Object-space bounding sphere the views were framed on.
A unit quad (x, y in -1..1); the material places and turns it.
Camera-facing, lit, alpha-tested material. Reads the scatterPosition / scatterParams instance attributes. Disposing it also frees both atlases.
Normal atlas (object-space normals packed to 0..1, alpha = the bake's mask), used to light the impostor like the mesh.
Options used for the bake.
TSL nodes of the impostor's surface, for re-shading material (e.g. weather, the way the mesh materials apply
it): set material.colorNode / roughnessNode from them. The default is colorNode = vec4(color, alpha).
Colour atlas (alpha = coverage).
Result of bakeImpostor: an atlas plus a material and quad ready to be a
ScatterLayer.impostor.