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

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.