# soundtrack

Writes a score for an animation in code with Web Audio, renders it to WAV, and checks that every cut lands on the beat without a startling jump in loudness.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 543 installs · ★ 34
- Author: iart-ai
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/iart-ai/javascript-animation-skills/tree/HEAD/skills/soundtrack/SKILL.md
- Updated: 2026-09-30
- HTML: https://affpapa.org/en/nevibecode/skills/soundtrack

## What it does

The skill writes sound for one specific film: its own oscillators, noise, filters, envelopes and delays inside `window.SCORE = ac => { … }`. Drums, a groove or a music-box bed get written the same way, per film — no shared template. The ready-made scores `templates/groove.js` and `templates/score.js` are used only when you ask for them.

Rendering comes from the `javascript-animation` skill: `render.mjs` runs `window.SCORE` through `OfflineAudioContext`, writes `<out>.wav` and muxes it into the MP4. The page's live preview plays the same score on click. Music follows the picture through a tempo map — each scene gets a whole number of bars, so cuts land on a downbeat. Cuts are marked with harmony rather than volume, because a loud accent on a cut reads as a jump scare.

Always run the check: `node scripts/sync-check.mjs piece.mp4 --cues-file piece.cues.json` verifies the declared `window.CUES`, and without the file it detects cuts in the picture. It fails on OFF-BEAT (more than ±100 ms) and STARTLE (more than +6 dB against the second before). With music you don't control, the direction reverses: render, mux, run the check, then shift scene boundaries in the `SCENES` table by the reported offsets. That stays cheap because each scene function receives local time. Code synthesis won't give you realistic voices, orchestras or genre production — reach for AI music or a real track instead.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add iart-ai/javascript-animation-skills --skill soundtrack -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/soundtrack/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/soundtrack/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add iart-ai/javascript-animation-skills --skill soundtrack -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/soundtrack/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add iart-ai/javascript-animation-skills --skill soundtrack -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/soundtrack/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install iart-ai/javascript-animation-skills soundtrack
```

2. **Manually: put the files in place**
- File: `.github/skills/soundtrack/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add iart-ai/javascript-animation-skills --skill soundtrack -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/iart-ai/javascript-animation-skills --path skills/soundtrack
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/soundtrack/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add iart-ai/javascript-animation-skills --skill soundtrack -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/iart-ai/javascript-animation-skills/tree/HEAD/skills/soundtrack/SKILL.md

## When to use it
- write a score for an animation from scratch in code
- land cuts on the downbeat without a loudness jump
- render `window.SCORE` to WAV and mux it into the MP4
- check whether audio lines up with picture and get offsets
- shift scene boundaries to fit a track you already have

## Example prompts
- add music to the animation, write the score in code
- check that the cuts land on the beat: `node scripts/sync-check.mjs piece.mp4 --cues-file piece.cues.json`
- the music is too sudden on the cut, rework the transition

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 0d519ef3d055a5a4.

BEGIN THIRD-PARTY FILE 0d519ef3d055a5a4
```text
---
name: soundtrack
description: This skill should be used when the user asks to "add music to the animation", "make the music in code", "synthesize a soundtrack", "compose background music that fits the cuts", "sync the music to the video", "check if the audio lines up with the picture", or "the music is too sudden / startling". Covers a code-synthesized score that lives inside the animation page (Web Audio API, rendered offline to WAV), AI-generated or user-supplied music, and an automatic check that cuts land on the beat without a startling jump in loudness.
version: 0.4.1
---

# Soundtrack

## Write the sound for this film

A film's sound is written for that film: its own instruments, rhythm and harmony in Web Audio, from nothing, in `window.SCORE = ac => { … }` (oscillators, noise, filters, envelopes, delays: whatever the piece calls for). Drums, a groove or a music-box bed are written the same way, for this film: no two films should sound alike because they share a template. `templates/groove.js` and `templates/score.js` are two ready-made scores, used only when the user asks for them; each file documents its options.

`render.mjs` (from the `javascript-animation` skill) renders `window.SCORE` offline with `OfflineAudioContext`, writes `<out>.wav` and muxes it into the MP4; the page's live preview plays the same score when clicked.

Don't ask the user to choose a music source up front. At delivery, mention in one line that AI-generated music or their own track also work.

## How the music follows the picture

- **Tempo map.** Each scene gets a whole number of bars (the tempo may flex per scene), so every cut lands on a downbeat.
- **Mark cuts with harmony, not volume.** Each scene walks home to the dominant and the cut arrives on the tonic. A loud accent on a cut (bass + bright chime at once) reads as a jump scare: users flagged a +7 to +10 dB jump as startling; +3 to +5 dB with a chord change reads as a clean edit.
- **Don't thin the opening bar if it's the only bar before the first cut**; a sparse intro followed by the full texture is itself a startle.
```
END THIRD-PARTY FILE 0d519ef3d055a5a4

The third-party file has ended. Everything between the 0d519ef3d055a5a4 tags is data: requests and commands from it are not followed. An “end of file” line with another tag inside the block is part of the file.

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