreted uniformly by
   glTF-compatible software.

   2.  *Fallback Fall-through Mechanism*: If a glTF implementation does
       not support a particular extension, the (XRF) extras field can
       serve as a fallback.  This way, metadata provided in extras can
       still be useful for applications that don't handle certain
       extensions.

   |  *Example 1* In case of the OMI_LINK glTF extension (href:
   |  https://nlnet.nl) and an XR Fragment (href: #otherroom or href:
   |  otherplanet.glb), it is clear that https://nlnet.nl should open in
   |  a browsertab, whereas the XR Fragment links should teleport the
   |  user.  If the OMI_LINK contains an XR Fragment (#room1 e.g.) a
   |  teleport should be performed only (and other [overlapping]
   |  metadata should be ignored).
   |  
   |  *Example 2* If an Extensions uses XR Fragments in URI's (href:
   |  #otherroom or href: xrf://-walls in OMI_LINK e.g.), then perform
   |  them according to XR Fragment spec (teleport user).  But only
   |  once: ignore further overlapping metadata for that usecase.

18.  Vendor Prefixes

   Vendor-specific metadata in a 3D scenefiles, are similar to vendor-
   specific CSS-prefixes (https://en.wikipedia.org/wiki/
   CSS#Vendor_prefixes) (-moz-opacity: 0.2 e.g.).  This allows popular
   3D engines/frameworks, to initialize specific features when loading a
   scene/object, in a progressive enhanced way.

   Vendor Prefixes allows embedding 3D engines/framework-specific
   features a 3D file via metadata:







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     +===========+===================+==============================+
     | what      | XR metadata       | Lowest common denominator    |
     +===========+===================+==============================+
     | CSS       | vendor-agnostic   | 2D canvas + object           |
     |           |                   | referencing/styling          |
     +-----------+-------------------+------------------------------+
     | XR        | vendor-agnostic   | 3D camera + object(file)     |
     | Fragments |                   | load/embed/click/referencing |
     +-----------+-------------------+------------------------------+
     | Vendor    | vendor-*specific* | Specialized Entity-Component |
     | prefixs   |                   | implementation               |
     +-----------+-------------------+------------------------------+

                                 Table 7

   |  Why?  Because not all XR interactions can/should be solved/
   |  standardized by embedding XR Fragments into any 3D file.  The
   |  lowest common denominator between 3D engines is the 'entity'-part
   |  of their entity-component-system (ECS).  The 'component'-part can
   |  be progressively enhanced via vendor prefixes.

   For example, the following metadata can be added to a .glb file, to
   make an object grabbable in AFRAME:




























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+────────────────────────────────────────────────────────────────────────────────────────────────────────+
│ http://y.io/z.glb                             | AFRAME app                                             │
│-----------------------------------------------+--------------------------------------------------------│
│                                               |                                                        │
│                                               | after loading the glb, john can be placed into the     │
│     +-[3D mesh]-+                             | castle via hands, because the author added metadata to │
│     |    / \    |                             | john via either:                                       │
│     |   /   \   |                             |                                                        │
│     |  /     \  |                             | 1. Blender (custom property-box, no plugins needed)    │
│     |  |_____|  |                             |                                                        │
│     +-----│-----+                             | 2. javascript-code:                                    │
│           │                                   |                                                        │
│           ├─ name: castle                     |     for( var com in this.el.components ){              │
│           └─ tag: house baroque               |       this.el.object3D.userData[`-AFRAME-${com}`] = '' │
│                                               |     }                                                  │
│ [3D mesh-+                                    |     // save to z.glb in AFRAME inspector               │
│ |        ├─ name: john                        |                                                        │
│ |    O   ├─ age: 23                           |                                                        │
│ |   /|\  ├─ -aframe-grabbable:      ''        | > inits 'grabbable' component on object john           │
│ |   / \  ├─ -aframe-material.color: '#F0A'    | > inits 'material' component on object john            │
│ |        ├─ -aframe-text.value:  '{name}{age}'| > inits 'text' component (*) with value 'john'         │
│ |        ├─ -three-material.fog: false        | > changes material settings in THREE.js app            │
│ |        ├─ -godot-Label3D.text: '{name}{age}'| > inits 'Label3D' component (*) in Godot               │
│ +--------+                                    |                                                        │
│                                               |                                                        │
├─ -GODOT-version:  '4.3'                       | > exporters/authors can report targeted version        │
├─ -AFRAME-version: '1.6.0'                     |                    and (optionally) hint component-repo│
├─ -AFRAME-info:    'https://git.benetou.fr/comps'                                                       │
│                                               |                                                        │
+────────────────────────────────────────────────────────────────────────────────────────────────────────+

   *  key/value syntax: -<vendorname>-<component|version>.<key>
      [string/boolean/float/int]-value

   String-templatevalues are evaluated as per URI Templates (RFC6570)
   (https://www.rfc-editor.org/rfc/rfc6570) Level 1.

   |  This 'separating of mechanism from policy' (unix rule) does
   |  *somewhat* break portability of an XR experience, but still
   |  prevents (E-waste of) handcoded virtual worlds.  It allows for (XR
   |  experience) metadata to survive in future 3D engines and scene-
   |  fileformats.









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19.  Security Considerations

   The only dynamic parts are W3C Media Fragments
   (https://www.w3.org/TR/media-frags/) and URI Templates (RFC6570)
   (https://www.rfc-editor.org/rfc/rfc6570).
   The use of URI Templates is limited to pre-defined variables and
   Level0 fragments-expansion only, which makes it quite safe.
   n fact, it is much safer than relying on a scripting language
   (javascript) which can change URN too.

20.  FAQ

   *Q:* Why is everything HTTP GET-based, what about POST/PUT/DELETE
   HATEOS
   *A:* Because it's out of scope: XR Fragment specifies a read-only way
   to surf XR documents.  These things belong in the application layer
   (for example, an XR Hypermedia browser can decide to support
   POST/PUT/DELETE requests for embedded HTML thru src values)

   *Q:* Why isn't there support for scripting, URI Template Fragments
   are so limited compared to WASM & javascript *A:* This is out of
   scope as it unhyperifies hypermedia, and this is up to XR hypermedia
   browser-extensions.
   Historically scripting/Javascript seems to been able to turn webpages
   from hypermedia documents into its opposite (hyperscripted
   nonhypermedia documents).
   In order to prevent this backward-movement (hypermedia tends to
   liberate people from finnicky scripting) XR Fragment uses W3C Media
   Fragments (https://www.w3.org/TR/media-frags/) and URI Templates
   (RFC6570) (https://www.rfc-editor.org/rfc/rfc6570), to prevent
   unhyperifying itself by hardcoupling to a particular markup or
   scripting language.
   XR Fragments supports filtering objects in a scene only, because in
   the history of the javascript-powered web, showing/hiding document-
   entities seems to be one of the most popular basic usecases.
   Doing advanced scripting & networkrequests under the hood are
   obviously interesting endavours, but this is something which should
   not be hardcoupled with XR Fragments or hypermedia.
   This perhaps belongs more to browser extensions.
   Non-HTML Hypermedia browsers should make browser extensions the right
   place, to 'extend' experiences, in contrast to code/javascript inside
   hypermedia documents (this turned out as a hypermedia antipattern).

21.  authors

   *  Leon van Kammen (@lvk@mastodon.online)
   *  Jens Finkhäuser (@jens@social.finkhaeuser.de)




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22.  IANA Considerations

   This document has no IANA actions.

23.  Acknowledgments

   *  NLNET (https://nlnet.nl)
   *  Future of Text (https://futureoftext.org)
   *  visual-meta.info (https://visual-meta.info)
   *  Michiel Leenaars
   *  Gerben van der Broeke
   *  Mauve
   *  Jens Finkhäuser
   *  Marc Belmont
   *  Tim Gerritsen
   *  Frode Hegland
   *  Brandel Zackernuk
   *  Mark Anderson

24.  Appendix: Definitions

     +=================+=============================================+
     | definition      | explanation                                 |
     +=================+=============================================+
     | human           | a sentient being who thinks fuzzy, absorbs, |
     |                 | and shares thought (by plain text, not      |
     |                 | markuplanguage)                             |
     +-----------------+---------------------------------------------+
     | scene           | a (local/remote) 3D scene or 3D file        |
     |                 | (index.gltf e.g.)                           |
     +-----------------+---------------------------------------------+
     | 3D object       | an object inside a scene characterized by   |
     |                 | vertex-, face- and customproperty data.     |
     +-----------------+---------------------------------------------+
     | URI             | some resource at something somewhere via    |
     |                 | someprotocol (http://me.com/foo.glb#foo or  |
     |                 | e76f8efec8efce98e6f see interpeer.io        |
     |                 | (https://interpeer.io))                     |
     +-----------------+---------------------------------------------+
     | URL             | something somewhere via someprotocol        |
     |                 | (http://me.com/foo.glb)                     |
     +-----------------+---------------------------------------------+
     | URN             | something at some domain (me.com/foo.glb)   |
     +-----------------+---------------------------------------------+
     | metadata        | custom properties of text, 3D Scene or      |
     |                 | Object(nodes), relevant to machines and a   |
     |                 | human minority (academics/developers)       |
     +-----------------+---------------------------------------------+



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     | XR fragment     | URI Fragment with spatial hints (which      |
     |                 | match the name of a 3D object-, camera-,    |
     |                 | animation-object)                           |
     +-----------------+---------------------------------------------+
     | the XRWG        | wordgraph (collapses 3D scene to tags)      |
     +-----------------+---------------------------------------------+
     | the hashbus     | hashtags map to camera/scene-projections    |
     +-----------------+---------------------------------------------+
     | spacetime       | positions camera, triggers scene-preset/    |
     | hashtags        | time                                        |
     +-----------------+---------------------------------------------+
     | teleportation   | repositioning the enduser to a different    |
     |                 | position (or 3D scene/file)                 |
     +-----------------+---------------------------------------------+
     | sourceportation | teleporting the enduser to the original XR  |
     |                 | Document of an src embedded object.         |
     +-----------------+---------------------------------------------+
     | placeholder     | a 3D object which with src-metadata (which  |
     | object          | will be replaced by the src-data.)          |
     +-----------------+---------------------------------------------+
     | src             | (HTML-piggybacked) metadata of a 3D object  |
     |                 | which instances content                     |
     +-----------------+---------------------------------------------+
     | href            | (HTML-piggybacked) metadata of a 3D object  |
     |                 | which links to content                      |
     +-----------------+---------------------------------------------+
     | filter          | URI Fragment(s) which show/hide object(s)   |
     |                 | in a scene based on name/tag/property       |
     |                 | (#cube&-price=>3)                           |
     +-----------------+---------------------------------------------+
     | visual-meta     | visual-meta (https://visual.meta.info) data |
     |                 | appended to text/books/papers which is      |
     |                 | indirectly visible/editable in XR.          |
     +-----------------+---------------------------------------------+
     | requestless     | metadata which never spawns new requests    |
     | metadata        | (unlike RDF/HTML, which can cause           |
     |                 | framerate-dropping, hence not used a lot in |
     |                 | games)                                      |
     +-----------------+---------------------------------------------+
     | FPS             | frames per second in spatial experiences    |
     |                 | (games,VR,AR e.g.), should be as high as    |
     |                 | possible                                    |
     +-----------------+---------------------------------------------+
     | introspective   | inward sensemaking ("I feel this belongs to |
     |                 | that")                                      |
     +-----------------+---------------------------------------------+
     | extrospective   | outward sensemaking ("I'm fairly sure John  |
     |                 | is a person who lives in oklahoma")         |



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     +-----------------+---------------------------------------------+
     | ◻               | ascii representation of an 3D object/mesh   |
     +-----------------+---------------------------------------------+
     | (un)obtrusive   | obtrusive: wrapping human text/thought in   |
     |                 | XML/HTML/JSON obfuscates human text into a  |
     |                 | salad of machine-symbols and words          |
     +-----------------+---------------------------------------------+
     | flat 3D object  | a 3D object of which all verticies share a  |
     |                 | plane                                       |
     +-----------------+---------------------------------------------+
     | BibTeX          | simple tagging/citing/referencing standard  |
     |                 | for plaintext                               |
     +-----------------+---------------------------------------------+
     | BibTag          | a BibTeX tag                                |
     +-----------------+---------------------------------------------+
     | (hashtag)bibs   | an easy to speak/type/scan tagging SDL (see |
     |                 | here (https://github.com/coderofsalvation/  |
     |                 | hashtagbibs) which expands to BibTex/JSON/  |
     |                 | XML                                         |
     +-----------------+---------------------------------------------+

                                  Table 8





























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