InstancedMesh: Honor instanceColor in toJSON() and ObjectLoader.

[Michael4d45]

Description

I was having troubles transferring InstancedMesh data due to the Object3D.toJSON and ObjectLoader.parseObject functions not accounting for the instanceColor attribute.

I've added instanceColor to the InstancedMesh cases in the respective functions.
Since instanceColor is optional, I've mimicked the behavior in InstanceMesh.copy() to check if it's null.

[mrdoob]

I was not sure about this one yet.
What if we wanted to add support for alpha? 🤔

[Mugen87]

Since setColorAt() already assumes three component colors, I think have this issue independently of this PR.

three.js/src/objects/InstancedMesh.js

Line 102 in 8323ee0

this.instanceColor = new BufferAttribute( new Float32Array( this.count * 3 ), 3 );

Maybe we always assume RGBA when vertex color alpha is supported?

We could change the signature of setColorAt() so it supports a third alpha parameter or introduce setColorAlphaAt().

[mrdoob]

I guess what I meant is that we should do this to be safe:

if ( instanceColor !== undefined ) object.instanceColor = new BufferAttribute( new Float32Array( instanceColor.array ), instanceColor.itemSize );

[mrdoob]

2f1fa4e

[mrdoob]

Hmm... Maybe we could introduce a fromJSON() static method to BufferAttribute?

if ( instanceColor !== undefined ) object.instanceColor = BufferAttribute.fromJSON( instanceColor );

I guess such change would then apply to maaany other classes 😬

[Mugen87]

Related #11266.

It's definitely worth to investigate this idea. I just wonder if fromJSON() should always return BufferAttribute or the more concrete types e.g. Float32BufferAttribute or Uint16BufferAttribute? I guess it does not matter a lot but there could be the case that somebody relies on using these classes.