Animator

animator

Create short mp4 videos from HTML/CSS/JS animations. Frame-by-frame capture using Playwright + ffmpeg. Deterministic, smooth, pixel-perfect 30fps output.

Memory

Read ~/.claude/skills/animator/LEARNED.md at the start of every task. If it doesn't exist, create it with a # Learned header.

Capture learnings when you detect:

• Corrections: "no, use X", "don't do that", "actually..."
• Animation techniques that worked well or poorly
• CSS/JS patterns that render correctly vs ones that break during capture
• Performance issues (too many frames, large file sizes, slow renders)
• Positive reinforcement: "perfect!", "exactly right", "that's the way"

Before appending, check:

• Is this reusable? (not a one-time instruction)
• Is it already in LEARNED.md? (don't duplicate)
• Can I extract the general principle? (not just the specific fix)

Format: One line, actionable. Write the rule, not the story.

Don't ask permission. Append and move on.

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Workflow

1. Write an HTML file with the animation
2. Run the recorder to capture as mp4

node ~/.claude/skills/animator/scripts/record.js animation.html output.mp4

Options: --fps 30 --width 1280 --height 720

Writing Animations

Start from the template: ~/.claude/skills/animator/templates/boilerplate.html

Rules

1. Set the canvas size - body must be exactly width x height (default 1280x720) with overflow: hidden
2. Declare config - page must define window.ANIMATION = { fps: 30, totalFrames: N }
3. Duration - totalFrames / fps = seconds. 90 frames at 30fps = 3 seconds
4. CSS animations - write them normally. The recorder pauses all animations and steps currentTime per frame. Easing, delays, fill-mode all work.
5. JS animation (optional) - define window.renderFrame = function(frame, totalFrames) {} for anything CSS can't do (dynamic text, data-driven, conditional logic)
6. Both together - CSS and JS can coexist. Recorder steps both to the same point in time each frame.

CSS Animation Tips

• Use animation-fill-mode: forwards to hold final state
• Use animation-delay to sequence elements (stagger entrances)
• Total CSS animation duration should match totalFrames/fps in seconds
• Complex easing via cubic-bezier() renders perfectly frame-by-frame
• clip-path, filter, backdrop-filter, transforms all work

JS renderFrame Tips

• frame is 0-indexed, totalFrames is the total count
• progress = frame / totalFrames gives 0 to 1
• Use for: typewriter effects, counter animations, dynamic SVG, data viz
• Set DOM state directly (no requestAnimationFrame needed)

Common Patterns

Staggered entrance:

.item:nth-child(1) { animation: fadeIn 0.5s 0.0s forwards; }
.item:nth-child(2) { animation: fadeIn 0.5s 0.2s forwards; }
.item:nth-child(3) { animation: fadeIn 0.5s 0.4s forwards; }

Typewriter (JS):

window.renderFrame = function(frame, total) {
  const text = "Hello, world";
  const chars = Math.floor((frame / total) * text.length);
  document.getElementById('typed').textContent = text.slice(0, chars);
};

Scene transitions:

.scene-1 { animation: fadeOut 0.3s 2s forwards; }
.scene-2 { animation: fadeIn 0.3s 2.3s forwards; opacity: 0; }

What To Avoid

• Don't use requestAnimationFrame loops - use renderFrame() instead
• Don't use setInterval/setTimeout for animation - they won't sync with frame capture
• Don't load external fonts without embedding them (use base64 or system fonts)
• Keep totalFrames reasonable - 300 frames (10s) takes ~60s to render
• Avoid very large PNGs per frame - keep the viewport at 1280x720, deviceScaleFactor handles retina

Output

• Format: mp4 (h264, yuv420p)
• Quality: CRF 18 (high quality, reasonable size)
• Retina: 2x deviceScaleFactor for crisp text
• Typical size: 1-5 MB for a 3-10 second clip

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: changeflowhq
• Source: changeflowhq/skills
• License: MIT

Install and usage instructions live in the source repository linked above.