hair-fit

Hair made for one head, worn on another. A hairstyle library such as Quaternius' is modelled around ITS base head, in world space; a character builder body has a different skull for every slider setting. Two steps carry a style across, both pure:

  1. fitToScalp(reference, scalp) fits a scale and an offset per axis that lay a REFERENCE that hugs the library's scalp (a buzz cut) onto this body's scalp, by iterated nearest points (ICP without rotation: both heads face +Z, up +Y). The same fit then moves every style, eyebrows and beards too, because they were all modelled on that one head.
  2. conformOutside(positions, scalp, normals, margin) pushes any vertex that ends up inside the skin (or closer than margin to it) back out along the skin's normal, so a shape the fit cannot capture exactly (a longer skull, a heavier brow) never shows skin through hair.

vertexNormals(positions, triangles) gives the skin's normals for the second step (counter-clockwise outward, as OBJ and glTF wind).

const fit = fitToScalp(buzzCut, scalp)
const placed = applyFit(styleVertices, fit)
conformOutside(placed, scalp, scalpNormals, 0.003)