# Erudit — VK loading-screen logo (Lottie)

Animated logo for the **VK app loading screen** (shown before the SPA assets load).
The Erudit «Э» tile (score `8`) **drops in under gravity, lands with a soft squash —
its left/right edges bulging into a cushion — then springs back up**, looping. A light
"glint" is caught at the moment of the bounce.

| File | Purpose |
|------|---------|
| `erudit-loader.json` | The Lottie to upload to VK. |
| `erudit-loader-preview.gif` | Looping preview. Rendered on a green "baize" background **only in the preview** — the real asset has a **transparent** background. |
| `build/` | The reproducible build pipeline (see *Regenerate*). |

**VK requirements met:** 96×96 px · vector Lottie JSON · ≤ 24 KB (~11 KB) · seamless
~1.1 s loop · transparent background.

Design reference: a photo of the physical wooden Erudit tile.

---

## How it works

A single flat layer holds the tile — a rounded-rect **face path**, the two glyphs, and
a thin border — and everything is driven by that layer's transform plus a few colour /
shape tracks. The anchor sits on the tile's **bottom edge**, so the squash happens
against the floor.

### Bounce (gravity)
`position.y` of the bottom edge goes apex → floor → apex with **no dwell** (the apex is
an instantaneous turn-around). The fall uses a strong **ease-in** (accelerate) and the
rise a strong **ease-out** (decelerate), so it reads as real gravity. While falling fast
the tile slightly **stretches** (tall+thin); that is the squash-and-stretch setup.

### Soft cushion (squash)
On contact the layer **squashes** (scaleY↓, scaleX↑) about the bottom anchor, with a
touch of skew. Crucially the squash/cushion is **decoupled from the fall speed** — it
eases in and out *slowly* (`ES`), so the landing feels soft even though the fall is
fast. The squash is deliberately gentle (≈ 86 % / 112 %).

### The cushion shape (the hard part)
The face is **not** a plain rounded rect — it is a path whose left/right contour bows
out into a convex cushion on impact:
- the rest rounded-rect outline is sampled (fine corner arcs + edge mids), and on impact
  every point is pushed outward by a smooth **barrel** profile `f(y) = b·(1 − (y/HH)²)`
  (max at the middle, fading to zero at the top/bottom);
- so the **whole side — corners included — bows out as one piece**, never just the
  straight middle;
- bezier handles are computed as a **chordal spline** (handle length ∝ the adjacent
  chord), which stays smooth across the uneven corner/edge point spacing. This is what
  keeps the corner↔cushion junction kink-free — both at rest and fully bulged — which a
  naïve uniform Catmull-Rom (or moving discrete points with fixed tangents) does not.

### Glint
At the bounce the face flashes a little lighter (`FACE_GLINT`) and the glyphs warm
toward brown-red (`BLACK_LIT`), then settle back. A cheap, warm "catch the light".

### Border & background
The tile carries a thin static **border** a touch darker than the face (`BORDER`) so it
reads on any background. The **green baize background is added only when rendering the
preview GIF** — the Lottie itself is transparent.

### Player compatibility
Only plain 2-D shapes (`ddd:0`) — no 3-D layers, expressions or effects — so every
Lottie player (lottie-web on the web, rlottie on native) renders it identically.

---

## Glyphs

`Э` (U+042D) and `8` are **real outlines from LiberationSans-Regular** — metric-
compatible with Arial, matching the game's `--font: system-ui … Arial` stack
(see `ui/src/app.css`). `build/extract.js` pulls the contours into `build/glyphs.json`
as Lottie cubic paths; a hair of same-colour stroke adds a touch of weight.

---

## Regenerate

Requirements: Node ≥ 18. `extract.js` additionally needs `opentype.js`
(`npm i opentype.js`); **`generate.js` has no dependencies**.

```sh
cd assets/vk

# 1. (optional) re-extract the glyphs — only if you change the font:
#    default font: /usr/share/fonts/truetype/liberation/LiberationSans-Regular.ttf
node build/extract.js [/path/to/font.ttf]      # -> build/glyphs.json

# 2. build the animation (reads build/glyphs.json):
node build/generate.js erudit-loader.json      # -> erudit-loader.json

# 3. (optional) live preview — needs lottie-web (npm i lottie-web):
node build/build-preview.js erudit-loader.json preview.html
#    then open preview.html in a browser.
```

The preview GIF is assembled by rendering the Lottie frame-by-frame (lottie-web canvas,
filled with the green baize) and stitching with `ffmpeg`; the live HTML preview is the
quickest way to eyeball changes.

## Tunables (top of `build/generate.js`)

| Symbol | Meaning |
|--------|---------|
| `OP`, `FR` | loop length (frames) / fps — overall speed |
| `T_HIT / T_PEAK / T_LIFT / T_REC` | beats: contact / squash peak / lift-off / cushion recovered |
| `EI / EO / ES` | easings: fast fall / fast rise / slow soft cushion |
| `APEX`, `FLOOR` | bottom-edge screen-y at the top of the bounce / at rest |
| `HW`, `HH`, `CR` | tile half-width / half-height / corner radius |
| `scl` squash values | the squash amount (scaleX↑ / scaleY↓) |
| `bulge` `b` | cushion depth (how far the sides bow out) |
| `FACE / FACE_GLINT` | wood face / glint flash |
| `BORDER` | tile rim colour |
| `BLACK / BLACK_LIT` | glyph / glyph-at-glint (brown-red) |
| preview bg `#3C7858` | the green-baize colour used **only** in the GIF |