chore: import upstream snapshot with attribution

skills/remotion-to-hyperframes/references/timing.md

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+# Timing translation: interpolate, spring, easing
+
+The single highest-leverage reference. Easings and timings are what readers
+notice; getting them wrong costs more SSIM than any other translation choice.
+Empirically validated against tiers T1–T3.
+
+## Conversion: frames → seconds
+
+HF's timeline is in seconds. Remotion is frame-based. Always:
+
+```
+time_seconds = frame / fps
+```
+
+So at fps=30:
+
+- frame 15 → 0.5 s
+- frame 30 → 1.0 s
+- frame 90 → 3.0 s
+
+Do this conversion once when translating, not at runtime.
+
+## interpolate — linear
+
+```tsx
+const opacity = interpolate(frame, [0, 30], [0, 1], { extrapolateRight: "clamp" });
+```
+
+Translates to:
+
+```js
+gsap.to(target, { opacity: 1, duration: 1.0, ease: "none" }, 0);
+// fromTo if the property starts at 0 and CSS doesn't already set it
+gsap.fromTo(target, { opacity: 0 }, { opacity: 1, duration: 1.0, ease: "none" }, 0);
+```
+
+`ease: "none"` matches Remotion's default linear interpolation. CSS sets the
+`from` value if your initial state is in CSS; otherwise use `fromTo`.
+
+`extrapolateLeft`/`extrapolateRight` defaults to `"extend"` in Remotion but
+`"clamp"` is what the agent will see most often. GSAP doesn't extend — values
+hold at the start and end of the tween. So for `clamp`, GSAP matches; for
+`extend`, you'd need to extend the input range manually before emitting.
+
+## interpolate — multi-segment
+
+```tsx
+const opacity = interpolate(frame, [0, 15, 75, 90], [0, 1, 1, 0]);
+```
+
+Three keyframed tweens at offsets `[0]/fps`, `[1]/fps`, `[2]/fps`:
+
+```js
+const tl = gsap.timeline({ paused: true });
+tl.to(target, { opacity: 1, duration: 0.5, ease: "none" }, 0);
+tl.to(target, { opacity: 1, duration: 2.0, ease: "none" }, 0.5);
+tl.to(target, { opacity: 0, duration: 0.5, ease: "none" }, 2.5);
+```
+
+Validated in T1 — mean SSIM 0.974 against Remotion baseline.
+
+## spring → GSAP back.out
+
+Remotion's `spring()` is the most lossy translation. The mapping is approximate
+but close enough that real-world compositions hold ≥ 0.92 SSIM (T2: 0.985, T3: 0.953).
+
+| Remotion `spring` config                          | GSAP equivalent                                      | Validated in                     |
+| ------------------------------------------------- | ---------------------------------------------------- | -------------------------------- |
+| `{damping: 12, stiffness: 100, mass: 1}` (snappy) | `back.out(1.4)` over ~0.7 s                          | T2, T3 (TitleScene)              |
+| `{damping: 14, stiffness: 90, mass: 1}` (calmer)  | `back.out(1.2)` over ~0.7 s                          | T3 (StatCard)                    |
+| `{damping: 8, stiffness: 200}` (very bouncy)      | `back.out(2.0)` or `elastic.out(1, 0.5)` over ~0.6 s | not validated; budget ~0.05 SSIM |
+| `{overshootClamping: true}`                       | `power3.out` over ~0.6 s (no overshoot)              | not validated                    |
+
+**Rule of thumb**: `back.out(N)` overshoot ratio ≈ `(stiffness / damping^2) * 1.4`. For
+`damping:12, stiffness:100` that gives `1.4 * 100/144 = 0.97`, which is close to
+the validated 1.4 (the formula is rough; tune by visual). Default duration is
+~0.7 s for the typical config.
+
+When the spring's `delay`/`from`/`to` are non-default, scale the duration
+proportionally.
+
+## interpolate with custom easing
+
+```tsx
+import { Easing } from "remotion";
+interpolate(frame, [0, 30], [0, 1], { easing: Easing.out(Easing.cubic) });
+```
+
+| Remotion                     | GSAP                                                                                   |
+| ---------------------------- | -------------------------------------------------------------------------------------- |
+| `Easing.in(Easing.linear)`   | `ease: "none"`                                                                         |
+| `Easing.out(Easing.cubic)`   | `ease: "power3.out"`                                                                   |
+| `Easing.inOut(Easing.cubic)` | `ease: "power3.inOut"`                                                                 |
+| `Easing.out(Easing.poly(N))` | `ease: "power<N>.out"` (N=2 quad, 3 cubic, 4 quart, 5 quint)                           |
+| `Easing.bezier(a,b,c,d)`     | `CustomEase.create("c", "M0,0 C${a},${b} ${c},${d} 1,1")` (requires CustomEase plugin) |
+| `Easing.elastic(bounciness)` | `ease: "elastic.out(${bounciness}, 0.3)"`                                              |
+| `Easing.bounce`              | `ease: "bounce.out"`                                                                   |
+| `Easing.back(overshoot)`     | `ease: "back.out(${overshoot * 1.7})"` (Remotion's overshoot scale differs)            |
+
+## interpolate driving non-numeric properties
+
+```tsx
+const color = interpolateColors(frame, [0, 30], ["#ff0000", "#0000ff"]);
+```
+
+GSAP does color tweens natively:
+
+```js
+gsap.to(target, { color: "#0000ff", duration: 1.0, ease: "none" }, 0);
+```
+
+Same for `backgroundColor`, `borderColor`. The `from` value is read from CSS
+or the inline style.
+
+## Custom count-up / number tweens
+
+When Remotion uses a frame-driven number ramp (`Math.round(value * eased)`):
+
+```tsx
+const t = interpolate(frame, [0, 45], [0, 1]);
+const eased = 1 - (1 - t) ** 3; // cubic ease-out
+const value = Math.round(target * eased);
+return <div>{value.toLocaleString()}</div>;
+```
+
+GSAP equivalent — tween a counter object, write `textContent` on update:
+
+```js
+const counter = { v: 0 };
+tl.to(
+  counter,
+  {
+    v: target,
+    duration: 1.5,
+    ease: "power3.out",
+    onUpdate: () => {
+      el.textContent = Math.round(counter.v).toLocaleString();
+    },
+  },
+  0,
+);
+```
+
+`power3.out` matches `1 - (1-t)^3` exactly. Validated in T3 (mean SSIM 0.953).
+Per-frame digit mismatches occur on sub-frame timing offsets but final values
+converge — no SSIM impact above the noise floor.
+
+## Stagger via per-instance prop
+
+When custom subcomponents take a `delayInFrames` prop:
+
+```tsx
+<StatCard delayInFrames={i * 12} value={...} />
+```
+
+Translate to GSAP timeline offsets:
+
+```js
+cards.forEach((card, i) => {
+  const start = base + i * (12 / fps); // i * 0.4s at fps=30
+  tl.to(card, { ... }, start);
+});
+```
+
+Validated in T3 — three StatCards staggered at 0.0/0.4/0.8 s.