Hair and fur grooming (hair curves, Blender 5.2)

Expert grooming means a few evenly spaced guides that set direction and silhouette. Stacked, masked procedural layers add density, clumping, frizz, length variation and strays, judged in a lit render at full density and in pose. Write guides as data (no curve brush stroke can be scripted in 5.2.1) and make every look decision through the Essentials hair node-group assets. If a sibling skill named here is missing from your available skills, ask the user to install it (npx skills add scenario-labs/skills --skill <name>); unattended, proceed from tool schemas and flag the gap.

REQUIRED BACKGROUND: scenario-blender-expert (execution channel, review loop, 5.2 API changes).

Stance (the expert delta)

• Guides get 2 points until every direction is right (Schmidbauer). More points make combing tangle and clip. Use 3 for short fur, 8 only for long hair. Bystedt resamples long human hair to 15 points before shaping.
• Space guides evenly, and always mask Interpolate (Schmidbauer). Children spawn over the whole surface, and a child far from any guide "does whatever". Gaps turn into bald spots that grow under deformation.
• Density is counted per square meter and must be huge. Radius drops with it (Thommes). Long fur uses 1,000,000/m² at about 0.2 mm radius. Work at a Viewport Amount of 4%.
• Randomize per curve, not per point (Thommes). Give 80% of strands no frizz. Strays are 10% of curves with strong noise.
• Trim with Replace Length off plus a Random Offset (Schmidbauer, Thommes). It is the cheapest realism gain. Mask it, or short-fur regions lose their guides and go bald.
• Grow a human hairstyle on a separate hair cap split along the part (Bystedt): body topology stops mattering, interpolation stops crossing the part. The part still needs extra density and a lift.
• Shape works the same way on every asset: 1 affects the tip only, 0 the whole strand, negative the roots. Use a negative Frizz Shape so curl and clump tips survive (Thommes, Bystedt).
• Judge renders at full density, and in pose (Thommes, Schmidbauer). A harsh mesh deformation "looks ten times worse" in the hair.

Establish first

Input	Changes	Default when the brief is silent
Target	Cycles strands vs EEVEE or game cards	Cycles strands (Principled Hair BSDF)
Subject	human hairstyle (hair cap, part) vs animal fur (body mesh, landmarks)	follow the reference
Length class	points per guide, clump and curl layers	short fur: 3 points, long: 8, long human: 15
Deformation	animated means Hair Dynamics (Animation) stays last, and you check in pose	animated
Budget	render curves, card triangles	say the number: evaluated curves at full density
Reference	same subspecies, season, ideally the same individual (Schmidbauer)	ask for it; draw landmarks and flow first

Finish topology and non-overlapping UVs before grooming (hair binds to UV space) and apply scale: density counts the mesh's local area (verified).

Workflow

Task	Headless	Live session
Curves object, surface binding, guides as data, snap, subdivide or resample, asset modifiers, inputs, masks, counts, renders, cards	yes (scripts/bx_hair.py)	yes, same code in Object Mode
Enter Sculpt Curves, activate the 11 curve brushes, set brush settings, sculpt_curves.select_random	n/a	yes (bx_gui.activate_brush("SCULPT_CURVES", "Comb"))
Comb, Add, Density, Grow/Shrink, Pull Tips strokes	no	no. sculpt_curves.brush_stroke has no exec, so bx_gui.stroke("SCULPT_CURVES", ...) returns PASS_THROUGH and changes nothing. Set the brush up and ask the user to stroke, or use the substitutes below.

1. Prepare the surface. H.uv_report(surface): stacked UVs make curves ignore the rig, UDIMs are fine, an unmerged mirror seam needs UV Merge by Distance (Schmidbauer). Humans: a hair cap split into islands along the part. Animals: separate the mouth interior. Masks: vertex groups (fast) or textures (detail on low-poly meshes). GATE: overlap_faces == 0, scale applied, one mask per region.
2. Plan flow and objects. Turn the draw-over into regions (vertex groups) with one flow direction each. Schmidbauer: one curves object per landmark (face, cheeks, ears, neck, body, tail, feet) for performance. Thommes: one object with masks. Bystedt: split front and back where combing one disturbs the other. GATE: written list of regions, flows, objects.
3. Guides. hair = H.new_groom(surface), P, N = H.sample_roots(surface, spacing, mask=...), H.add_guides(hair, P, N, length, points=2, flow=..., flow_blend=...) (snaps, writes surface_uv_coordinate). Later: H.subdivide_guides or H.resample_guides(hair, 15). Brush substitutes: a direction field for Comb, droop for gravity, delete and re-add guides for Density and Add; Bystedt's "grow long, comb, shrink back" is long guides plus Trim. GATE: in H.groom_report, roots_off_surface and spacing_gaps_2x are 0 and length_outliers is 0. A guides-only render shows every direction right.
4. Density. Add Set Hair Curve Profile first, then Interpolate Hair Curves with a Density Mask. Bystedt paints 50% gray and 100% at the part, then remaps. The 5.2 Interpolate also takes a Mask Texture image. GATE: H.count(hair) (full density) is within budget, root_area_fraction is well under 0.5, and a render shows no bald spots.
5. Procedural layers, each masked, A/B-tested by toggling show_viewport and rendering.
   • Long fur (Thommes, preset H.FUR_STACK_LONG, applied with H.build_stack): Noise, Curl, strays, Frizz with negative shape, Clump, Clump 2, final Frizz (Cumulative off), Trim, Rotate.
   • Animal (Schmidbauer): Trim, Clump (cheeks, Tip Spread, Clump Offset), low Frizz, each with its own vertex group.
   • Human (Bystedt): Noise on guides (Cumulative off, Offset per Curve on), Roll at the tip only, then per population Interpolate, Profile, Clump on the groomed guides; wide and thin layers need different seeds.
   • Per-curve randomness: H.random_curve_mask(hair, "frizz_mask", 0.8) then input "attr:frizz_mask"; root-to-tip ramps: H.along_curve_attr. GATE: each layer earns its place in an A/B render.
6. Part and hairline (human). Leave Part by Mesh Islands on (default): in the test, 21% of children near the part crossed it with islands off and 0% with islands on. For the lift, H.distance_weights(cap, "part_lift", "part", radius, "bump") feeds Displace Hair Curves (Surface Normal on, small distance, Shape near 0, Factor attr:part_lift). GATE: in a top view there is no scalp gap at the part and the crown is not flat.
7. Shading and render. Use H.hair_material(name, "CYCLES", melanin=(root, tip)) or "EEVEE" with explicit colors, then H.setup_render(scene, engine, close_up). GATE: H.review([surface, hair], out, engine) passes references/critique.md.
8. Deformation. Key 2 or 3 extreme poses and render each frame from the same camera. GATE: no bald spots or clipping in any pose.
9. Games: cards. Run H.add_cards(hair, width, material=H.card_material(...)), then convert a copy with object.convert(target='MESH') and export glTF. Put a textured hair cap under the cards. GATE: triangles within budget, UVs in the half tile for cards, roots on the scalp.

Numbers

Item	Value	Relative to, source
Guide points	2 while placing, 3 short fur, 8 long hair	Schmidbauer [00:20:24], [00:21:34]
Long human hair	resample to 15 points	Bystedt [00:08:38]
Add brush length	about 0.01 m for fur (default 0.3 m)	Schmidbauer [00:17:59]
Density, human layer	5,000 per m² per clump population	Bystedt [00:17:28]
Density, long fur	1,000,000/m², radius 0.2 mm, viewport 4%	Thommes [00:09:02], [00:16:44]
Density map	part 1.0, base 0.5, bald 0; ramp black stop just above 0.5	Bystedt [00:22:04], [00:23:46]
Wide clumps	Shape about 0.3, reduced Factor, groomed guides	Bystedt [00:19:47]
Curl	Radius 2 cm, Curl Start 1, Random Offset 0.5, Guide Distance 1 cm, Guide Mask 0.2	Thommes [00:12:04], [00:14:18]
Sub-clumps	Guide Distance 1 mm, Mask 0.1; layer 2: 4 mm, 0.25, Clump Offset 5 mm	Thommes [00:16:12], [00:31:01]
Frizz	2 mm, negative Shape; final: -0.2, Cumulative off, on 20% of curves	Thommes [00:15:04], [00:32:08], [00:26:03]
Strays	10% of curves, Noise distance 5 mm	Thommes [00:30:28]
Trim, Rotate	Replace Length off, Random Offset 5 mm; Rotate random 5°	Thommes [00:22:20]
Card subdivision	1 per 20 cm of card length	Bystedt g_ZIOafq4QU [00:05:08]
Card and tube profile	arc: 1 segment flat, 3 preferred; tube: flat spiral, resolution 6, 1 turn, height 0	Matsumoto [00:19:54], [00:22:40]
Card UV width	cards use half the tile of tubes	Matsumoto [00:31:27]
Coverage sanity	density × π × radius²: 0.13 for Thommes' fur	[added] check

Quality gates

In code:

import sys; sys.path.append("<skills>/scenario-blender-hair/scripts"); import bx_hair as H
print(H.uv_report(surface))                 # overlap_faces == 0
rep = H.groom_report(hair); print(H.verdict(rep))   # [] or a list of fixes
print(H.count(hair))                        # full-density curves and points vs budget
print(H.card_stats(hair))                   # cards: tris, UV range

groom_report checks attachment UVs, roots on the surface, spacing gaps, length dents (over 50% off neighbors), Hair Dynamics last, Trim Replace Length, unmasked Frizz or Noise, missing Density Mask, radius vs density, Strand display, and a Cycles hair shader under EEVEE.

Visually: H.review(objs, out, engine, views=("front", "threequarter", "top")) renders a lit full-density sheet with Thommes' rig (warm top fill, side key, cool rim, dark bluish world). Score it with references/critique.md: bald spots, part coverage, clumps, strays, length variation, tips, root-to-tip color, highlights.

Common mistakes

Mistake	What it looks like	Fix
Guides at the brush default of 8 points while blocking	tangles, clipping, fights while combing	2 points, raise later (Schmidbauer)
Interpolate without a Density Mask	hair in the mouth or eyes, drifting children far from guides	vertex-group mask; separate the mouth interior
Uneven guide spacing	bald spots, worse in pose	sample_roots spacing; check spacing_gaps_2x
High density at default radius	a solid mass or "a mess"	Profile first, about 0.2 mm at 1M/m² (Thommes)
Trim at defaults	every strand becomes 1 m long	Replace Length off (Thommes)
Positive Frizz on curls or clumps	curl tips destroyed, fuzzy clump tips	negative Shape, Cumulative Offset off on the final Frizz
Duplicated layer with the same seed	wide and thin clumps stack identically	change seed and frequency (Bystedt)
Straight tips	"bad CG hair"	Roll Hair Curves at the tip only (Bystedt)
Surface edited after grooming	floating or sunken roots	snap_to_surface(hair, "DEFORM"), or NEAREST after topology changes
Judging at 4% viewport density	uniform clumps and equal lengths unnoticed	render at full density
Principled Hair BSDF in EEVEE	dark and flat (measured 0.09 vs 0.26 center luminance)	Principled BSDF with explicit colors
Scripting a Comb stroke	nothing happens, PASS_THROUGH	write positions, or hand the stroke to the user

Blender 5.2 notes

• Empty Hair and Quick Fur add a pinned Hair Dynamics modifier (Mode Animation or Physics (Experimental)). It replaces the old Surface Deform and moves roots with the deforming surface (shape keys and armature verified). Keep it last.
• Interpolate has no Surface or UV Map inputs; it reads the curves object's surface. Newer inputs are Part by Mesh Islands, Follow Surface Normal, Mask Texture and Resting Surface (on). With Resting Surface on, Thommes' "Mask modifier on the surface" patch preview has no effect. Use a patch vertex group as the Density Mask instead, or turn Resting Surface off while previewing.
• Modifier inputs are set with md.properties.inputs.<id>.value or .type = 'ATTRIBUTE'. Headless, evaluation stays stale until obj.update_tag(). H.set_inputs resolves UI names and tags for you. Background evaluation applies Viewport Amount, so use H.count(full=True).
• Brushes are assets: Snake Hook is Pull Tips, Selection Paint is Select; Add defaults to 8 points, 0.3 m; settings in brush.curves_sculpt_settings.
• Factory render.hair_type is STRAND: use STRIP with hair_subdiv >= 1. Cycles shapes: RIBBONS, THICK, THICK_LINEAR. "Curves Info" is ShaderNodeHairInfo; Principled Hair offers Chiang and Huang. Hair Curves Noise writes resolution 12, which curves render with since 5.0.
• Cards: Curve to Mesh has a Scale input (4.5), and Arc Resolution counts points. EEVEE alpha uses surface_render_method = 'DITHERED'. Keep the GeometrySet alive when reading obj.evaluated_geometry().mesh.
• Quick Fur density scales with object size (MEDIUM on a 10 cm sphere: 80,220/m², 12 points per guide).

References

• references/expert-notes.md: load for judgment calls, per expert with timestamps (human hairstyle, animal fur, procedural fur, cards, mesh hair).
• references/procedures.md: load before writing code. It has full tested procedures P1 to P8 with raw bpy equivalents of the bx_hair calls.
• references/critique.md: load when reviewing a render or report, as the scoring rubric.
• references/sources.md: load to cite or re-watch a source, with the best timestamps.
• scripts/bx_hair.py: the toolkit (tested on 5.2.1). Tests are in tests/code/blender-hair/.