Add utility class for generating AO Map

[gkjohnson]

Related to #32. Something like the following:

const uvGenerator = new UVGenerator();
uvGenerator.attributeName = 'uv2';
uvGenerator.generate( targetMeshes );

const aoGenerator = new AOMapGenerator();
aoGenerator.samples = 1000;
aoGenerator.attributeName = 'uv2';

const targetMeshes = [ ... ];
const texture = await aoGenerator.generate( renderer, targetMeshes, aoBvh );

Untested UV unwrapping logic using xatlas is here:

https://github.com/gkjohnson/three-gpu-pathtracer/blob/main/src/utils/UVUnwrapper.js

Originally from this Mozilla Spoke commit:

https://github.com/mozilla/Spoke/commit/9701d647020e09d584885bd457eb225e9995c12f

TODO

• Write uv generator and ao map generator utilities
• AO map must be flooded between triangles to avoid seams
• Update example to show mesh with unwrapped UVs and live generating ao map
• Update example to show 2d projection of ao map
• Update example to take target map resolution

[donmccurdy]

/cc @robertlong – Hi! Hope it's OK to ask a couple questions here. I'm wondering about using xatlas-web for this. I think we'd ideally need two workflows supported:

• (A) 1 mesh - unwrap and pack

  • load 1 mesh, providing position and normal attributes
  • generate a new UV atlas
  • use xref vertex IDs to rewrite the original mesh since some vertices have been duplicated. Original mesh may have arbitrary other attributes not used in the atlasing process.

• (B) N meshes - repack only

  • load N meshes that already have UVs, providing position, normal, and uv attributes
  • generate a shared atlas, packing all UVs into the same [0,1] coordinate space
  • use xref vertex IDs to rewrite all original meshes, as above

At a glance I'm having trouble seeing if these workflows are supported by xatlas itself, the xatlas-web bindings, or neither... is that something you happen to know?

Related: https://github.com/donmccurdy/glTF-Transform/issues/534

[hybridherbst]

A nice workflow would also be (kind of an extension of B):

• C)
  • load N meshes that may or may not have UVs
  • generate UV2 if UV1 is overlapping or doesn't exist
  • atlas all UV2s into 1-M AO maps (depends on target texel count)
  • generate AO map(s) in UV2
  • write out meshes with new UV2 + AO, no need to repack existing textures

This is generally what game engines (Unity/Unreal at least) do for lightmaps.

[gkjohnson]

@donmccurdy Looks like there's a new xatlas wrapper being developed -- maybe it alleviates some of your concerns?

https://twitter.com/repalash/status/1521165176086487040