nx_nxa_groom.nx

nx_nxa_groom.nx -- GENERATE THE GROOM: full-scalp procedural hair strands written into the asset's HSTR section, replacing the sampled partial cap that left the back of the head bare (operator-reported 2026-08-26: "the back of the head is shaved"). INFINIGEN-2 PRINCIPLE, APPLIED (operator standing order: those principles for everything): hair is GENERATED from parameters and the mesh's own measured geometry -- no groom asset, no donor, no hand-placed strand. The industry's own groom pipelines (XGen, Ornatrix, Houdini) are procedural for the same reason: a scalp map plus guide growth beats any sampled cap. METHOD (every number derived or parameterized, never tasted): 1. Axes derived from vertex extents (the nx_nxa_dyna derivation, same tree of assumptions). 2. HEAD TOP = highest vertex; HEAD CENTER = centroid of verts in the top head band (the band spans from crown down by a face-height fraction derived from stature). 3. ROOTS: every scalp vertex above the EAR LINE (head centre height) whose radial direction is not into the face cone, deterministically thinned to <= GRO_MAX_STRANDS with a seeded hash (Infinigen principle: seeded, reproducible diversity). 4. GROWTH: 8 points per strand (the HSTR contract, NXHCH_W=24 words). Point i steps seg_len along a direction that starts RADIAL (off the scalp) and blends toward GRAVITY (down the stature axis) as the strand extends -- long hair hangs, crown hair lifts then falls, back hair drapes the nape. Length = stature * length_permil / 1000. 5. Writes HSTR via the proven section-replace machinery (nx_nxa_dyna pattern): input never modified, an existing HSTR is REPLACED, re-runs identical. usage: nx_nxa_groom <in.nxa> <out.nxa> [length_permil=280] [strands=1400] [seed=7] exit 0 OK | 2 usage | 3 bad-nxa | 1 io license_tier: ORIGINAL No hw writes (Rule 26). expect_exit: 0