FollicleScript Expressions

FollicleScript is FollicleFX's expression language for procedural hair control. Write simple expressions that are evaluated in real time on 100,000+ strands.

Quick Start

Click the ƒx button next to any parameter to open expression mode.

0.8                          # Constant value
rand(index)                  # Random 0-1 per strand
rand3(0.7, 1.0, index)      # Random in range per strand
lerp(0.5, 1.0, v)           # UV-based gradient
v > 0.5 ? 1.0 : 0.5         # Conditional

Variables

Strand Properties

Variable	Description	Range
`index`	Strand index	0 to count-1
`count`	Total strand count	—
`length`	Strand length	World units
`maxLength`	Maximum strand length in system	World units
`width`	Strand width	World units
`t`	Position along strand	0 (root) to 1 (tip)

Spatial Coordinates

Variable	Description	Range
`u`	UV X coordinate	0–1
`v`	UV Y coordinate	0–1
`x`	World X position	World units
`y`	World Y position	World units
`z`	World Z position	World units

Clumping

Variable	Description
`clump_id`	Clump index (when using Clumping modifier)

Constants

Constant	Value
`pi`	3.14159...
`tau`	6.28318... (2π)
`e`	2.71828...

Operators

Arithmetic

+ - * / ^ (or **) ()

Comparison

< > <= >= == !=

Returns 1.0 (true) or 0.0 (false).

Ternary

condition ? value_if_true : value_if_false

Functions

Random & Noise

Function	Description
`rand(seed)`	Deterministic random 0–1
`rand3(min, max, seed)`	Random in range [min, max]
`noise(x)` / `noise(x, y)` / `noise(x, y, z)`	Smooth organic noise

Math

Function	Description
`abs(x)`	Absolute value
`sqrt(x)`	Square root
`pow(x, y)`	Power
`exp(x)`	e^x
`log(x)`	Natural logarithm
`sign(x)`	Returns -1, 0, or 1
`mod(x, y)`	Modulo
`min(a, b)`	Minimum
`max(a, b)`	Maximum

Trigonometry

sin(x) cos(x) tan(x) — Input in radians.

Rounding

floor(x) ceil(x) frac(x)

Interpolation

Function	Description
`clamp(x, min, max)`	Clamp to range
`lerp(a, b, t)`	Linear interpolation
`smoothstep(edge0, edge1, x)`	Smooth S-curve
`select(cond, a, b)`	If cond > 0 then a, else b

Texture Sampling

map(u, v)    # Sample bound texture at UV coordinate

Custom Variables (Dynamic Sliders)

Use $varName syntax to create UI sliders automatically:

$base + rand(index) * $variation

This creates "base" and "variation" sliders that you can adjust in real-time. Up to 16 custom variables supported.

Common Patterns

Per-Strand Randomization

rand(index)                      # Random 0-1
rand3(0.8, 1.2, index)          # Random range
clamp(rand(index), 0.2, 0.8)   # Clamped random

Gradients

v                                # Bottom to top (0→1)
1.0 - v                          # Top to bottom (1→0)
u                                # Left to right
lerp(0.5, 1.5, v)               # Custom range gradient

Along-Strand Effects

t                                # Root to tip (0→1)
1.0 - t                          # Tip to root
lerp(1.0, 0.1, t)               # Taper
smoothstep(0.0, 1.0, t)         # Smooth taper

Center/Edge Falloff

1.0 - abs(u - 0.5) * 2.0        # Dense in center
abs(u - 0.5) * 2.0              # Dense at edges

Per-Clump Variation

rand(clump_id)                   # Random per clump
0.5 + rand(clump_id) * 0.5      # Clumps vary 0.5–1.0
clump_id % 2                    # Alternating clumps

Conditional Logic

v > 0.5 ? 1.0 : 0.5             # Top/bottom split
rand(index) * 0.5               # Per-strand variation
index < 100 ? 0.8 : 1.0         # First 100 strands different

Noise Patterns

noise(u * 10, v * 10)            # 2D organic pattern
noise(u * 5, v * 5, index)      # Per-strand noise
0.7 + noise(u * 3, v * 3) * 0.3 # Noise in range

Modifier-Specific Examples

Clumping — Per-Clump Tightness

rand3(0.6, 0.9, clump_id)

Frizz — Per-Strand Variation

rand(index) * 0.5

Cut — Random Trim (Keep 80–100%)

rand3(0.8, 1.0, index)

Cut — Remove 1–3% Per Strand

1.0 - rand3(0.01, 0.03, index)

Density — UV-Based

lerp(0.5, 1.0, v)

Width — Taper to Tips

lerp(1.0, 0.1, t)

Coil — Per-Strand Count Variation

5.0 + rand(index) * 3.0

Tips

Use index for per-strand randomization — consistent, deterministic
Use t for along-strand effects — 0 at root, 1 at tip
Use clump_id for per-clump variation — all strands in a clump get the same value
Clamp extreme values — clamp(expr, 0.2, 0.8) avoids outliers
Start simple — test with constants, add complexity gradually
Custom variables for tweaking — $varName creates real-time sliders
Most parameters expect 0–1 — check the parameter's range

Performance Notes

Expressions are evaluated in real time on all strands simultaneously
Simple expressions (arithmetic, rand) are very fast
Complex expressions (noise, trig) are slightly slower
No practical limit on expression complexity