Add KHR_interactivity draft

[lexaknyazev]

Rendered version: https://github.com/KhronosGroup/glTF/blob/interactivity/extensions/2.0/Khronos/KHR_interactivity/Specification.adoc

[bhouston]

Great work! What sections are next? Also what could we release as a draft extension for SIGGRAPH 2023?

[aaronfranke]

If we have Pi, we should also have Tau. https://tauday.com/tau-manifesto The value Tau is the circle constant (equal to 2*Pi, or rather, Pi is half of Tau). Using Tau usually results in more readable code. Tau is supported in many programming languages such as C#, Java, Python, GDScript, Rust, Unreal Blueprints, and more, so it's useful for interoperability with other languages, especially Unreal Blueprints which is conceptually similar.

===== Tau

[cols="1h,1,2"]
|===
| Type | `math/tau` | The circle constant, the circumference of the unit circle in radians.
| Output value sockets | `float value` | 6.2831853071795862
|===

[aaronfranke]

In case more justification is needed, the KHR_interactivity document currently contains Degrees-To-Radians (math/rad) and Radians-To-Degrees (math/deg). These allow converting between degrees and radians. Tau can be used for a similarly important purpose, dividing or multiplying by this number allows converting between turns and radians, where 1 turn is τ radians. Tau can be used for more than just this, but the point is, if math/rad and math/deg are justified as a part of KHR_interactivity for converting angles, then math/tau is also justified for converting angles.

[aaronfranke]

Why are variables accessed by index instead of by name? It would make more sense to me to set a variable called "money" than one called 27.

[warrenm]

3. For interpolating rotations, how would the most-used quaternion interpolation – slerp – be implemented? I'm not sure a bezier curve (p1/p2) can be mapped to a spherical interpolation.

My understanding is that the control points specified by an interpolation node are to be used as the control points of a Bézier spline that is used as an easing function, à la cubic Bézier easing functions in CSS and elsewhere.

If that's correct, then the implementation would pass the normalized time parameter (on [0, 1]) to the easing function to get the eased progress fraction, then use that as the "time" parameter of the lower-level interpolation routine (whether lerp or slerp).

[hybridherbst]

Thanks! I found it now. The spec states that it's linear interpolation but also has this remark:

If the Object Model property is a quaternion, spherical linear interpolation expression SHOULD be used.

[Hackn0214]

Thank you for your response.
These parameters seem a bit challenging for designers to handle.

[Hackn0214]

Regarding the question I asked earlier, it seems it might have gotten buried and become hard to notice, so I’d like to ask again.
I couldn't find node/OnSelect(event/OnSelect) in the specification, but is it possible to handle events when an object is selected?

[lexaknyazev]

I couldn't find node/OnSelect(event/OnSelect) in the specification, but is it possible to handle events when an object is selected?

This event is defined in the KHR_node_selectability extension, see #2422.

[Hackn0214]

I couldn't find node/OnSelect(event/OnSelect) in the specification, but is it possible to handle events when an object is selected?

This event is defined in the KHR_node_selectability extension, see #2422.

Oh, I thought this extension was an attribute to specify whether an object can be selected. I’ll read through the draft. Thanks!

[Hackn0214]

How would a connected graph with multiple output value sockets, as shown in the diagram, be represented in glTF?

[Hackn0214]

It's not a self-solution, but is there any issue with this kind of definition?

  "nodes": [
    {
      "type": "math/sign",
      "values": [
        {
          "id": "a",
          "node": 2,
          "socket": "1"
        }
      ]
    },
    {
      "type": "math/sign",
      "values": [
        {
          "id": "a",
          "node": 2,
          "socket": "0"
        }
      ]
    },
    {
      "type": "math/extract2",
      "values": [
        {
          "id": "a",
          "type": 1,
          "value": [
            0.0,
            0.0
          ]
        }
      ]
    }
  ],

[hybridherbst]

I believe this matches the output of https://github.com/KhronosGroup/glTF-InteractivityGraph-AuthoringTool/tree/initial-work-merge (minus the missing type declarations in your partial file).

[Hackn0214]

When defining a custom event as shown below, if event/receive and event/send nodes refer to this custom event.

 "events": [
    {
      "id": "MyEvent",
      "values": [
        {
          "id": "Val_1",
          "type": 0
        },
        {
          "id": "Val_2",
          "type": 0
        }
      ]
    }
  ],

I think the generated node will have sockets as shown below, but what do you think?

[Hackn0214]

Is it possible to return the value pointed to by a pointer with the behavior graph?
For example, I would like to change the base color of a selected object's material.

/nodes/{nodeIdx}/mesh -> meshIdx
/meshes/{meshIdx}/primitibes/0/material -> materialIdx
/materials/{materialIdx}/pbrMetallicRoughness/baseColorFactor

[boguscoder]

If configuration value is array of length 2,3,4 or 16 it could be ambiguous if its vec2/vec3/vec4/vec4x4 or int[], this might not be a problem for JS but is a problem for strongly-typed implementations, is there any recommendation for them how to disambiguate?

[Hackn0214]

For example, how about specifying the type at the time of the call?
In fact, I would also like to have a feature that returns the number of elements as shown below.
/materials -> total material number in the scecne
/meshes/0/primitives -> Total primitive number

[boguscoder]

@Hackn0214 you already can get number of materials when using appropriate pointer , check out core pointers

[Hackn0214]

That's wonderful!
Thank you!

[Hackn0214]

I would appreciate it if you could add /meshes/{}/primitives.length

[RaananW]

The last sentence is a bit hard to understand. Would it be right to say "until a node with any number of flow sockets is executed." ?

[javagl]

Is there a (reasonably up-to-date) machine-processable representation of this? (I.e. something like a schema or a "repository" of node structure descriptions)?

[lexaknyazev]

Is there a (reasonably up-to-date) machine-processable representation of this? (I.e. something like a schema or a "repository" of node structure descriptions)?

Not yet, this document is the only normative source for now. We'll publish regular JSON schemas after confirming that the current early implementations are aligned with them.

That said, JSON schemas alone are not enough since most operations have their own predefined socket types and ids. Suggestions on the machine-readable node spec representations are welcome.

[javagl]

It could be something simple as a starter....

output-2025-01-21.json

One could/should probably group this based on the Section structure, maybe even include the headers like ===== Cross Product or so. One could also think about parsing and understanding the types...

[RaananW]

Each property of the configuration object should have a value property that is an array