Quick reference

1. Abstract
2. Index
3. Introduction
4. How does it work
5. What does it solve
6. HFL (Hypermediatic Feedback Loop) for XR Browsers
7. Conventions and Definitions
   1. XR Fragment URL Grammar
8. Spatial Referencing 3D
9. Level0: Files
   1. via href metadata
   2. via chained extension
   3. via subdocuments/xattr
   4. JSON sidecar-file
10. Level1: URI
    1. List of URI Fragments
    2. List of explicit metadata
11. Level2: href links
    1. Interaction behaviour
    2. XR Viewer implementation
12. Level3: Media Fragments
    1. Animation(s) timeline
    2. Specify playback loopmode
    3. Controlling embedded content
13. Level4: prefix operators
    1. Object teleports
    2. Object multipliers
    3. De/selectors (+ and -)
    4. Sharing object or file (#|)
    5. xrf:// URI scheme
14. Level5: URI Templates (RFC6570)
15. Top-level URL processing
    1. UX
16. Example: Navigating content href portals
    1. Walking surfaces
17. Example: Virtual world rings
18. Additional scene metadata
19. Accessibility interface
    1. Two-button navigation
    2. Overlap with fileformat-specific extensions
20. Vendor Prefixes
21. Security Considerations
22. FAQ
23. Authors
24. IANA Considerations
25. Acknowledgments
26. Appendix: Definitions

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

How does it work

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

What does it solve

It solves:

1. addressibility and hypermediatic navigation of 3D scenes/objects: URI Fragments using src/href spatial metadata
2. Interlinking text & spatial objects by collapsing space into a Word Graph (XRWG) to show 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

XR Fragments views XR experiences through the lens of 3D deeplinked URI’s, rather than thru code(frameworks) or protocol-specific browsers (webbrowser e.g.). To aid adoption, the standard comprises of various (optional) support-levels, which incorporate existing standards like W3C Media Fragments and URI Templates (RFC6570) to promote spatial addressibility, sharing, navigation, filtering and databinding objects for (XR) Browsers.

XR Fragments is in a sense, a heuristical 3D format 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 higher interop between fileformats, 3D editors, viewers and game-engines.

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:

• 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 and HTML-agnostic, though pseudo-XR Fragment browsers can be implemented on top of HTML/Javascript.

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()).

Conventions and Definitions

See appendix below in case certain terms are not clear.

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

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).

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:

  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.

Level0: Files

Compatible 3D fileformats: glTF, usdz, obj, collada, THREE.json, X3D e.g.

A 3D scene-file can be considered XR Fragment-compatible when it contains metadata: 1. implicit: there’s at least one object with a name (*) 2. explicit: (optional) object(s) have (level2) href extras.

* = last wins in case of non-unique names

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

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:

• 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)

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 without editing them.

heuristics:

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

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 for lowlevel access.

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:

• experience.glb
• experience.json <----

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

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 

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.

  +───────────────────────────────────────────────────────────────────────────────────────────────+
  │                                                                                               │
  │                          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 or NextGraph )

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:

 +───────────────────────────────────────────────────────────────────────────────────────────────+
 │                                                                                               │
 │   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.

List of URI Fragments

List of *explicit metadata

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

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
|

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.

XR Viewer implementation

| spec | action | feature | |-|-|-| | level0+1 | hover 3D file href | show the preview PNG thumbnail (if any). | | level0+1 | launch 3D file href | replace the current scene with a new 3D file (href: other.glb e.g.) | | level2 | click internal 3D file href (#roomB e.g.) | teleport the camera to the origin of object(name roomB). See [[teleport camera]].| | level2 | click external 3D file href (foo.glb e.g.) | replace the current scene with a new 3D file (href: other.glb e.g.) | | level2 | hover external 3D file href | show the preview PNG thumbnail (if any sidecar, see level0) | | level2 | click href | hashbus: execute without changing the toplevel URL location (href: xrf://#someObjectName e.g.) | | level3 | click href | 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 hashbus for more info.

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 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

Animation(s) timeline

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

| fragment | type | functionality | | #t=start,stop | [vector2] | 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) |

Specify playback loopmode

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

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

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

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

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]]

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

Clicking the href-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#!

Clicking the href-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-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)

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-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.)

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 which infinitely clones the referenced object (instead of repositioning the object).

De/selectors (+ and -)

How to show/hide/group material- or object- or animations by name?

Clicking href-values below will show/hide the targeted material- or animation or object-name (incl. children):

• #-welcome
• #+welcome
• #-VR*
• https://foo.glb#bar&-welcome

Matching logic:

• - and + prefix for exact matches (welcome e.g.)
• * postfix for match beginning (VR_skybox VR_skyboxmat e.g.)

NOTE: to hide a skybox when importing/loading a 3D file (force AR) is possible by linking to `https://my.org/foo.glb#-skybox or https://my.org/foo.glb#-skyboxmaterial

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 or xdg-open

• href: xrf://#|bar

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

xrf:// URI scheme

Prefixing the xrf: to href-values will prevent level2 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, 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

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.

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 step 1 or 2 assume 0,0,0 as camera coordinate (XR: add user-height) (example)
3. IF a camera-object exists with name cam assume that user(camera) position

UX

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

1. the current (toplevel) URL (an URLbar etc)
2. URL-history (a back/forward 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).

Example: Navigating content href portals

navigation, portals & mutations

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 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 for an example wearable)

6. make sure that the “back-button” of the “browser-history” always refers to the previous position (see [here)

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
» example 3D asset
» discussion

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.

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

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.

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 license information
• ARIA attributes (aria-*: .....)
• datapackage.json 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 like this example or via glTF’s KHR_xmp_json_ld extension
• Open Graph attributes (og:*: .....)
• Dublin-Core attributes(dc:*: .....)
• BibTex 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).

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

* = 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.

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:

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
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://.../...

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)

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.

Vendor Prefixes

Vendor-specific metadata in a 3D scenefiles, are similar to vendor-specific CSS-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:

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:

+────────────────────────────────────────────────────────────────────────────────────────────────────────+ 
│ 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) 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.

Security Considerations

The only dynamic parts are W3C Media Fragments and URI Templates (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.

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 and URI Templates (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).

authors

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

IANA Considerations

This document has no IANA actions.

Acknowledgments

• NLNET
• Future of Text
• visual-meta.info
• Michiel Leenaars
• Gerben van der Broeke
• Mauve
• Jens Finkhäuser
• Marc Belmont
• Tim Gerritsen
• Frode Hegland
• Brandel Zackernuk
• Mark Anderson

Appendix: Definitions

Index