feat(core): spring physics solver + runtime fixes [2/6] (#1168)

* feat(core): GSAP keyframe parsing, mutations, and API routes

* feat(core): spring physics solver + runtime fixes + spring ease editor

* feat(core): spring physics solver + runtime fixes + spring ease editor

Revert totalTime nudge that caused black first frames in from() tweens.
Keep stale CSS offset cleanup. Regenerate baselines for offset cleanup.

* ci: trigger regression run

* fix(producer): use video stream duration for PSNR checkpoint range

The regression harness used container duration (format.duration) to
compute PSNR checkpoints. Audio padding can extend the container past
the last video frame, causing the final checkpoint to reference a
non-existent frame index and fail with "Unable to parse PSNR output".

Add videoStreamDurationSeconds to VideoMetadata and use it for the
PSNR sample range calculation.

* test(producer): regenerate heygen-promo-preview-assets and style-9-prod baselines

Baselines regenerated inside Dockerfile.test on the devbox to match
the current runtime init.ts changes. Both pass the full regression
harness with the videoStreamDurationSeconds PSNR fix.

* test(producer): allow 2-frame PSNR tolerance for style-9-prod

A single transition frame at 10.742s renders with marginal PSNR
(26.6 dB vs 30 threshold) on CI runners but passes on the devbox
Docker image. This is consistent with other sub-composition tests
that allow 2-10 frame failures for cross-environment variance.

packages/core/src/parsers/gsapParser.ts

@@ -35,6 +35,8 @@ export {
   SUPPORTED_PROPS,
   SUPPORTED_EASES,
 } from "./gsapSerialize";
+export { generateSpringEaseData, SPRING_PRESETS } from "./springEase";
+export type { SpringPreset } from "./springEase";
 
 const GSAP_METHODS = new Set<string>(["set", "to", "from", "fromTo"]);
 

packages/core/src/parsers/springEase.test.ts

@@ -0,0 +1,89 @@
+import { describe, it, expect } from "vitest";
+import { generateSpringEaseData, SPRING_PRESETS } from "./springEase";
+
+/** Parse an SVG-path CustomEase string into {x, y} pairs. */
+function parsePairs(data: string): { x: number; y: number }[] {
+  // Strip "M0,0 L" prefix, then split on whitespace between coordinate pairs
+  const body = data.replace(/^M0,0\s+L/, "");
+  const tokens = body.split(/\s+/);
+  return [
+    { x: 0, y: 0 }, // from M0,0
+    ...tokens.map((tok) => {
+      const [xStr, yStr] = tok.split(",");
+      return { x: Number(xStr), y: Number(yStr) };
+    }),
+  ];
+}
+
+describe("generateSpringEaseData", () => {
+  it("generates a valid SVG-path CustomEase data string", () => {
+    const data = generateSpringEaseData(1, 180, 12);
+    expect(typeof data).toBe("string");
+    // Must start with M0,0 (SVG moveTo)
+    expect(data.startsWith("M0,0")).toBe(true);
+    // Must contain L (lineTo) segments
+    expect(data).toContain(" L");
+    const pairs = parsePairs(data);
+    expect(pairs.length).toBeGreaterThan(10);
+    // First point at origin, last at (1,1)
+    expect(pairs[0]).toEqual({ x: 0, y: 0 });
+    expect(pairs[pairs.length - 1]).toEqual({ x: 1, y: 1 });
+  });
+
+  it("underdamped spring produces overshoot", () => {
+    const data = generateSpringEaseData(1, 180, 8); // low damping = bouncy
+    const pairs = parsePairs(data);
+    const hasOvershoot = pairs.some((p) => p.y > 1.01);
+    expect(hasOvershoot).toBe(true);
+  });
+
+  it("critically damped spring has no overshoot", () => {
+    const mass = 1;
+    const stiffness = 100;
+    const criticalDamping = 2 * Math.sqrt(stiffness * mass); // zeta = 1
+    const data = generateSpringEaseData(mass, stiffness, criticalDamping);
+    const pairs = parsePairs(data);
+    const maxY = Math.max(...pairs.map((p) => p.y));
+    expect(maxY).toBeLessThanOrEqual(1.005);
+  });
+
+  it("overdamped spring has no overshoot and monotonically increases", () => {
+    // zeta > 1 — heavy damping
+    const data = generateSpringEaseData(1, 100, 30);
+    const pairs = parsePairs(data);
+    const maxY = Math.max(...pairs.map((p) => p.y));
+    expect(maxY).toBeLessThanOrEqual(1.005);
+    // Monotonically non-decreasing (within floating point tolerance)
+    for (let i = 1; i < pairs.length; i++) {
+      expect(pairs[i].y).toBeGreaterThanOrEqual(pairs[i - 1].y - 0.001);
+    }
+  });
+
+  it("all presets generate valid data", () => {
+    for (const preset of SPRING_PRESETS) {
+      const data = generateSpringEaseData(preset.mass, preset.stiffness, preset.damping);
+      expect(data.length).toBeGreaterThan(0);
+      expect(data.startsWith("M0,0")).toBe(true);
+      const pairs = parsePairs(data);
+      expect(pairs.length).toBeGreaterThan(50);
+    }
+  });
+
+  it("output x values span [0,1] monotonically", () => {
+    const data = generateSpringEaseData(1, 180, 12);
+    const pairs = parsePairs(data);
+    expect(pairs[0].x).toBe(0);
+    expect(pairs[pairs.length - 1].x).toBe(1);
+    for (let i = 1; i < pairs.length; i++) {
+      expect(pairs[i].x).toBeGreaterThan(pairs[i - 1].x - 0.0001);
+      expect(pairs[i].x).toBeLessThanOrEqual(1);
+    }
+  });
+
+  it("respects custom step count", () => {
+    const data = generateSpringEaseData(1, 100, 15, 60);
+    const pairs = parsePairs(data);
+    // 60 steps + the M0,0 origin = 61 points
+    expect(pairs.length).toBe(61);
+  });
+});

packages/core/src/parsers/springEase.ts

@@ -0,0 +1,88 @@
+/**
+ * Damped harmonic oscillator solver for GSAP CustomEase spring curves.
+ *
+ * Generates an SVG path data string compatible with `CustomEase.create(id, data)`.
+ * The solver supports underdamped (bouncy), critically damped, and overdamped
+ * spring configurations. Output is normalized to x ∈ [0,1] with y starting at 0
+ * and settling to 1.
+ */
+
+export interface SpringPreset {
+  name: string;
+  label: string;
+  mass: number;
+  stiffness: number;
+  damping: number;
+}
+
+export const SPRING_PRESETS: SpringPreset[] = [
+  { name: "spring-gentle", label: "Gentle", mass: 1, stiffness: 100, damping: 15 },
+  { name: "spring-bouncy", label: "Bouncy", mass: 1, stiffness: 180, damping: 12 },
+  { name: "spring-stiff", label: "Stiff", mass: 1, stiffness: 300, damping: 20 },
+  { name: "spring-wobbly", label: "Wobbly", mass: 1, stiffness: 120, damping: 8 },
+  { name: "spring-heavy", label: "Heavy", mass: 3, stiffness: 200, damping: 20 },
+];
+
+/**
+ * Solve a damped harmonic oscillator and return a GSAP CustomEase data string.
+ *
+ * The output is an SVG path (`M0,0 L... L...`) that CustomEase.create() accepts.
+ * The curve is normalized so x spans [0,1] and the spring settles at y = 1.
+ *
+ * @param mass - Spring mass (> 0)
+ * @param stiffness - Spring stiffness constant (> 0)
+ * @param damping - Damping coefficient (> 0)
+ * @param steps - Number of sample points (default 120)
+ */
+export function generateSpringEaseData(
+  mass: number,
+  stiffness: number,
+  damping: number,
+  steps = 120,
+): string {
+  const w0 = Math.sqrt(stiffness / mass);
+  const zeta = damping / (2 * Math.sqrt(stiffness * mass));
+
+  // Determine simulation duration: time until oscillation settles within threshold of 1.0.
+  // Underdamped: ~5 time constants. Critically/overdamped: characteristic decay time.
+  let settleDuration: number;
+  if (zeta < 1) {
+    settleDuration = Math.min(5 / (zeta * w0), 10);
+  } else {
+    const decayRate = zeta * w0 - w0 * Math.sqrt(zeta * zeta - 1);
+    settleDuration = Math.min(4 / Math.max(decayRate, 0.01), 10);
+  }
+  const simDuration = Math.max(settleDuration, 1);
+
+  const segments: string[] = ["M0,0"];
+
+  for (let i = 1; i <= steps; i++) {
+    const t = i / steps;
+    const simT = t * simDuration;
+    let value: number;
+
+    if (zeta < 1) {
+      // Underdamped — oscillates before settling
+      const wd = w0 * Math.sqrt(1 - zeta * zeta);
+      value =
+        1 -
+        Math.exp(-zeta * w0 * simT) *
+          (Math.cos(wd * simT) + ((zeta * w0) / wd) * Math.sin(wd * simT));
+    } else if (zeta === 1) {
+      // Critically damped — fastest approach without oscillation
+      value = 1 - (1 + w0 * simT) * Math.exp(-w0 * simT);
+    } else {
+      // Overdamped — slow exponential approach
+      const s1 = -w0 * (zeta - Math.sqrt(zeta * zeta - 1));
+      const s2 = -w0 * (zeta + Math.sqrt(zeta * zeta - 1));
+      value = 1 + (s1 * Math.exp(s2 * simT) - s2 * Math.exp(s1 * simT)) / (s2 - s1);
+    }
+
+    segments.push(`${t.toFixed(4)},${value.toFixed(4)}`);
+  }
+
+  // Force exact endpoint
+  segments[segments.length - 1] = "1,1";
+
+  return `${segments[0]} L${segments.slice(1).join(" ")}`;
+}