Plugins: Consider plugin that uses / enables use of BatchedMesh

[gkjohnson]

Provide a variant of 3d tiles renderer that uses three.js' BatchedMesh to reduce draw calls. Related to gkjohnson/batched-3d-tiles-demo.

Useful for things like google earth and mars tiles.

For an initial set of requirements:

• Requires all geometry to be of a similar size or maximum-size.
• Requires that all geometry use a similar size of texture and at most one texture.
• Requires that all tiles must use a common tile material / shader.

Process:

• On geometry load copy the texture into a texture array and geometry into the BatchedMesh in a pre-determined buffer size along with the transform.
• Discard the original geometry on load and store the BatchedMesh and texture index.
• If we find a mesh that is too large to fit in the pre-allocated buffer length then create a new BatchedMesh.
  • instead we can just ask for a larger buffer allocation than is typical and optimize / expand later if we reach capacity
• Implement a custom traversal algorithm for raycasting against batched mesh.
  • Specifically we'll want to support individually intersecting each tile from within the batched mesh to take advantage of the 3d tiles hierarchy

Open Questions:

• Interoperability with other plugins (custom fade shader, etc)?
  • Unsupported for now
• What happens if something like bvh is used under the hood?
  • Separate plugin required

[gkjohnson]

From #628

Related to #604

Add a plugin that fills in a BatchedMesh with the loaded tiles geometry, uses a custom material with support for reading from a texture array, and uploading textures to an array.

See proof of concept here.

Approach

• Create a preallocated BatchedMesh with enough space and large enough slots for the number of tiles to be rendered.
• Preallocate a texture array with enough space for each texture needed.
• Auto expand allocations if needed.
• Custom materials for rendering with albedo from texture array.
• We can lose original references to loaded geometry.

Changes Needed

• Ray casting must change
• Approach for storing internally references to geometry must change
• Lots of internal callbacks and functions that rely on "scene" would have to change

Limitations

• Will not work with Fading tiles.
• Will only work with tile albedo.

[gkjohnson]

To start this could be developed to only work in simple cases (eg earth globe tiles)

[gkjohnson]

BatchedMesh optimizing and resizing will help here when we want to resize the gometry to support more / larger tiles than originally anticipated:

• BatchedMesh: add support for resizing instance count, geometry size mrdoob/three.js#29577
• BatchedMesh: Add optimize(). mrdoob/three.js#29527

[gkjohnson]

Options

{
  instanceCount,
  vertexCount,
  indexCount,

  expansionPercent,
  maxExpansionCount,
  
  // possibly just inherit?
  textureSize, generateMipmaps, wrapping, etc,
  material,
}