chore: import upstream snapshot with attribution

skills/hyperframes-animation/rules/split-tilt-cards.md

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+---
+name: split-tilt-cards
+description: Two cards side-by-side with opposing Y-rotation creating a symmetric 3D split-screen layout for comparisons or feature pairs.
+metadata:
+  tags: 3d, cards, split, tilt, comparison, symmetric, layout
+---
+
+# Split Tilt Cards
+
+Two cards positioned side-by-side, each rotated in opposite Y directions. Creates a symmetric "book-open" 3D effect — natural fit for comparisons, before/after, or feature pairs.
+
+## How It Works
+
+- Left card rotates `+Y` (faces toward the right viewer angle)
+- Right card rotates `-Y` (faces toward the left viewer angle)
+- Both share the same `perspective` parent → opposing rotations balance visually
+- Each card enters from outside (left card slides in from the left, right card from the right) to reinforce its identity
+- Idle phase: gentle counter-phase float (`Math.PI` offset on sine) — cards bob in opposition
+
+## HTML
+
+```html
+<div
+  class="scene"
+  id="split-scene"
+  data-composition-id="split-scene"
+  data-start="0"
+  data-duration="4"
+  data-track-index="0"
+>
+  <div class="split-stage">
+    <div class="card card-left">
+      <div class="card-eyebrow">{leftEyebrow}</div>
+      <div class="card-headline">{leftHeadline}</div>
+      <div class="card-body">{leftBody}</div>
+    </div>
+    <div class="card card-right">
+      <div class="card-eyebrow">{rightEyebrow}</div>
+      <div class="card-headline">{rightHeadline}</div>
+      <div class="card-body">{rightBody}</div>
+    </div>
+  </div>
+</div>
+```
+
+## CSS
+
+```css
+.scene {
+  position: relative;
+  width: 100%;
+  height: 100%;
+  display: grid;
+  place-items: center;
+  background: {bgGradient};
+  perspective: SCENE_PERSPECTIVE; /* REQUIRED — without perspective rotateY flattens */
+}
+.split-stage {
+  display: flex;
+  gap: STAGE_GAP;
+  transform-style: preserve-3d;
+}
+.card {
+  width: CARD_WIDTH;
+  min-height: CARD_MIN_HEIGHT;
+  padding: CARD_PADDING;
+  display: flex;
+  flex-direction: column;
+  gap: CARD_INNER_GAP;
+  border-radius: CARD_RADIUS;
+  background: {cardSurface};
+  border: 1px solid {cardBorder};
+  color: {textColor};
+  font-family: {font};
+  transform-style: preserve-3d;
+  will-change: transform;
+}
+.card-left {
+  /* Faces right → shadow falls right */
+  box-shadow:
+    -CARD_SHADOW_OFFSET CARD_SHADOW_DROP CARD_SHADOW_BLUR {shadowColor},
+    0 0 CARD_GLOW_BLUR {accentGlowColor};
+}
+.card-right {
+  /* Faces left → shadow falls left */
+  box-shadow:
+    CARD_SHADOW_OFFSET CARD_SHADOW_DROP CARD_SHADOW_BLUR {shadowColor},
+    0 0 CARD_GLOW_BLUR {accentGlowColor};
+}
+.card-eyebrow {
+  font-size: EYEBROW_FONT_SIZE;
+  font-weight: 800;
+  letter-spacing: EYEBROW_LETTER_SPACING;
+  text-transform: uppercase;
+  color: {accentColor};
+}
+.card-headline {
+  font-size: HEADLINE_FONT_SIZE;
+  font-weight: 900;
+  line-height: 1;
+  letter-spacing: HEADLINE_LETTER_SPACING;
+}
+.card-body {
+  font-size: BODY_FONT_SIZE;
+  font-weight: 500;
+  line-height: 1.3;
+  color: {bodyColor};
+  opacity: BODY_OPACITY;
+}
+```
+
+## GSAP Timeline
+
+```html
+<script src="https://cdn.jsdelivr.net/npm/gsap@3.14.2/dist/gsap.min.js"></script>
+<script>
+  window.__timelines = window.__timelines || {};
+  const tl = gsap.timeline({ paused: true });
+
+  // Phase 1 — entry from outside
+  tl.fromTo(
+    ".card-left",
+    { x: -ENTRY_SLIDE_DIST, rotateY: TILT + TILT_OVERSHOOT, opacity: 0 },
+    { x: 0, rotateY: TILT, opacity: 1, duration: ENTRY_DUR, ease: "power3.out" },
+    LEFT_AT,
+  );
+  tl.fromTo(
+    ".card-right",
+    { x: ENTRY_SLIDE_DIST, rotateY: -TILT - TILT_OVERSHOOT, opacity: 0 },
+    { x: 0, rotateY: -TILT, opacity: 1, duration: ENTRY_DUR, ease: "power3.out" },
+    RIGHT_AT,
+  );
+
+  // Phase 2 — counter-phase idle bob (cards move in opposition for dynamism)
+  tl.to(
+    ".card-left",
+    { y: -FLOAT_AMP, duration: FLOAT_DURATION / 2, ease: "sine.inOut", yoyo: true, repeat: 1 },
+    IDLE_START,
+  );
+  tl.to(
+    ".card-right",
+    { y: FLOAT_AMP, duration: FLOAT_DURATION / 2, ease: "sine.inOut", yoyo: true, repeat: 1 },
+    IDLE_START,
+  );
+
+  // Phase 3 — gentle copy reveal (body slides up + fades after cards arrive)
+  tl.from(
+    ".card-eyebrow, .card-headline, .card-body",
+    { opacity: 0, y: COPY_RISE, stagger: COPY_STAGGER, duration: COPY_DUR, ease: "power2.out" },
+    COPY_REVEAL_AT,
+  );
+
+  window.__timelines["split-scene"] = tl;
+</script>
+```
+
+## How to Choose Values
+
+### Layout / typography
+
+- **SCENE_PERSPECTIVE** — perspective on the scene root.
+  - Range: 1000-2400 px
+  - Effects: lower exaggerates the tilt (more cone-like); higher reads as a near-isometric, flatter rotation
+- **STAGE_GAP** — horizontal gap between the two cards.
+  - Range: 40-120 px (≈0.06-0.15× `CARD_WIDTH`)
+  - Effects: small gap reads "fused pair"; large gap reads "compared but separate"
+- **CARD_WIDTH** — width of each card.
+  - Range: 480-820 px at 1920×1080
+  - Constraints: `2 * CARD_WIDTH + STAGE_GAP ≤ 0.95 * stageWidth` so both cards stay on-screen at full tilt
+- **CARD_MIN_HEIGHT** — minimum card height.
+  - Range: ≈0.75-1.05× `CARD_WIDTH` (square-ish reads as balanced; very tall reads as poster)
+- **CARD_PADDING / CARD_INNER_GAP / CARD_RADIUS** — interior chrome.
+  - Range: padding 40-72 px; inner gap 24-48 px; radius 24-40 px
+- **CARD_SHADOW_OFFSET / CARD_SHADOW_DROP / CARD_SHADOW_BLUR** — drop-shadow geometry.
+  - Constraints: offset's sign matches tilt direction (see Key Principles); drop > 0 grounds the card on the scene
+  - Range: offset 16-28 px; drop 20-32 px; blur 40-80 px
+- **CARD_GLOW_BLUR** — secondary inner glow blur radius (`box-shadow` 0 0 blur).
+  - Range: 16-32 px (subtle accent rim; larger competes with the card content)
+- **EYEBROW_FONT_SIZE / EYEBROW_LETTER_SPACING** — small uppercase label.
+  - Range: 22-32 px; spacing 6-12 px (uppercase reads cleaner with positive letter-spacing)
+- **HEADLINE_FONT_SIZE / HEADLINE_LETTER_SPACING** — the main one-line punch.
+  - Range: 72-104 px at 1920×1080; spacing -1 to -3 px (tight tracking for display weight)
+- **BODY_FONT_SIZE / BODY_OPACITY** — supporting copy.
+  - Range: 28-40 px; opacity 0.8-0.92
+  - Constraints: body limited to ≤2 lines (see Critical Constraints — tilted long paragraphs blur)
+
+### Entry / tilt
+
+- **TILT** — static `rotateY` magnitude in degrees (left `+`, right `−`).
+  - Range: 10-18° (under 10 reads almost flat; over 18 the cards fold shut and body copy becomes hard to read)
+- **TILT_OVERSHOOT** — extra degrees added to the starting `rotateY` before settling to `TILT`.
+  - Range: 4-12°
+  - Effects: gives the entry a slight pivot-into-place feel
+- **ENTRY_SLIDE_DIST** — pixels each card slides from off-axis.
+  - Range: 200-500 px (≈0.3-0.6× `CARD_WIDTH`)
+- **ENTRY_DUR** — per-card slide-in duration.
+  - Range: 0.6-1.2 s
+- **LEFT_AT** — left card entry start.
+  - Range: 0.0-0.4 s
+- **RIGHT_AT** — right card entry start.
+  - Range: `LEFT_AT + 0.0` to `LEFT_AT + 0.3` s (zero stagger feels mechanical; large stagger fragments the pair)
+
+### Idle bob
+
+- **FLOAT_AMP** — sine amplitude on idle y bob (px).
+  - Range: 3-8 px (see sine-wave-loop rule — subtle is the point)
+- **FLOAT_DURATION** — full yoyo round-trip duration (one breath).
+  - Range: 1.6-3.2 s (≈breathing cadence)
+- **IDLE_START** — when idle bob begins.
+  - Constraints: `≥ max(LEFT_AT, RIGHT_AT) + ENTRY_DUR` so idle doesn't fight the entry tail
+
+### Copy reveal
+
+- **COPY_REVEAL_AT** — when eyebrow/headline/body fade in.
+  - Constraints: usually starts during the cards' settle (overlaps the entry tail) — content shouldn't pop in after the cards are already idle
+- **COPY_DUR / COPY_STAGGER / COPY_RISE** — fade-up shape.
+  - Range: duration 0.4-0.7 s; stagger 0.04-0.10 s; rise 12-24 px
+
+### Color / typography tokens
+
+- **{bgGradient}** — radial or linear gradient behind the cards; darker than `{cardSurface}` so cards lift
+- **{cardSurface}** — card background (typically a low-saturation gradient layered over the scene)
+- **{cardBorder}** — 1 px border color, usually `{accentColor}` at low alpha
+- **{shadowColor}** — drop-shadow color, typically near-black at 0.5-0.7 alpha
+- **{accentGlowColor}** — inner glow color, typically `{accentColor}` at low alpha
+- **{accentColor}** — eyebrow + accent rim color (single hue per scene)
+- **{textColor}** — primary headline color, high contrast against `{cardSurface}`
+- **{bodyColor}** — body copy color, slightly desaturated vs `{textColor}`
+- **{font}** — display font stack for all card copy
+
+## Variations
+
+### Mid-tilt zoom-through (combined with camera move)
+
+If a separate camera tween scales `.split-stage`, the cards' tilt reads as the viewer crossing through the gap between them.
+
+### Asymmetric content density (badge / label / icon)
+
+Add a floating badge near each card for additional context. Position absolutely on the parent — not inside the card, so the badge doesn't inherit the 3D rotation:
+
+```html
+<div class="badge badge-left">{leftBadge}</div>
+<div class="badge badge-right">{rightBadge}</div>
+```
+
+### Stacked variants (3+ cards)
+
+For 3 cards, the center card stays flat (`rotateY 0`) and the outer two tilt inward — useful for "your old way / nothing in between / our way" comparisons.
+
+## Key Principles
+
+- **`perspective` on scene root REQUIRED** — without it rotateY flattens and the split-tilt collapses to a flat side-by-side layout
+- **`transform-style: preserve-3d`** on both the stage and each card — preserves the 3D plane as cards have their own transforms
+- **Shadow direction must match tilt** — left card faces right, shadow falls right (positive X), and vice versa. Wrong shadow direction reads as "broken 3D"
+- **Symmetric content weight** — both cards same width, same vertical center, similar line counts. Asymmetric content breaks the comparison metaphor
+- **Counter-phase float (`Math.PI` offset)** — left bobs up while right bobs down. Synchronized bob looks like both cards are on the same conveyor belt; counter-phase looks alive
+- **Slide-in from the outside** — left card from left, right card from right — reinforces "they came from their own worlds and met here"
+- **❗ Tilt magnitude 10-15°** — under 10° looks like a slight perspective offset (almost flat), over 18° looks like the cards are folding shut and copy becomes hard to read
+
+## Critical Constraints
+
+- **Timeline must be paused**: `gsap.timeline({ paused: true })`
+- **Registry key = `data-composition-id`**
+- **No `requestAnimationFrame`** for the idle float — drive it inside the timeline so seek is deterministic
+- **Don't put badges inside the card divs** — they'd inherit the rotateY and tilt off-axis with the card. Float them on the parent
+- **Body copy ≤ 2 lines per card** — tilted text becomes hard to read; long paragraphs collapse into a perspective blur
+
+## Combinations
+
+- [card-morph-anchor.md](card-morph-anchor.md) — both cards could morph into a single unified shape afterward
+- [counting-dynamic-scale.md](counting-dynamic-scale.md) — numbers as the headline content for each side
+
+## Pairs with HF skills
+
+- `/hyperframes-animation` — timeline + `yoyo` for the idle bob
+- `/hyperframes-core` — composition wiring
+- `/hyperframes-cli` — `hyperframes lint`