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Spring easing for transitions and motion (stiffness/damping/mass) #196
Description
Why
The dock (react + motion) uses type: "spring" on every transition (stiffness 400–700, damping 12–30, mass 0.8–0.9), including overshooting entrance/squeeze effects. Our easing model (MotionEase in packages/react/src/types/host.ts, interpolation in packages/native/src/motion.rs:593-840) is keyword + cubic-bezier over a fixed duration only. A spring is physics-integrated with no fixed duration and can overshoot — not expressible as a single bezier.
Shape
MotionEasegains a spring variant:{ type: "spring", stiffness, damping, mass, velocity? }accepted by bothStyleTransition.easingandMotionTransition.ease(duration ignored/derived when spring, matching motion's semantics).- Native side: closed-form damped-spring solution in motion.rs (or settle-threshold integration), driving the existing interruptible track machinery. Interruption must carry velocity into the new spring, or it loses the main feel benefit.
- Browser-mirror analogue: CSS
linear()easing is the standard web encoding of sampled springs (and motion/framer-motion is itself a web library). Document the mapping.
Sequencing
Depends-on-in-spirit: #195 (scale) — the dock's springs animate scale; springs land with or after it. Implementation after wave 2, paused if defects come back.
Activity
Ernxst commentedon Sep 1, 2026
Upstream prior art: remorses/gpuix#34 (open, unmerged) adds a native spring integrator + GELATIN preset. Assessment (2026-09-01): adopt its WIRE FORMAT (
{ type: "spring", stiffness, damping, mass, velocity }— matches motion's API) for future-reconciliation convergence, but not its implementation. Compromises we don't take: parallel JS frame loop (onFrame/pumpFrames+ JSstepSpring) alongside the native integrator; unit-blind rest threshold (0.05 — 5% on the opacity channel); springs only on motion.* transitions with nothing forStyleTransition(state styles — the whileHover/whileTap path the dock needs); author-taste defaults (36/8/1.2 vs motion's 100/10/1); untagged serde spring arm doesn't validatetype; no scale/rotate.Ideal shape stands as described above: spring as an easing variant driving the existing native interruptible track machinery for BOTH MotionTransition and StyleTransition, velocity carried across interruption, unit-aware settling thresholds. Watch item: if upstream merges #34, the next reconciliation round converges on the shared wire format.
Stamp 0.6.0-fork.2