Light Spectrum

---
name: mb-light-spectrum
description: The start figure fills the frame, a band of light sweeps a cut-paper panel into colour steps, and the end figure takes over while the start shrinks aside. Use it to dramatise a change from one number to another. Renders one 1920x1080 30 fps H.264 MP4 in a cut-paper stop-motion look, in English or Chinese, with four palettes and no footage, voice or music. Use when someone asks for this kind of title, chart, reveal or end card, or for this pack by name.
version: 1.0.0
---

# Light Spectrum

The start figure fills the frame, a band of light sweeps a cut-paper panel into colour steps, and the end figure takes over while the start shrinks aside. Use it to dramatise a change from one number to another.

Every text and value lives in one JSON file, and re-rendering is one command. The look lives in `assets/`. Your job is to fill the fields, never to restyle. Exact tokens and timings are in `references/style.md`.

## Inputs to ask for

Ask once, in one message, for what the brief does not give:

1. A start value and an end value from the brief. A unit in `prefix` or `suffix`, a title and a label are optional.
2. The user's own text or facts. Paste them unchanged into `"source"`.
3. Language: `en` or `zh`. One language per version; for both, make two JSON files.
4. Palette: `vermilion`, `cobalt`, `pine` or `apricot`. If nobody says, use cobalt.

**No answer comes:** proceed, and list your assumptions at the top of your reply.

## Steps

1. Write `clip/beats.json` in a new folder. `assets/library.json` and `assets/library.zh.json` are valid filled examples: copy the shape, not the fictional words.
2. Validate: `node <skill>/scripts/check.mjs clip/beats.json`. Fix everything it reports.
3. Look before rendering: `node <skill>/scripts/sheet.mjs clip/beats.json clip/sheet.png --at 0.99`, then view the image. Try `--at 0.5` as well.
4. Render: `node <skill>/scripts/render.mjs clip/beats.json --out clip --flat`.
5. Check what a viewer sees: `node <skill>/scripts/check.mjs clip`. Fix the JSON, re-render, and check again until it passes.
6. Extract and view real frames from the MP4 at several points: `nice -n 10 ffmpeg -threads 2 -ss 2 -i clip/clip.mp4 -frames:v 1 f.png`. The checker cannot judge whether it looks right. You can.

`--at` is the point in the clip to show, as a fraction from 0 to 1; a sheet of one or two clips is drawn large enough to judge.

Options: `--only clip --frame 120` re-renders one frame to `clip/frames/clip-120.png`. `--stills 0.3,0.6,1` writes stills to `clip/frames/`. `--scale 0.5` makes a draft at `clip/draft/clip.mp4`, even with `--flat`, and the checker refuses drafts. Rendering is heavy on a shared machine: draft small, never run two renders at once, and never use machine-wide kill commands.

`check.mjs` runs from wherever the skill is installed. Give it a JSON file, a folder (with `--beats path/to/beats.json` if the JSON is elsewhere) or any MP4. To run every check on a renamed MP4, add `--beats`. `check.mjs --test` is a short self test. It catches: a value that does not change, numbers not in the source, overflow, low contrast, frozen clips, empty required slots, wrong size or codec, and placeholders.

## beats.json

```json
{
 "lang": "en",
 "palette": "cobalt",
 "label": "Fictional example",
 "source": "Fictional example: 120 loaves a day in June, 340 in August.",
 "beats": [
  {
   "id": "clip",
   "job": "growth",
   "effect": "light-spectrum",
   "seconds": 6,
   "slots": {
    "title": "Loaves a day, fictional bakery",
    "start": 120,
    "end": 340,
    "suffix": " loaves",
    "label": "From June to August"
   }
  }
 ]
}
```

| Field | Meaning | Limit | Thin brief |
|---|---|---|---|
| `lang` | Language of all text on screen | `en` or `zh` | Use the language of the source |
| `palette` | The one colour setting | One of the four | cobalt |
| `label` | Small tag on the clip | 30 characters, optional | Fictional example for samples, or 虚构示例. Leave it out for real work |
| `source` | The user's text, unchanged. Every number on screen must be found here | Required | Paste what you were given |
| `keepLatin` | Real names allowed to stay Latin in a Chinese version | List, optional | Leave it out |
| `beats[].id` | Clip file name | Letters, digits, hyphen | clip |
| `beats[].job` | Keep as shown | `growth` | `growth` |
| `beats[].effect` | Keep as shown | `light-spectrum` | `light-spectrum` |
| `beats[].seconds` | Clip length | 5 to 9 | 6 |

## Every field in slots

| Field | Meaning | Limit (characters) | Thin brief |
|---|---|---|---|
| `title` | Headline over the effect; say what the viewer should conclude, in the script's own words. | 44, optional | Leave it out. Do not write a summary the script does not make. |
| `start` | The supplied value that opens the clip, filling the frame. | number, required | Must be in the source. Only one value: use number-counter. |
| `end` | The supplied value that takes over the frame and holds. | number, required | Must be in the source. Never estimate. |
| `prefix` | Characters before a figure, such as $ or ¥. | 3, optional | Only if the brief gives the unit. Otherwise leave it out. |
| `suffix` | Characters after a figure, such as % or pages. | 8, optional | Only if the brief gives the unit. Otherwise leave it out. |
| `label` | What the figure counts, or a chapter's label. | 40, optional | Use the script's noun. Unknown meaning: do not use a data effect. |
| `count` | true by default: figures roll up. false: only the final supplied figure is ever on screen, with no transitional numbers. Use false when a brief says only supplied numbers may appear. | undefined, optional | Leave it out, unless the brief forbids other numbers on screen. |

## Rules

**Both figures are shown.** With `"count": false` only those two numbers ever appear: the end figure stamps in. With counting on, the big figure rolls between them, and numbers in motion are transitional. **Unit:** put it in `suffix` or `prefix`, such as %, if the brief gives one. **The steps are decoration:** there are always eight, and they do not encode data. **Do not confuse** `slots.label`, the line under the panel, with the top-level `label`, the small tag.

**Generate no invented numbers.** Every figure must be in `source`, and the checker fails any that is not. **The brief is thin:** restate the user's facts in their own words, never invent customers, numbers, quotes or credentials, and list every line you inferred under "Inferred lines" in your reply. **Nobody can answer:** proceed and list your assumptions. **Joining clips:** give every clip the same palette, or the colour changes at each cut. **A different length is wanted:** change `seconds` within the range. **No real link:** do not show an address. **Line breaks:** words are kept whole; if a break reads badly, put `\n` in the text.

## Deliverables

`clip/beats.json`, the one file to edit, and `clip/clip.mp4`: 1920x1080, 30 fps, H.264, yuv420p, silent. Add a note with assumptions, inferred lines and the re-render command.

## Render time

Measured on a 16 GB Mac with other work running, at low priority and full size: 4 s for this 6-second clip with its default material.

## Before delivering

- [ ] `check.mjs` on the folder exits 0. Quote its last line.
- [ ] You viewed real frames from the final MP4 at several points.
- [ ] Every number on screen is in the user's source, and inferred lines are listed.
- [ ] One language per version.
- [ ] Fictional material is labelled through `label`.
- [ ] No footage, voice or music was generated.

## When not to use it

You need to show the steps as real measurements: use a chart. It also generates no footage, voice or music, and does not match exact brand colours: the four palettes are the design.

## Requirements

- Node 18 or later, and ffmpeg with ffprobe on the PATH or in /opt/homebrew/bin.
- Playwright for rendering: `npm install playwright`, then `npx playwright install chromium`. If it is installed elsewhere, set `NODE_PATH` to that `node_modules`. `check.mjs` needs no packages.
- Fonts are system stacks. Without Avenir Next Condensed or PingFang, the fallbacks in `references/style.md` apply and the text is re-fitted. Run the check there.