Vid Motion Graphics Agent Skill

vid-motion-graphics

Generates multi-scene motion graphics as MP4. Renders HTML/CSS animations in headless Chromium via Playwright (Web Animations API frame-seeking), assembles PNG frames with FFmpeg. No React, no AI APIs, no Python — zero new dependencies beyond the graphic-gif family.

CDN fonts only. No external libraries in HTML.

Critical Rules (read before every generation)

• Use window.renderFrame(t) — no CSS @keyframes for scene transitions. CSS animation currentTime seeking is silently ignored for backward seeks in Chromium. The renderFrame approach: a pure JS function computes opacity/transform directly from milliseconds. Playwright calls it once per frame. Deterministic, race-free.
• No animation-delay on ANY element. Not needed with renderFrame. If you catch yourself writing animation-delay, stop — you're using the wrong architecture.
• window.__videoReady = true only inside document.fonts.ready.then(...). Never set synchronously — fonts must load before Playwright captures frame 1 or text renders with fallback fonts.
• Expose window.__stopPreview(). The browser's rAF preview loop races with Playwright's evaluate/screenshot calls. capture-frames.mjs calls __stopPreview() before the frame loop. Always include it.
• Use t < startMs (not <=) in scene boundary checks. t <= 0 at frame 0 makes scene 1 black. The correct guard is if (t < startMs || t >= endMs) return hidden.
• Body = exact pixel dimensions. Width and height are integers (1080px, 1920px). No %, vw/vh, or responsive units.
• No two scenes visible simultaneously (except 10% enter overlap). All scenes opacity: 0 outside their renderFrame window.
• Transitions use opacity only. No display toggle, no visibility — GPU-composited opacity is frame-perfect.
• Never dump HTML in chat. Save to file, show summary only.

10. Title states the message, not the topic. "3 Reasons Q4 Crushed Targets" not "Q4 2024 Performance Video".

11. Read references/scene-library.md before generating ANY HTML. Use exact HTML structure and CSS class names from that file.

Step 1: Intake

Required: content_brief

Optional parameters and defaults:

| Parameter | Default | Description |

|---|---|---|

| content_brief | — | Text describing what the video communicates (required) |

| scenes | auto | Number of scenes (1–6). Auto = derived from brief. |

| duration_per_scene | 3s | Duration per scene in seconds (1–8s) |

| style | kinetic-dark | kinetic-dark / editorial-light / data-pulse / bold-type / minimal-clean |

| aspect_ratio | 1:1 | 1:1 (1080×1080) / 16:9 (1920×1080) / 9:16 (1080×1920) |

| fps | 30 | Frames per second (24, 30, or 60) |

| music | none | Path to audio file for background track (mp3/m4a/wav) |

| source | — | Source attribution shown in final frame footer |

If content_brief is missing, ask exactly:

> "To create the video, I need a content brief — what should the video communicate?

>

> Example: 'Show 3 reasons why Q4 revenue grew 85%: new enterprise deals, reduced churn, price increase. Use bold numbers. Style: data-pulse.'

>

> Optional: style (default: kinetic-dark), aspect ratio (default: 1:1), seconds per scene (default: 3s)"

If content_brief is present → proceed to Step 2 immediately.

Step 2: Internal Architecture (never shown to user)

1. Parse brief into scenes (max 6):

• Scene 1 = hook or title (always)
• Scenes 2–N-1 = supporting points, metrics, or story beats
• Scene N = CTA or closing summary (always, if more than 1 scene)
• One key idea per scene — if brief has 7+ ideas, consolidate the weakest ones

2. Read references/scene-library.md — choose scene type for each scene:

• Hook/opening → title-card
• Single metric → stat-reveal
• List of 2–4 points → bullet-list
• Before vs after / two values → split-screen
• Testimonial / quote → quote-card
• Final / CTA → cta-card

3. Read references/style-presets.md — load CSS tokens + animation personality for chosen style.

4. Calculate timing:

totalDuration = sceneCount × duration_per_scene  (seconds)
totalFrames   = totalDuration × fps

Each scene occupies (100 / sceneCount)% of the total @keyframes range.

| Scene | Start % | End % |

|---|---|---|

| 1 | 0% | (100/N)% |

| 2 | (100/N)% | (200/N)% |

| … | … | … |

| N | ((N-1)×100/N)% | 100% |

Within each scene's range:

• Enter: first 10% of scene range
• Hold: 10% to 85% of scene range
• Exit: 85% to 100% of scene range

5. Determine pixel dimensions:

• 1:1 → W=1080, H=1080
• 16:9 → W=1920, H=1080
• 9:16 → W=1080, H=1920

Step 3: HTML Generation

Read references/scene-library.md AND references/style-presets.md before writing any code.

Required HTML structure:

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
[font CDN link from style preset]
<style>
:root {
  [all CSS tokens from style preset]
}

*, *::before, *::after { margin: 0; padding: 0; box-sizing: border-box; }
html, body {
  width: [W]px; height: [H]px;
  overflow: hidden;
  background: var(--bg);
  font-family: var(--font-body);
  position: relative;
}

.scene {
  position: absolute;
  inset: 0;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  padding: 80px;
  opacity: 0;
  will-change: opacity, transform;
}
.scene-inner {
  width: 100%;
  max-width: 960px;
}

/* Scene-type CSS from scene-library.md */
[paste scene-type CSS here]
</style>
</head>
<body>

<div class="scene scene-1">
  <div class="scene-inner">
    [scene 1 HTML from scene-library.md template]
  </div>
</div>

[repeat for each scene]

<script>
window.__videoReady = false;
window.TOTAL_DURATION_MS = [totalDuration * 1000];

// ── Animation helpers ─────────────────────────────────────────────────────────
function lerp(a, b, p) { return a + (b - a) * p; }
function clamp(v, lo, hi) { return Math.max(lo, Math.min(hi, v)); }
function easeOutCubic(t) { return 1 - Math.pow(1 - clamp(t, 0, 1), 3); }

function sceneState(t, startMs, endMs) {
  if (t < startMs || t >= endMs) return { opacity: 0, ty: 0 };
  const prog = (t - startMs) / (endMs - startMs);
  if (prog < 0.10) {
    const p = easeOutCubic(prog / 0.10);
    return { opacity: p, ty: lerp(24, 0, p) };
  }
  if (prog < 0.85) return { opacity: 1, ty: 0 };
  const p = (prog - 0.85) / 0.15;
  return { opacity: 1 - p, ty: lerp(0, -12, p) };
}

function applySceneState(el, state) {
  el.style.opacity = state.opacity;
  el.style.transform = state.ty !== 0 ? `translateY(${state.ty.toFixed(2)}px)` : '';
}

// ── Main render function — called by Playwright once per frame ────────────────
window.renderFrame = function(t) {
  // Scene 1: 0ms – [D]ms
  applySceneState(document.querySelector('.scene-1'), sceneState(t, 0, [D]));
  // Scene 2: [D]ms – [2D]ms
  applySceneState(document.querySelector('.scene-2'), sceneState(t, [D], [2D]));
  // ... repeat per scene ...
};

// ── Preview loop — stopped by Playwright before frame capture ─────────────────
let __previewActive = false;
let __previewRafId = null;
window.__stopPreview = function() {
  __previewActive = false;
  if (__previewRafId !== null) { cancelAnimationFrame(__previewRafId); __previewRafId = null; }
};

document.fonts.ready.then(() => {
  window.renderFrame(0);
  window.__videoReady = true;
  __previewActive = true;
  const startTime = performance.now();
  function previewTick() {
    if (!__previewActive) return;
    const elapsed = performance.now() - startTime;
    if (elapsed < window.TOTAL_DURATION_MS) {
      window.renderFrame(elapsed);
      __previewRafId = requestAnimationFrame(previewTick);
    } else {
      window.renderFrame(window.TOTAL_DURATION_MS - 1);
      __previewActive = false;
    }
  }
  __previewRafId = requestAnimationFrame(previewTick);
});
</script>
</body>
</html>

Design quality rules:

• Headline font size: never smaller than 60px — text must read at mobile thumbnail size
• Numbers and stats: always the largest element on screen (120–200px)
• Tight letter-spacing on display type: -0.02em to -0.04em
• One accent color per scene — don't scatter accent across multiple elements
• Dark presets: dividers rgba(255,255,255,0.10), never solid
• Light presets: dividers rgba(0,0,0,0.10), never solid
• transform-origin: center center on every element that uses transform
• Padding inside .scene: minimum 80px — never let text touch viewport edges

Step 4: Self-QA (fix every failure before Step 5)

renderFrame correctness:

• [ ] window.renderFrame(t) defined — pure function, no side effects outside style writes
• [ ] Scene boundary uses t < startMs (not t <= startMs) — avoids black frame 0
• [ ] Zero instances of animation-delay or @keyframes for scene transitions
• [ ] No two scenes have overlapping opacity: 1 windows (except 10% enter overlap)
• [ ] window.__stopPreview() exposed and preview rAF loop checks __previewActive

Readiness signal:

• [ ] window.__videoReady = false declared before document.fonts.ready
• [ ] window.__videoReady = true set ONLY inside document.fonts.ready.then(...)
• [ ] window.renderFrame(0) called inside document.fonts.ready.then(...) before setting __videoReady = true

Layout:

• [ ] html, body use exact pixel dimensions ([W]px, [H]px)
• [ ] No %, vw, vh, rem units on body width/height
• [ ] overflow: hidden on html, body
• [ ] All scenes position: absolute; inset: 0

Design:

• [ ] All colors from style preset tokens — no free hex values
• [ ] All fonts from style preset — no free font-family strings
• [ ] Headline font size ≥ 60px
• [ ] Source in final scene if source param provided

Step 5: Export

Determine slug from brief content (kebab-case, ≤30 chars):

mkdir -p chart/[slug]

Save HTML: chart/[slug]/video.html

Browser preview:

open chart/[slug]/video.html

Run export (replace [skill-root] with path to this skill's directory):

bash [skill-root]/scripts/export-video.sh \
  chart/[slug]/video.html \
  chart/[slug]/video.mp4 \
  --duration [totalDuration] \
  --fps [fps] \
  --width [W] \
  --height [H] \
  [--music path/to/audio.mp3]

The script installs Playwright on first run (~200MB Chromium), captures all frames, then assembles MP4 with FFmpeg.

Step 6: Output Summary

## Video: [title from brief]
Date: [YYYY-MM-DD] | Scenes: [N] | Style: [style] | Aspect: [ratio]
Duration: [N]s | FPS: [fps] | Frames: [N]

Files
  Source:   chart/[slug]/video.html
  Output:   chart/[slug]/video.mp4
  Size:     [X] MB

Checklist
- [ ] All scenes appear in sequence with no blank frames
- [ ] Text legible at mobile thumbnail size
- [ ] Scene transitions smooth (no jump cuts)
- [ ] Final scene includes CTA or closing message
- [ ] Source attribution present in final frame (if provided)

Prompt Tips (show when user asks for guidance)

> "Provide a content brief — bullet points of what each scene should say."

>

> "Name the insight directly. '3 reasons Q4 grew 85%' gives the video a spine."

>

> "Mention the style if you have a preference: kinetic-dark (default), editorial-light, data-pulse, bold-type, minimal-clean."

>

> "Specify aspect ratio for the platform: 1:1 for LinkedIn/Instagram feed, 9:16 for Stories/Reels, 16:9 for YouTube/presentations."

>

> ✅ Good: "Create a 9-second video. Q4 revenue hit $4.2M (85% growth). Drivers: enterprise deals, churn 1.2%, price increase. CTA: acme.com/q4. Style: data-pulse. Aspect: 1:1."

>

> ❌ Bad: "make a video about our company"