Struct SpringJoint 

pub struct SpringJoint {
    pub body_a: usize,
    pub body_b: usize,
    pub local_anchor_a: Vec3Fix,
    pub local_anchor_b: Vec3Fix,
    pub rest_length: Fix128,
    pub stiffness: Fix128,
    pub damping: Fix128,
    pub break_force: Option<Fix128>,
}

Spring joint: a spring-damper between two anchor points

The joint applies the force (on B; A receives the opposite)

• rest_length == 0: F = −k d − c u, with d = anchor_b − anchor_a and u = d' the relative anchor velocity. Spring and damper are both isotropic, so the damper removes the tangential relative velocity as well as the radial one and a body held by the spring does not orbit its anchor.
• rest_length > 0: F = −(k (|d| − L) + c (u · n)) n, n = d / |d|, the usual spring with a dashpot along its axis. The direction across the axis is free (a pendulum stays a pendulum), as for a physical spring-damper element; use rest_length = 0 to hold a body at a point.

For a body of mass m held against a static or kinematic anchor (or two bodies with m_eff = 1 / (1/m_a + 1/m_b)), angular frequency ω and damping ratio ζ, use stiffness = m_eff ω² and damping = 2 ζ m_eff ω; ζ = 1 (damping = 2 √(k m_eff)) is critical damping, x(t) = (x0 + (v0 + ω x0) t) e^(−ωt) with no overshoot.

It is solved once per substep as an XPBD constraint with damping (Macklin, Müller, Chentanez 2016, eq. 26), which is a backward Euler step of the force law: the result converges to the same continuous solution for any substeps, does not depend on iterations, and is stable for any k and c. Each joint carries its own stiffness / damping, so springs of different ω can share one world. SolverConfig::damping and the per-body damping factors are applied on top, once per frame.

Fields

body_a: usize

Index of the first body

body_b: usize

Index of the second body

local_anchor_a: Vec3Fix

Anchor point in body A’s local space

local_anchor_b: Vec3Fix

Anchor point in body B’s local space

rest_length: Fix128

Rest length L of the spring in metres. 0 makes the spring and the damper isotropic (see the type doc)

stiffness: Fix128

Spring stiffness k in newtons per metre (m_eff ω² for angular frequency ω). Independent of the step length and of substeps

damping: Fix128

Damping coefficient c in newton-seconds per metre, acting on the relative anchor velocity (every component for rest_length = 0, the component along the spring otherwise). Critical damping is c = 2 √(k m_eff)

break_force: Option<Fix128>

Maximum spring force |k (|d| − L)| in newtons before the joint breaks (None = unbreakable), see solve_joints_breakable

Implementations

impl SpringJoint

pub const fn new( body_a: usize, body_b: usize, anchor_a: Vec3Fix, anchor_b: Vec3Fix, rest_length: Fix128, stiffness: Fix128, damping: Fix128, ) -> Self

Create a new spring joint

pub const fn with_break_force(self, force: Fix128) -> Self

Set break force threshold

Trait Implementations

impl Clone for SpringJoint

fn clone(&self) -> Self

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for SpringJoint

impl Debug for SpringJoint

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Eq for SpringJoint

impl PartialEq for SpringJoint

fn eq(&self, other: &Self) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for SpringJoint