GLTFExporter: CubicSpline interpolation support

[takahirox]

This PR adds CubicSpline interpolation support to GLTFExporter.

CubicSpline interpolation isn't in Three.js core yet, so exporting as CUBICSPLINE only when a custom interpolation GLTFCubicSplineInterpolant declared in GLTFLoader is used.

As GLTFCubicSplineInterpolant is a custom interpolation track.getInterpolation() doesn't return a valid value, it returns Three.js global interpolation type like THREE.InterpolateLinear but there's no global type for GLTFCubicSplineInterpolant.

Then I named GLTFCubicSplineInterpolant factory function and check that name to detect if interpolation is CubicSpline instead of calling track.getInterpolation().

[donmccurdy]

.name will disappear in some minified builds, I think... :/

[takahirox]

Ahhh, right. Hmmmmmm

[donmccurdy]

Related: I imagine we could support exporting three.js's cubic spline interpolant by filling in some inferred tangents? (does not need to be in this PR, though)

[takahirox]

I haven't seen CubicInterpolant (InterpolateSmooth) in the detail yet but probably we could. Yeah let's do that in another PR.

[takahirox]

Related: I'm thinking to add an option to GLTFLoader which loads CUBICSPLINE as CubicInterpolant. Pros, it should always work well with other Three.js stuffs, ex: (de)serialization, because it's in Three.js core. (Custom interpolant sometimes wouldn't work for a certain purpose.) Cons, less precision. Getting (GLTF)CubicSplineInterpolant into core would be another option.

[donmccurdy]

I wonder if we could add support for in/out tangents in THREE.CubicInterpolant without breaking backwards compatibility? 🤔

[takahirox]

Values layout and itemSize valueSize will be differ, so hard to do that without breaking compatibility maybe?
Adding an option to turn on/off in/out tangents could solve this issue but adding an new intepolant type would be easier.

[takahirox]

Updated. Added isInterpolantFactoryMethodGLTFCubicSpline property to the factory method. Workaround indeed.

[donmccurdy]

Adding an option to turn on/off in/out tangents could solve this issue but adding an new intepolant type would be easier.

Yeah, it just seems weird to have two different cubic spline implementations, especially when one is a superset of the other. We could deprecate but not remove the version without tangents, I suppose, or automatically create tangents when they aren't provided? Would want to verify that interpolation is as fast or faster when tangents are used.

Meanwhile, this change looks fine to me.

[takahirox]

or automatically create tangents when they aren't provided

Ah, probably we could detect old version by checking valueSize(values.length / items.length) and generate tangents maybe?

Another concern is increased memory consumption of sample values, 3x larger, if we create tangents at initialization.

[mrdoob]

Thanks!