Purpose

Strand simulates hair, ropes and fibers with stretch and bend constraints. Roots can follow transforms, a velocity field can move the strands, and contact settings control collisions. Groom rules shape the displayed result; Rest rules change the shape the simulation tries to hold.

Use

Place Strand in the Geometry view, under Solvers. Connect a small set of root points to geometry. Input normals provide the growth direction; Grow Dir supplies it when N is absent. Start with 16 Points Per Strand and Strand Length at 1.2.

On Resources, add a velocity Connection and choose Force Field’s velocity resource. Drag Coupling starts at 3. Use modest wind while checking the roots and floor. Add an sdf resource for an obstacle, and xforms when roots need to follow a transform set.

Inputs and outputs

Outputs include P, v, Cd and id, a line output with strand topology, and an optional contact set. Use the line output downstream when you need the strand connections rather than isolated points.

Connections accepts sdf, velocity, xforms and contacts. Publish Contact Set starts off. Enable it if another solver needs Strand’s contacts, then connect that resource on the receiver.

Controls

• Simulate pauses advancement; Reset restores the starting strands. Points Per Strand and pool limits affect the build. Auto-Rebuild applies those edits, or Rebuild commits them when automatic rebuilding is off.
• Length and Length Jitter set the starting variation. Root Offset moves the live anchors.
• Substeps starts at 4 and Constraint Iterations at 12. Bend Stiffness resists folding; Max Stretch limits strain. Damping and Max Velocity control motion.
• Groom Rule Stack applies clump, curl, frizz, lean and wave offsets after the solve. These display changes do not feed back into collision physics.
• Rest Shape Stack bakes the same shaping vocabulary into the rest shape on rebuild. Raise Shape Stiffness from 0 when you want the strands to hold that shape. Twist Rate controls twist along the strand.
• Self Collision starts on and changes require a rebuild. Contact Radius, Friction and Damping tune strand contacts. Contact Stride reduces the density of published contacts.

Notes

Max Strands and Max Points/Strand reserve the pools; Points Per Strand selects the active length of each chain. Increase counts after checking a sparse groom.

A strong display Groom offset can move the visible strand away from its simulated collision position. Use Rest rules for a shape that should participate in the solve.

Ports

Inputs

• geometry points
• sdf sdf collider
• velocity velocity field
• xforms transforms
• contacts contacts

Outputs

• P points
• v points
• Cd points
• id points
• contact contacts
• line lines

Parameters

Strand

Auto-Rebuild on Build Params (Autorebuild) Toggle op('strand').par.Autorebuild

Default:

On

Options:

Off, On

Rebuild (Rebuildnow) Pulse op('strand').par.Rebuildnow

Default:

None

Simulate (Simulate) Toggle op('strand').par.Simulate

Default:

On

Options:

Off, On

Reset (Resetsolver) Pulse op('strand').par.Resetsolver

Default:

None

Groom (Hdrgroom) Str op('strand').par.Hdrgroom

Read only.

Default:

"" (Empty String)

Max Strands (pool) (Maxstrands) Int op('strand').par.Maxstrands

Default:

16384

Range:

64 to 131072

Max Points/Strand (pool) (Maxpoints) Int op('strand').par.Maxpoints

Default:

32

Range:

2 to 256

Points Per Strand (Pointsperstrand) Int op('strand').par.Pointsperstrand

Default:

16

Range:

2 to 256

Strand Length (Length) Float op('strand').par.Length

Default:

1.2

Range:

0.01 to 8

Length Jitter (Lengthjitter) Float op('strand').par.Lengthjitter

Default:

0.15

Range:

0 to 1

Roots

Roots (from input geo + N) (Hdrroots) Str op('strand').par.Hdrroots

Read only.

Default:

"" (Empty String)

Grow Dir (fallback when input has no N) (Growdirx) Float op('strand').par.Growdirx

Default:

0

Grow Dir (fallback when input has no N) Y (Growdiry) Float op('strand').par.Growdiry

Default:

-1

Grow Dir (fallback when input has no N) Z (Growdirz) Float op('strand').par.Growdirz

Default:

0

Root Offset (live anchor) (Rootoffsetx) Float op('strand').par.Rootoffsetx

Default:

0

Root Offset (live anchor) Y (Rootoffsety) Float op('strand').par.Rootoffsety

Default:

0

Root Offset (live anchor) Z (Rootoffsetz) Float op('strand').par.Rootoffsetz

Default:

0

Per-root attrs: mass/stiffness/length/twist/pscale/Cd/N (Hdrattrs) Str op('strand').par.Hdrattrs

Read only.

Default:

"" (Empty String)

Solver

Dt (Dt) Float op('strand').par.Dt

Default:

0.0166667

Range:

0.0001 to 0.5

Substeps (Substeps) Int op('strand').par.Substeps

Default:

4

Range:

1 to 16

Constraint Iterations (Iterations) Int op('strand').par.Iterations

Default:

12

Range:

1 to 64

Forces

Forces (Hdrforces) Str op('strand').par.Hdrforces

Read only.

Default:

"" (Empty String)

Gravity (Gravityx) Float op('strand').par.Gravityx

Default:

0

Gravity Y (Gravityy) Float op('strand').par.Gravityy

Default:

-9.81

Gravity Z (Gravityz) Float op('strand').par.Gravityz

Default:

0

Damping (Damping) Float op('strand').par.Damping

Default:

0.985

Range:

0 to 1

Max Velocity (Maxvelocity) Float op('strand').par.Maxvelocity

Default:

20

Range:

0.1 to 100

Drag Coupling (vel: field) (Dragcoupling) Float op('strand').par.Dragcoupling

Default:

3

Range:

0 to 1000

Collision

Constraints & Collision (Hdrcoll) Str op('strand').par.Hdrcoll

Read only.

Default:

"" (Empty String)

Max Stretch (strain limit) (Maxstretch) Float op('strand').par.Maxstretch

Default:

0.1

Range:

0.01 to 1

Bend Stiffness (Bendstiffness) Float op('strand').par.Bendstiffness

Default:

0.35

Range:

0 to 1

Collision Radius (SDF/floor) (Collisionradius) Float op('strand').par.Collisionradius

Default:

0.02

Range:

0.001 to 0.5

Moving Boundary Slip (Boundaryslip) Float op('strand').par.Boundaryslip

Default:

0

Range:

0 to 1

Floor Enabled (Floorenabled) Toggle op('strand').par.Floorenabled

Default:

On

Options:

Off, On

Floor Y (Floory) Float op('strand').par.Floory

Default:

-1.5

Range:

-10 to 10

Strand-Strand Contact (Hdrself) Str op('strand').par.Hdrself

Read only.

Default:

"" (Empty String)

Self Collision (rebuilds) (Selfcollision) Toggle op('strand').par.Selfcollision

Default:

On

Options:

Off, On

Contact Radius (Contactradius) Float op('strand').par.Contactradius

Default:

0.02

Range:

0.001 to 0.5

Contact Stiffness (Contactstiffness) Float op('strand').par.Contactstiffness

Default:

0.5

Range:

0 to 1

Contact Friction (Contactfriction) Float op('strand').par.Contactfriction

Default:

0.1

Range:

0 to 1

Contact Damping (1=inelastic) (Contactdamping) Float op('strand').par.Contactdamping

Default:

1

Range:

0 to 1

Coupling

Coupling (Hdrcouple) Str op('strand').par.Hdrcouple

Read only.

Default:

"" (Empty String)

Publish Contact Set (Publishcontacts) Toggle op('strand').par.Publishcontacts

Default:

Off

Options:

Off, On

Couple Weight (Coupleweight) Float op('strand').par.Coupleweight

Default:

1

Range:

0 to 2

Max Foreign Contacts (cap) (Maxforeign) Int op('strand').par.Maxforeign

Default:

65536

Range:

1024 to 4000000

Contact Stride (Contactstride) Int op('strand').par.Contactstride

Default:

4

Range:

1 to 64

Shape

Shape (LRA + local shape + twist) (Hdrshape) Str op('strand').par.Hdrshape

Read only.

Default:

"" (Empty String)

Shape Stiffness (hold rest shape) (Shapestiffness) Float op('strand').par.Shapestiffness

Default:

0

Range:

0 to 1

Twist Rate (rad / unit length) (Twistrate) Float op('strand').par.Twistrate

Default:

0

Range:

-25 to 25

Color

Color (Hdrcolor) Str op('strand').par.Hdrcolor

Read only.

Default:

"" (Empty String)

Base Hue (Basehue) Float op('strand').par.Basehue

Default:

0.08

Range:

0 to 1

Hue Spread (per-strand) (Huespread) Float op('strand').par.Huespread

Default:

0.12

Range:

0 to 1

Rules

Groom Rules (Hdrrules) Str op('strand').par.Hdrrules

Read only.

Default:

"" (Empty String)

Groom Rule Stack (display) (Groomrules) Str op('strand').par.Groomrules

Default:

"" (Empty String)

Rest Shape Stack (baked, rebuilds) (Restrules) Str op('strand').par.Restrules

Default:

"" (Empty String)