blender-hair-grooming

Hair grooming

The execution model is in blender-core. Hair physics details are in blender-simulation, and look-dev in blender-lighting-lookdev.

One call each

scripts/groom.py -- --surface Head --mask Scalp --length 0.15 --children 150000 --clump 0.5
scripts/groom.py -- --surface Head --mask Beard --length 0.02 --droop 0.2 --curl 0.004 --melanin 0.9 --name Beard
scripts/groom.py -- --surface Body --length 0.025 --droop 0.3 --clump 0.3 --name Fur
scripts/groom.py -- --cards Hair --card-count 150 --atlas-res 2048 --out //textures      # game hair cards

How pros groom

Surface first: the scalp is a mesh (or the head itself) with UVs (hair attaches by UV) and a vertex group painted where hair grows. That group becomes the density mask, so soft edges give a natural hairline.
Guides → children: a few hundred to a few thousand guides carry the shape, and thousands of interpolated children give the volume. Style the guides, never the children.
Shape in layers: length, then clump (strands gather into locks), then curl/wave, then frizz/noise, then profile (thin tips). Keep each a separate modifier you can toggle.
Comb by hand for anything hero: Curves Sculpt mode brushes (Comb, Add, Puff, Pinch, Grow/Shrink, Smooth, Slide, Density, Delete, Pull Tips). Procedural droop gives a believable base, not a hairstyle.
Look-dev under the final lights. Hair reads mostly from specular and depth (clumps catch highlights), not albedo.

groom.py

Runs 5.2's Add Empty Hair (object.curves_empty_hair_add). It parents to the surface, sets surface/surface_uv_map, adds Capture Rest Geometry to the surface, and adds the node Hair Dynamics modifier (pinned last, experimental). Then it stacks the bundled assets: Generate (--density guides/m²) → Interpolate (--children/m², --viewport fraction in the viewport) → Clump → Curl → Displace + Shrinkwrap (--droop × length down, kept 2 mm above the surface) → Noise → Set Hair Curve Profile (--radius, tip taper).
Clump/curl guide distance = 10% of --length. The asset's default 0.1 m collapses a whole head into a few tufts.
Each asset stays a modifier with live inputs, so tweak there. Inputs by name: getattr(md.properties.inputs, identifier).value; vertex-group masks use .type = "ATTRIBUTE", .attribute_name.
It reports curves, mean length, the max root gap to the surface (must be ~0) and the modifier stacks. Evaluated counts in background mode use the viewport amount.

Numbers

	Value
Human scalp	~100–150k hairs, ~200/cm². CG: 30k–150k rendered strands, 300–3000 guides
Strand radius	real 0.03–0.05 mm. Use 0.05 mm (`--radius 0.00005`), thicker with fewer strands
Melanin (Principled Hair, Chiang)	black 1.0 · dark brown 0.8 · brown 0.65 · light brown 0.45 · blond 0.25 · platinum 0.1. Red: 0.5 + redness 0.9. Grey: 0 + tint
Roughness	0.25–0.35 (glossy) … 0.5 (dry). Radial roughness 0.3
Fur	length 1–5 cm, density 200k–2M/m², little clump
`HUANG` (4.0+) models elliptical cross-sections and is better for close-ups. `CHIANG` is the reference default.

Game hair cards (`--cards HAIR`)

Picks --card-count well-spread strands (farthest-point sampling) of the evaluated groom and builds a ribbon along each: --card-width m, tapering to 30% at the tip, lying flat on the surface (side = tangent × surface normal). UV v runs root 0 → tip 1, one atlas column per card (--atlas-columns).
Renders the atlas with Cycles from flat strand clumps: T_<name>_BC (darker roots, coverage in alpha, Standard view), plus T_<name>_RootTip and T_<name>_ID (Raw, linear data). The card material uses Dithered alpha with no backface culling.
Pros layer cards from the scalp outward: dense, wide, opaque cards on the scalp to hide skin; mid layers; then thin flyaway cards on top. Budgets: hero 20k–60k tris, NPC 5k–15k. Bend cards in S/C shapes, never flat planes, and check from every angle. Engines use alpha test/dither + depth prepass, root-tip for darkening/AO and ID for per-strand variation. Add a tangent (flow) map along V for anisotropic specular.

Export grooms

Strand hair for UE Groom/Houdini: Alembic (wm.alembic_export) or USD (wm.usd_export(export_curves=True)). Both write the evaluated curves (measured: 246/246 curves round-trip). Bake the modifier stack first if the target needs guides only.

Gotchas

No UVs on the surface means Generate has nothing to attach to. Add UVs before grooming.
Changing the surface topology after grooming breaks attachment. Groom last, or re-run.
Scale: apply the head's scale. Lengths and densities are world meters.
Viewport: keep --viewport ≤ 0.2 on full heads. Final strand counts only appear in renders.
Hair Dynamics simulates on playback. For stills on frame 1 it's a pass-through. Bake it (blender-simulation) before animated renders.