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This draft is a specification for 4D URLs & navigation, which links together space, time & text together, for hypermedia browsers with- or without a network-connection.<br>
The specification promotes spatial addressibility, sharing, navigation, query-ing and annotating interactive (text)objects across for (XR) Browsers.<br>
XR Fragments allows us to enrich existing dataformats, by recursive use of existing proven technologies like [URI Fragments](https://en.wikipedia.org/wiki/URI_fragment) and BibTags notation.<br>
1. Interlinking text/& 3D by collapsing space into a Word Graph (XRWG) to show [visible links](#visible-links) (and augmenting text with [bibs](https://github.com/coderofsalvation/tagbibs) / [BibTags](https://en.wikipedia.org/wiki/BibTeX) appendices (see [visual-meta](https://visual-meta.info) e.g.)
1. unlocking spatial potential of the (originally 2D) hashtag (which jumps to a chapter) for navigating XR documents
XR Fragments tries to seek to connect the world of text (semantical web / RDF), and the world of pixels.<br>
Instead of combining them (in a game-editor e.g.), XR Fragments is opting for a more integrated path **towards** them, by describing how to make browsers **4D URL-ready**:
> XR Fragments does not look at XR (or the web) thru the lens of HTML.<br>But approaches things from a higherlevel feedbackloop/hypermedia browser-perspective:
> 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.
1. the Y-coordinate of `pos` identifies the floorposition. This means that desktop-projections usually need to add 1.5m (average person height) on top (which is done automatically by VR/AR headsets).
1. set the position of the camera accordingly to the vector3 values of `#pos`
1.`rot` sets the rotation of the camera (only for non-VR/AR headsets)
1.`t` sets the playbackspeed and animation-range of the current scene animation(s) or `src`-mediacontent (video/audioframes e.g., use `t=0,7,7` to 'STOP' at frame 7 e.g.)
An XR Fragment-compatible browser viewing this scene, allows the end-user to interact with the `buttonA` and `buttonB`.<br>
In case of `buttonA` the end-user will be teleported to another location and time in the **current loaded scene**, but `buttonB` will **replace the current scene** with a new one, like `other.fbx`, and assume `pos=0,0,0`.
1. IF a `#cube` matches a custom property-key (of an object) in the 3D file/scene (`#cube`: `#......`) <b>THEN</b> execute that predefined_view.
2. IF scene operators (`pos`) and/or animation operator (`t`) are present in the URL then (re)position the camera and/or animation-range accordingly.
3. IF no camera-position has been set in <b>step 1 or 2</b> update the top-level URL with `#pos=0,0,0` ([example](https://github.com/coderofsalvation/xrfragment/blob/main/src/3rd/js/three/navigator.js#L31]]))
4. IF a `#cube` matches the name (of an object) in the 3D file/scene then draw a line from the enduser('s heart) to that object (to highlight it).
5. IF a `#cube` matches anything else in the XR Word Graph (XRWG) draw wires to them (text or related objects).
An XR Fragment-compatible browser viewing this scene, lazy-loads and projects `painting.png` onto the (plane) object called `canvas` (which is copy-instanced in the bed and livingroom).<br>
Also, after lazy-loading `ocean.com/aquarium.gltf`, only the queried objects `bass` and `tuna` will be instanced inside `aquariumcube`.<br>
Resizing will be happen accordingly to its placeholder object `aquariumcube`, see chapter Scaling.<br>
> Instead of cherrypicking objects with `#bass&tuna` thru `src`, queries can be used to import the whole scene (and filter out certain objects). See next chapter below.
1.<b>local</b>`src` values (URL **starting** with `#`, like `#cube&foo`) means **only** the mentioned objectnames will be copied to the instanced scene (from the current scene) while preserving their names (to support recursive selectors). [(example code)](https://github.com/coderofsalvation/xrfragment/blob/main/src/3rd/js/three/xrf/src.js)
1.<b>local</b>`src` values indicating a query (`#q=`), means that all included objects (from the current scene) will be copied to the instanced scene (before applying the query) while preserving their names (to support recursive selectors). [(example code)](https://github.com/coderofsalvation/xrfragment/blob/main/src/3rd/js/three/xrf/src.js)
1. the instanced scene (from a `src` value) should be <b>scaled accordingly</b> to its placeholder object or <b>scaled relatively</b> based on the scale-property (of a geometry-less placeholder, an 'empty'-object in blender e.g.). For more info see Chapter Scaling.
1.<b>external</b>`src` values should be served with appropriate mimetype (so the XR Fragment-compatible browser will now how to render it). The bare minimum supported mimetypes are:
1.`src` values should make its placeholder object invisible, and only flush its children when the resolved content can succesfully be retrieved (see [broken links](#links))
1.<b>external</b>`src` values should respect the fallback link mechanism (see [broken links](#broken-links)
|`href` | string (uri or predefined view) | `#pos=1,1,0`<br>`#pos=1,1,0&rot=90,0,0`<br>`://somefile.gltf#pos=1,1,0`<br> |
1. clicking an ''external''- or ''file URI'' fully replaces the current scene and assumes `pos=0,0,0&rot=0,0,0` by default (unless specified)
2. relocation/reorientation should happen locally for local URI's (`#pos=....`)
3. navigation should not happen ''immediately'' when user is more than 2 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 (in case of javascript: see [[here](https://github.com/coderofsalvation/xrfragment/blob/dev/src/3rd/js/three/navigator.js) for an example navigator).
5. In 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)
6. in case of navigating to a new [[pos)ition, ''first'' navigate to the ''current position'' so 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. portal-rendering: a 2:1 ratio texture-material indicates an equirectangular projection
[» example implementation](https://github.com/coderofsalvation/xrfragment/blob/main/src/3rd/js/three/xrf/href.js)<br>
[» example 3D asset](https://github.com/coderofsalvation/xrfragment/blob/main/example/assets/href.gltf#L192)<br>
How does the scale of the object (with the embedded properties) impact the scale of the referenced content?<br>
> Rule of thumb: visible placeholder objects act as a '3D canvas' for the referenced scene (a plane acts like a 2D canvas for images e, a cube as a 3D canvas e.g.).
1.<b>IF</b> an embedded property (`src` e.g.) is set on an non-empty placeholder object (geometry of >2 vertices):
* calculate the <b>bounding box</b> of the ''placeholder'' object (maxsize=1.4 e.g.)
* hide the ''placeholder'' object (material e.g.)
* instance the `src` scene as a child of the existing object
* calculate the <b>bounding box</b> of the instanced scene, and scale it accordingly (to 1.4 e.g.)
> REASON: non-empty placeholder object can act as a protective bounding-box (for remote content of which might grow over time e.g.)
| `:` | indicates an object-embedded custom property key/value |
| `>``<` | compare float or int number |
| `/` | reference to root-scene.<br>Useful in case of (preventing) showing/hiding objects in nested scenes (instanced by `src`) (*) |
> \* = `#q=-/cube` hides object `cube` only in the root-scene (not nested `cube` objects)<br> `#q=-cube` hides both object `cube` in the root-scene <b>AND</b> nested `skybox` objects |
[» example implementation](https://github.com/coderofsalvation/xrfragment/blob/main/src/3rd/js/three/xrf/q.js)
[» example 3D asset](https://github.com/coderofsalvation/xrfragment/blob/main/example/assets/query.gltf#L192)
1. detect excluders like `-foo`,`-foo:1`,`-.foo`,`-/foo` (reference regex: `/^-/` )
1. detect root selectors like `/foo` (reference regex: `/^[-]?\//` )
1. detect number values like `foo:1` (reference regex: `/^[0-9\.]+$/` )
1. for every query token split string on `:`
1. create an empty array `rules`
1. then strip key-operator: convert "-foo" into "foo"
1. add operator and value to rule-array
1. therefore we we set `id` to `true` or `false` (false=excluder `-`)
1. and we set `root` to `true` or `false` (true=`/` root selector is present)
1. we convert key '/foo' into 'foo'
1. finally we add the key/value to the store like `store.foo = {id:false,root:true}` e.g.
> An example query-parser (which compiles to many languages) can be [found here](https://github.com/coderofsalvation/xrfragment/blob/main/src/xrfragment/Query.hx)
The obvious approach for this, is to consult the XRWG ([example](https://github.com/coderofsalvation/xrfragment/blob/feat/macros/src/3rd/js/XRWG.js)), which basically has all these things already collected/organized for you during scene-load.
> The XR Fragments does this by collapsing space into a **Word Graph** (the **XRWG** [example](https://github.com/coderofsalvation/xrfragment/blob/feat/macros/src/3rd/js/XRWG.js)), augmented by Bib(s)Tex.
Instead of just throwing together all kinds media types into one experience (games), what about their tagged/semantical relationships?<br>
Perhaps the following question is related: why is HTML adopted less in games outside the browser?
Through the lens of constructive lazy game-developers, ideally metadata must come **with** text, but not **obfuscate** the text, or **spawning another request** to fetch it.<br>
> Why Bib(s)Tex? Because its seems to be the lowest common denominator for an human-curated XRWG (extendable by speech/scanner/writing/typing e.g, see [further motivation here](https://github.com/coderofsalvation/hashtagbibs#bibs--bibtex-combo-lowest-common-denominator-for-linking-data))
1. XR Fragments promotes (de)serializing a scene to the XRWG ([example](https://github.com/coderofsalvation/xrfragment/blob/feat/macros/src/3rd/js/XRWG.js))
5. Like Bibs, XR Fragments generalizes the BibTex author/title-semantics (`author{title}`) into **this** points to **that** (`this{that}`)
6. The XRWG should be recalculated when textvalues (in `src`) change
7. HTML/RDF/JSON is still great, but is beyond the XRWG-scope (they fit better in the application-layer)
8. Applications don't have to be able to access the XRWG programmatically, as they can easily generate one themselves by traversing the scene-nodes.
9. The XR Fragment focuses on fast and easy-to-generate end-user controllable word graphs (instead of complex implementations that try to defeat word ambiguity)
> both `#john@baroque`-bib and BibTex `@baroque{john}` result in the same XRWG, however on top of that 2 tages (`house` and `todo`) are now associated with text/objectname/tag 'baroque'.
> [hashtagbibs](https://github.com/coderofsalvation/hashtagbibs) potentially allow the enduser to annotate text/objects by **speaking/typing/scanning associations**, which the XR Browser saves to remotestorage (or localStorage per toplevel URL). As well as, referencing BibTags per URI later on: `https://y.io/z.fbx#@baroque@todo` e.g.
9. The XR Browser needs to adjust tag-scope based on the endusers needs/focus (infinite tagging only makes sense when environment is scaled down significantly)
10. The XR Browser should always allow the human to view/edit the metadata, by clicking 'toggle metadata' on the 'back' (contextmenu e.g.) of any XR text, anywhere anytime.
13. Default font (unless specified otherwise) is a modern monospace font, for maximized tabular expressiveness (see [the core principle](#core-principle)).
14. anti-pattern: hardcoupling an XR Browser with a mandatory **markup/scripting-language** which departs from onubtrusive plain text (HTML/VRML/Javascript) (see [the core principle](#core-principle))
> The simplicity of appending metadata (and leveling the metadata-playfield between humans and machines) is also demonstrated by [visual-meta](https://visual-meta.info) in greater detail.
* lines beginning with `@` will not be rendered verbatim by default ([read more](https://github.com/coderofsalvation/hashtagbibs#hashtagbib-mimetypes))
* the XRWG should expand bibs to BibTex occurring in text (`#contactjohn@todo@important` e.g.)
> This significantly expands expressiveness and portability of human tagged text, by **postponing machine-concerns to the end of the human text** in contrast to literal interweaving of content and markupsymbols (or extra network requests, webservices e.g.).
> additional tagging using [bibs](https://github.com/coderofsalvation/hashtagbibs): to tag spatial object `note_canvas` with 'todo', the enduser can type or speak `#note_canvas@todo`
To prime the XRWG with text from plain text `src`-values, here's an example XR Text (de)multiplexer in javascript (which supports inline bibs & bibtex):
> when an XR browser updates the human text, a quick scan for nonmatching tags (`@book{nonmatchingbook` e.g.) should be performed and prompt the enduser for deleting them.
**Q:** Why is everything HTTP GET-based, what about POST/PUT/DELETE HATEOS<br>
**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
**A:** This is out of scope, and up to the XR hypermedia browser. 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 Fragments should never unhyperify itself by hardcoupling to a particular markup or scripting language. [XR Macro's](https://xrfragment.org/doc/RFC_XR_Macros.html) are an example of something which is probably smarter and safer for hypermedia browsers to implement, instead of going full-in with a turing-complete scripting language (and suffer the security consequences later).
|src | (HTML-piggybacked) metadata of a 3D object which instances content |
|href | (HTML-piggybacked) metadata of a 3D object which links to content |
|query | an URI Fragment-operator which queries object(s) from a scene like `#q=cube` |
|visual-meta | [visual-meta](https://visual.meta.info) data appended to text/books/papers which is indirectly visible/editable in XR. |
|requestless metadata | metadata which never spawns new requests (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") |
|`◻` | 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 |
|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 |
