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.

[boguscoder]

closing this one as done, this will be in the text soon

[Hackn0214]

I apologize if this has already been mentioned somewhere.
I am currently checking the Interactivity Extension Specification.
How are p1 and p2 parameters used in node pointer/interpolate?

[hybridherbst]

A couple of practical questions based on my experiments:

1. For looping an animation, is the expected approach to set endTime to positive infinity?

2. For preventing animations overlap when multiple animations target the same node, is the expected approach to just stop all animations that might be playing, or to add an internal state of which animation was last played and then stop that?

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.
   Answer: quaternion interpolation is specified as using slerp already. The bezier curve is for the easing function for the t parameter going into the interpolation.

4. Can the same output flow socket be connected to multiple input flow sockets, or must I add a flow/sequence node inbetween? For me that's not clear from the spec. I would expect that similar to values, I can just connect those outputs to multiple inputs and they're all triggered in the order they appear in the JSON.
   Answer: one output flow socket can only be connected to one input flow socket.

5. I find flow/sequence to be confusingly named. It's not clear to me if "all output flows are activated one by one" means that each flow output first needs to finish before the next one is invoked, or that they are all fired immediately and just the order is specified here. For reference, other flow graph implementations that I'm aware of use sequence for "sequential" execution with waiting, and "parallel" for in-order parallel execution of the connected flows.

[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]

Thank you.
I'm comparing the behavior of my custom tool with the authoring tool as a reference.
By the way, is there a way to delete a node once it's placed on the graph in this tool? Currently, I'm restarting the tool to clear the graph.

[boguscoder]

@Hackn0214 backspace seems to remove just fine?

[Hackn0214]

@Hackn0214 backspace seems to remove just fine?
Ah... I was pressing the Delete key.

[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