SplatMesh.forEachSplat() returns 0 splats at onLoad when using paged .rad files

[Alecapra96]

When loading a .rad file with paged: true, onLoad fires before any splat data is available. At that point:

splatMesh.numSplats === 0
splatMesh.packedSplats === undefined
splatMesh.isInitialized === true
splatMesh.forEachSplat() iterates 0 splats

This makes it impossible to compute a bounding box (or do any per-splat work) at load time for paged .rad files.

Additionally, SplatMesh.getBoundingBox() throws "Bounding box requires PackedSplats or ExtSplats" in this state, even though isInitialized is true.

• Reproduction

const splat = new SplatMesh({
  url: 'my-file.rad',
  paged: true,
  onLoad: () => {
    console.log(splat.numSplats);      // 0
    console.log(splat.packedSplats);   // undefined
    console.log(splat.isInitialized);  // true
    splat.forEachSplat(() => {});      // iterates 0 times
    splat.getBoundingBox();            // throws: "Bounding box requires PackedSplats or ExtSplats"
  }
});

• Expected behavior

Either:

onLoad fires only after the first chunk of splat data is available (so numSplats > 0), or
A separate callback like onFirstChunkLoaded / onSplatsReady is provided for paged files, or
getBoundingBox() and forEachSplat() work correctly once isInitialized === true, regardless of paging mode

• Workaround

Currently working around this by polling numSplats in onFrame:

const splat = new SplatMesh({
  url: 'my-file.rad',
  paged: true,
  onFrame: ({ mesh }) => {
    if (mesh.numSplats > 0 && !boundingBoxComputed) {
      // compute bounding box here
      boundingBoxComputed = true;
    }
  }
});

• Environment

@sparkjsdev/spark version: 2.0.0
File format: .rad with paged: true
THREE.js version: r180

[mrxz]

Currently a SplatMesh that is paged is immediately initialized with 0 splats. Once the chunks are loaded and pages swapped in, its numSplats will increase. Regardless, neither getBoundingBox nor forEachSplat is implemented/supported for paged splats.

It could be implemented, but they'll both be based on the splats that "happen" to be available, which might cause unexpected outcomes. For example, the exact dimensions of the bounding box could vary depending on which splats are loaded in. But I can see how this would still be more useful than the current behaviour, especially as the first chunk for LoDs should mostly cover the entire splat and be a decent approximation of the bounding box.

I've created a PR to explore this direction: #324
In addition, it might be interesting to defer the isInitialized and onLoad callback until the first chunk is indeed loaded. That would bring it more in-line with the other splat sources. Not to mention that it would now even fire before any network requests have been made.

Currently working around this by polling numSplats in onFrame:

Unless I'm missing something, this workaround won't really work. Both .getBoundingBox() and forEachSplat would still fail to give any results, even when the paged SplatMesh has more than zero splats.