Spec — Motion: Physics

Facet *isual*do Koder Design. Material parity: https://m3.material.io/styles/motion/overview/how-it-works.

Principles

1. *otion has a reason*— state change, hierarchy, spatial cue.

   Decorative animation (idle particles, ambient parallax) prohibited in app UI; allowed in marketing landings only.

2. *ast enough to disappear*— UI animations are sub-300ms.

   Above that, the user notices "the app is animating" instead of "the app responded."

3. *asing matches intent*— entering uses decelerate (fast→slow,

   "arriving"); exiting uses accelerate (slow→fast, "leaving"); onscreen tweaks use easeinout. Detail: `easingduration.kmd`.

4. *educed motion respected*— every animation collapses to

   instant (0ms) under prefers-reduced-motion: reduce.

R1 — Reduced motion contract

When prefers-reduced-motion: reduce:

• All durations → 0ms (instant transitions)
• Scale animations → opacity only (no scaling)
• Continuous animations → static (loaders use indeterminate progress text instead of spinning)
• Parallax → disabled
• Auto-playing carousels → static, manual advance only
• Spring animations → snap to end state (0ms); no overshoot, no oscillation

CSS implementation:

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after {
    animation-duration: 0ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0ms !important;
    scroll-behavior: auto !important;
  }
}

R2 — Performance budget

• All transitions composited via GPU (transform, opacity)
• Avoid animating: width, height, top, left, padding, margin (causes

  layout reflow)

• Singleframe check: animations stay at 60fps on midtier mobile
• Coordinated multielement animations: stagger (50100ms offset) so

  the GPU doesn't paint all frames simultaneously

R3 — Forbidden patterns

• ❌ Animations > 700ms (except marketing hero)
• ❌ Bounce/elastic curves in UI (only marketing landings) — see R4 spring exception
• ❌ Auto-playing animations on tab return (cumulative motion blur)
• ❌ Animating colors via JS (use CSS transitions)
• ❌ animation: ... infinite outside loaders / static decoration in landings

R4 — Spring tokens (Material 3 Expressive)

Durationbased tokens (`easingduration.kmd`) describe *ow long*an animation takes; spring tokens describe *ow it moves*— physics-driven (stiffness, damping ratio, mass) instead of time-driven. Spring physics is what unlocks Expressive: interruptible gestures, natural overshoot/ bounce on movement, smooth chaining of state changes mid-animation.

Two families, separated by what they animate:

R4.1 — Spatial tokens (movement, size, position)

Used for any property that has a *patial meaning*— translate, scale, layout size, rotation, container shape morph. May overshoot slightly (natural physics) — bounded so the overshoot reads as "settled" not "jiggle."

R4.2 — Effect tokens (fade, color, opacity)

Used for properties that are *on-spatial*— opacity, color channel, blur radius, elevation alpha. High damping ratio (≥1.0) guarantees zero overshoot (a half-faded element bouncing back into visibility is wrong).

R4.3 — Decision tree: duration token vs spring token

Is the animation INTERRUPTIBLE mid-flight (drag, fling, gesture)?
├── YES → use spring (R4.1 for spatial, R4.2 for effect)
└── NO  → does it move position/size?
          ├── YES, and it has natural physics (FAB morph, sheet snap)
          │       → use spring (R4.1)
          └── NO, it's a deterministic timed transition (modal entry,
                  cross-fade, loader)
                  → use duration tokens (`easing-duration.kmd` R1-R2)

Spring tokens are *dditive*to duration tokens — duration tokens remain canonical for deterministic transitions. New code should prefer springs when the motion is gestural or spatial; legacy duration use stays valid.

R4.4 — Surface bindings

For Web, generate the linear(0, 0.1, 0.3, 0.6, 0.85, 1.0) form from the spring params at build time per preset; do NOT compute springs in JS at runtime.

R4.5 — Per-preset variation

Cross-link

• motion.kmd — index over all Motion sub-specs
• motion/easing-duration.kmd — duration tokens + easing curves
• motion/transitions.kmd — transition pattern catalog
• interaction/states.kmd — state-level motion details
• themes/ui-style.kmd — per-preset motion variation