Physics and constraints

Physics constraints add secondary motion: a bone lags behind the animated pose and springs back, so hair, skirts, tails, antennae and tree branches move on their own. The simulation is deterministic, so every runtime and every export shows the same motion.

Windswept: the first frame. Press play to run it live. — **Windswept**Click the buttons on the left of the artworklive

Everything that sways in Windswept (eight hair chains, the skirt, three levels of the tree) is a physics constraint. Press gust to see the wind input push them.

Add physics to a chain

Select the first bone of the chain (a lock of hair, the base of a tail).
Under Constraints in the Inspector press + Physics for that bone, or + Physics chain to add one constraint per bone down the chain in one step.
Press Space in Animate mode, or drag a parent bone in the viewport, and tune the springs.

The hair-A1-physics constraint selected on Windswept: bone hair-A1, rotate, move X and Y, stretch, inertia 0.5, strength 70, damping 2.5, mass, gravity 20, wind -30, mix and steps per second; the constraint order list on the right — A hair chain's physics: what it moves (rotate, move, stretch), how it springs (inertia, strength, damping, mass), and the forces (gravity and wind).

Setting	Effect
Rotate / Move X / Move Y / Stretch	Which of the bone’s properties the simulation drives, and how much
Inertia	How much the bone keeps its own momentum when the parent moves
Strength	How hard it springs back to the animated pose
Damping	How quickly the swinging dies out
Mass	Heavier bones react more slowly
Gravity, Wind	Constant forces; both are keyable, so a gust is just an animation
Mix	Blend between the animated pose (0) and the simulated one (1)
Steps/s	Simulation rate; higher is steadier for fast motion

Gravity, wind and IK softness follow the rig’s reference scale (in the document’s Inspector), so a rig scaled up or down behaves the same.

The Tail example with the antenna-l1 physics constraint selected: inertia 0.9, strength 90, damping 3, gravity 60 — The Tail's antennae are two-bone physics chains with high inertia, so they wobble when the head moves.

Other constraints

Constraint	Use
IK	1- and 2-bone inverse kinematics with mix, bend direction, softness, stretch and compression. See First rig.
Transform	Copy rotation, translation, scale and shear from another bone, in world or local space, absolute or relative (a wheel that turns with a lever, eyes that follow a target)
Path	Place and rotate a bone chain along a bezier path drawn with the Pen tool, with position, spacing and rotate modes and constant speed (the Tail example’s tail)
Sliders	Let a bone’s property or a number input scrub an animation (corrective shapes, a shadow that follows a jump, a dial)

The Constraint order list in the document’s Inspector sets which constraints run first; each one sees the result of the ones above it.

Tail: the first frame. Press play to run it live. — **Tail**Click the creaturelive

The Tail combines a path constraint (the tail follows a bezier path whose knots are animated), physics antennae and a camera pan on a second state-machine layer. Click the creature.

OpenBone is MIT licensed and made by UXCentury. The example art, voices and sounds are credited in the repository.