motion-principles

Motion design foundation - timing, easing, enter/exit patterns, accessibility, performance.

Install

npx skills add https://github.com/athevon/genjutsu --skill motion-principles

SKILL.md

---name: motion-principlesdescription: Motion design foundation - timing, easing, enter/exit patterns, accessibility, performance.metadata: internal: true---

Version-sensitive. Every API name, SDK gate and browser-support claim below was verified on 2026-09-08 against primary sources. What against, and when, is in _jutsu/VERSIONS.md. If that date is old, re-verify before acting on a version number.

Motion Principles

The foundation. Loaded by every creative skill invocation. Concise rules here. Deep-dive in references/.

Timing Rules

Context	Duration	Why
Micro-interaction (toggle, hover, focus)	100-150ms	Instant feedback, no perceived delay
UI transition (modal, drawer, tab switch)	200-300ms	Smooth but never sluggish
Page/route transition	300-500ms	Establishes spatial narrative
Scroll-driven / 3D	Free (progress-based)	Tied to user input, no fixed duration

Frequency rule: The more often an animation plays, the shorter and subtler it must be. A button hover (1000x/day) = 100ms opacity. An onboarding reveal (1x ever) = 600ms+ full choreography.

Easing Cheat Sheet

Action	Easing	Why
Element enters	`ease-out` / spring	Decelerates into resting position (natural arrival)
Element exits	`ease-in`	Accelerates away (gets out of the way)
Element moves between states	`ease-in-out`	Smooth start and stop
Scroll-synced	`linear` / `none`	Matches 1:1 with input, no lag perception
Bouncy/playful	spring (underdamped)	Overshoot creates life
Snappy UI	`cubic-bezier(0.2, 0, 0, 1)`	Fast start, smooth land

Exit is always more subtle than enter. Enter: 300ms ease-out, full choreography. Exit: 200ms ease-in, opacity only.

Native easing equivalents (cross-platform)

Web (CSS / JS)	SwiftUI	Compose
`cubic-bezier(0.2, 0, 0, 1)`	`.spring(response: 0.4, dampingFraction: 0.85)` or `.snappy`	`spring(stiffness = Spring.StiffnessMedium, dampingRatio = 0.85f)`
`ease-out`	`.easeOut(duration: 0.3)`	`tween(durationMillis = 300, easing = LinearOutSlowInEasing)`
`ease-in`	`.easeIn(duration: 0.2)`	`tween(durationMillis = 200, easing = FastOutLinearInEasing)`
spring (bouncy)	`.bouncy` (iOS 17+)	`spring(stiffness = Spring.StiffnessLow, dampingRatio = Spring.DampingRatioMediumBouncy)`
spring (smooth)	`.smooth` (iOS 17+)	`spring(stiffness = Spring.StiffnessMedium, dampingRatio = Spring.DampingRatioNoBouncy)`

Accessibility (Non-Negotiable)

Reduced motion - MANDATORY

Every animated component must respect the user's reduced-motion preference. No exceptions, regardless of platform.

Platform	API
Web CSS	`@media (prefers-reduced-motion: reduce)`
Web JS	`window.matchMedia('(prefers-reduced-motion: reduce)')`
SwiftUI	`@Environment(\.accessibilityReduceMotion) var reduceMotion`
UIKit	`UIAccessibility.isReduceMotionEnabled` (+ `reduceMotionStatusDidChangeNotification`)
Compose	Custom helper using `Settings.Global.ANIMATOR_DURATION_SCALE` (deep-dive in `mobile-principles/references/accessibility-mobile.md`)

CSS:

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
    scroll-behavior: auto !important;
  }
}

SwiftUI:

struct AnimatedView: View {
    @Environment(\.accessibilityReduceMotion) var reduceMotion
    @State var visible = false

    var body: some View {
        Text("Hello")
            .opacity(visible ? 1 : 0)
            .animation(reduceMotion ? .none : .spring(response: 0.4, dampingFraction: 0.85), value: visible)
    }
}

Compose:

@Composable
fun rememberReduceMotion(): Boolean {
    val context = LocalContext.current
    return remember {
        Settings.Global.getFloat(context.contentResolver, Settings.Global.ANIMATOR_DURATION_SCALE, 1f) == 0f
    }
}

@Composable
fun AnimatedComponent(visible: Boolean) {
    val reduce = rememberReduceMotion()
    val alpha by animateFloatAsState(
        targetValue = if (visible) 1f else 0f,
        animationSpec = if (reduce) snap() else spring(stiffness = Spring.StiffnessMedium)
    )
}

Other a11y requirements

Focus indicators must never be hidden by animations
Animated content must meet WCAG contrast ratios at every frame (no mid-transition fading to invisible text)
Looping animations: provide a pause mechanism

Universal "Do Not" Rules

1. Never animate width/height/top/left

Triggers layout recalculation every frame = jank.

/* BAD */
.drawer { transition: height 0.3s ease; }
.drawer.open { height: 400px; }

/* GOOD */
.drawer { transition: transform 0.3s ease-out; transform: translateY(100%); }
.drawer.open { transform: translateY(0); }

SwiftUI equivalent (BAD vs GOOD):

// BAD - animates frame (causes layout pass)
.frame(height: open ? 400 : 0)
.animation(.easeInOut, value: open)

// GOOD - animates transform via offset
.offset(y: open ? 0 : 400)
.animation(.spring(), value: open)

Compose equivalent (BAD vs GOOD):

// BAD - animates size (full layout pass)
val height by animateDpAsState(if (open) 400.dp else 0.dp)

// GOOD - animates Y offset via graphicsLayer
val translation by animateFloatAsState(if (open) 0f else 400f)
Box(modifier = Modifier.graphicsLayer { translationY = translation })

2. Never scale to 0

scale(0) causes elements to vanish in a black hole. Always keep a minimum.

/* BAD */
.modal-exit { transform: scale(0); }

/* GOOD */
.modal-exit { transform: scale(0.95); opacity: 0; }

SwiftUI equivalent:

// BAD
.scaleEffect(visible ? 1.0 : 0.0)

// GOOD
.scaleEffect(visible ? 1.0 : 0.95)
.opacity(visible ? 1.0 : 0.0)

Compose equivalent:

// BAD
Modifier.graphicsLayer { scaleX = if (visible) 1f else 0f; scaleY = if (visible) 1f else 0f }

// GOOD
Modifier.graphicsLayer {
    scaleX = if (visible) 1f else 0.95f
    scaleY = if (visible) 1f else 0.95f
    alpha = if (visible) 1f else 0f
}

3. Never ease-in on an entry

Ease-in = slow start. An entering element that hesitates feels broken.

/* BAD */
.card-enter { animation: fadeIn 0.3s ease-in; }

/* GOOD */
.card-enter { animation: fadeIn 0.3s ease-out; }
/* OR spring via JS for natural feel */

4. Never exceed 500ms on a UI interaction

Modals, dropdowns, tooltips, tabs -- users are waiting. Respect their time.

// BAD
gsap.to(modal, { opacity: 1, y: 0, duration: 0.8 });

// GOOD
gsap.to(modal, { opacity: 1, y: 0, duration: 0.25, ease: "power2.out" });