# Motion Design

## Duration: The 100/300/500 Rule

Timing matters more than easing. These durations feel right for most UI:

| Duration | Use Case | Examples |
|----------|----------|----------|
| **100-150ms** | Instant feedback | Button press, toggle, color change |
| **200-300ms** | State changes | Menu open, tooltip, hover states |
| **300-500ms** | Layout changes | Accordion, modal, drawer |
| **500-800ms** | Entrance animations | Page load, hero reveals |

**Exit animations are faster than entrances.** Use ~75% of enter duration.

## Easing: Pick the Right Curve

**Don't use `ease`.** It's a compromise that's rarely optimal. Instead:

| Curve | Use For | CSS |
|-------|---------|-----|
| **ease-out** | Elements entering | `cubic-bezier(0.16, 1, 0.3, 1)` |
| **ease-in** | Elements leaving | `cubic-bezier(0.7, 0, 0.84, 0)` |
| **ease-in-out** | State toggles (there → back) | `cubic-bezier(0.65, 0, 0.35, 1)` |

**For micro-interactions, use exponential curves.** They feel natural because they mimic real physics (friction, deceleration):

```css
/* Quart out - smooth, refined (recommended default) */
--ease-out-quart: cubic-bezier(0.25, 1, 0.5, 1);

/* Quint out - slightly more dramatic */
--ease-out-quint: cubic-bezier(0.22, 1, 0.36, 1);

/* Expo out - snappy, confident */
--ease-out-expo: cubic-bezier(0.16, 1, 0.3, 1);
```

**Avoid bounce and elastic curves.** They were trendy in 2015 but now feel tacky and amateurish. Real objects don't bounce when they stop; they decelerate smoothly. Overshoot effects draw attention to the animation itself rather than the content.

## Premium Motion Materials

Transform and opacity are reliable defaults, not the whole palette. Premium interfaces often need atmospheric properties: blur reveals, backdrop-filter panels, saturation or brightness shifts, shadow bloom, SVG filters, masks, clip paths, gradient-position movement, and variable font or shader-driven effects.

Use the right material for the effect:

- **Transform / opacity**: movement, press feedback, simple reveals, list choreography.
- **Blur / filter / backdrop-filter**: focus pulls, depth, glass or lens effects, softened entrances, atmospheric transitions.
- **Clip path / masks**: wipes, reveals, editorial cropping, product-like transitions.
- **Shadow / glow / color filters**: energy, affordance, focus, warmth, active state.
- **Grid-template rows or FLIP-style transforms**: expanding and reflowing layout without animating `height` directly.

The hard rule is not "transform and opacity only." The hard rule is: avoid animating layout-driving properties casually (`width`, `height`, `top`, `left`, margins), keep expensive effects bounded to small or isolated areas, and verify in-browser that the result is smooth on the target viewports. If blur/filter makes the interaction feel significantly more premium and remains smooth, use it.

## Staggered Animations

Use CSS custom properties for cleaner stagger: `animation-delay: calc(var(--i, 0) * 50ms)` with `style="--i: 0"` on each item. **Cap total stagger time**: 10 items at 50ms = 500ms total. For many items, reduce per-item delay or cap staggered count.

## Reduced Motion

This is not optional. Vestibular disorders affect ~35% of adults over 40.

```css
/* Define animations normally */
.card {
  animation: slide-up 500ms ease-out;
}

/* Provide alternative for reduced motion */
@media (prefers-reduced-motion: reduce) {
  .card {
    animation: fade-in 200ms ease-out;  /* Crossfade instead of motion */
  }
}

/* Or disable entirely */
@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after {
    animation-duration: 0.01ms !important;
    transition-duration: 0.01ms !important;
  }
}
```

**What to preserve**: Functional animations like progress bars, loading spinners (slowed down), and focus indicators should still work, just without spatial movement.

## Perceived Performance

**Nobody cares how fast your site is, just how fast it feels.** Perception can be as effective as actual performance.

**The 80ms threshold**: Our brains buffer sensory input for ~80ms to synchronize perception. Anything under 80ms feels instant and simultaneous. This is your target for micro-interactions.

**Active vs passive time**: Passive waiting (staring at a spinner) feels longer than active engagement. Strategies to shift the balance:

- **Preemptive start**: Begin transitions immediately while loading (iOS app zoom, skeleton UI). Users perceive work happening.
- **Early completion**: Show content progressively, don't wait for everything. Video buffering, progressive images, streaming HTML.
- **Optimistic UI**: Update the interface immediately, handle failures gracefully. Instagram likes work offline; the UI updates instantly, syncs later. Use for low-stakes actions; avoid for payments or destructive operations.

**Easing affects perceived duration**: Ease-in (accelerating toward completion) makes tasks feel shorter because the peak-end effect weights final moments heavily. Ease-out feels satisfying for entrances, but ease-in toward a task's end compresses perceived time.

**Caution**: Too-fast responses can decrease perceived value. Users may distrust instant results for complex operations (search, analysis). Sometimes a brief delay signals "real work" is happening.

## Performance

Don't use `will-change` preemptively, only when animation is imminent (`:hover`, `.animating`). For scroll-triggered animations, use Intersection Observer instead of scroll events; unobserve after animating once. Create motion tokens for consistency (durations, easings, common transitions).

---

**Avoid**: Animating everything (animation fatigue is real). Using >500ms for UI feedback. Ignoring `prefers-reduced-motion`. Using animation to hide slow loading.
