party-player/README.md

# Party Player

A YouTube playlist player for parties. Two browser windows run side by side:

- **Player** (`http://localhost:3000/`) — fullscreen poster background with a
  small video frame that moves around. Both the animation (see
  [Animations](#animations)) and the poster behind it (see
  [Backgrounds](#backgrounds)) change on every track. Consecutive videos
  overlap in a 5 second audio/video crossfade, so there is never silence
  between tracks.
- **Queue** (`http://localhost:3000/queue`) — add videos by URL, drag to
  reorder, delete, see how much playtime is left, and pick the background,
  animation and animation speed.

## Run

```sh
npm install
npm start
```

Put the player on the projector/TV screen (F11 for fullscreen) and open the
queue on a laptop or phone. On the same network, other devices reach both pages
at `http://<this-machine-ip>:3000`.

**The player page opens behind a "Start" button.** Browsers block unmuted
autoplay, so the first click is what unlocks audio. One click per session.

## Notes

- No YouTube API key is needed. Titles come from YouTube's public oEmbed
  endpoint; durations are measured by a hidden muted player on the player page,
  which is why a freshly added video shows `–:––` for a moment (and keeps
  showing it while the player page is closed). Totals are prefixed with `≥`
  whenever a length is still unknown.
- The queue is saved to `queue.json` on every change and restored at startup.
- Settings live in `config.js`: overlap length, frame size, drift speed, corner
  screenshot interval and margin, the drag animation's size range, the
  background zoom range, the low-res speed factor, port, and the animation list. Everything there except
  `PORT` is mirrored to the browser by the generated `/config.js` route in
  `server/index.js`, so a new constant needs no second list.

## Crossfades

Two decks, one video each, and a track change is an equal-power fade from the
one to the other. Three details are what keep it from being audible as a seam:

- **The next track is prerolled.** `PREROLL_SECONDS` before the fade is due, the
  idle deck loads it, plays until YouTube reports it running, then seeks back to
  0 and pauses. A deck told to play at the moment the fade opens would spend its
  first second or two fetching, and the fade — which runs on wall-clock time —
  would ramp the outgoing track down over that silence. The fade still will not
  open until the incoming deck is genuinely playing, so a preroll that did not
  take costs a moment of the outgoing track rather than a gap.
- **The fade lands before the end, not on it.** It runs for what is actually
  left of the outgoing track minus `END_GUARD_SECONDS`, rather than for a fixed
  `OVERLAP_SECONDS` — the tick that triggers it is already a little late, and
  `getDuration()` reports a length that runs slightly past the last of the
  audio. A fade that outlasts its own track ends in a clipped ending and a flash
  of YouTube's end screen, which shows the video's first frame and reads as the
  track restarting.
- **The player owns the hand-over until the server confirms it.** The queue
  pointer only moves once the server has processed `trackEnded`, so for one
  round trip the state still names the track that just finished. The player
  advances its own pointer at the same moment it sends, and reconciles when the
  broadcast arrives; without that, the jump check reads the stale pointer as a
  manual jump and cuts straight back to the track that just ended.

## Backgrounds

Every image file in `assets/` is a background — drop one in, restart, and it
joins the rotation with a button on the queue page named after the file. There
is no list to maintain.

Like the animations, the background changes on every track: a different poster,
at a random zoom between 1.15× and 1.9×, cropped around a randomly chosen point
of it, cross-fading over about a second. The posters are portrait prints and
the screen is wide, so the crop is mostly a question of which band of the
poster to keep — the zoom never goes below 1.15×, because at 1× a wide screen
shows the poster's full width, paper margin included.

Each crop is resized off the main thread and drawn into a screen-sized canvas
(`public/player/backdrop.js`), so the cross-fade never has to composite a
17-megapixel layer, and the full-size scans are never held in memory. This is
why `assets/` can hold 300 dpi print files without the player caring.

The queue page's **Background** buttons pin one poster the same way the
animation buttons pin one animation; **Auto** hands selection back to the same
shuffle bag the animations use.

## Animations

The video frame's movement changes with every track. Six animations ship
today:

- **bounce** — diagonal drift, reflecting off the screen edges.
- **corners** — the frame parks in a random corner and every 5 seconds a
  screenshot of it appears in the next of the three other corners, going
  clockwise. The frame never moves, so a screenshot can never cover it.
- **drag** — a drawn mouse pointer travels to the frame, grabs it near its top
  edge and drags it across the screen, lets go for a beat, then takes hold of
  one corner and pulls the frame to a new size. The aspect ratio stays locked
  and the frame stays on screen, so the resize reads as a window resize rather
  than a stretch.
- **pong** — the 1972 game with the frame cast as the paddle: it slides along
  the bottom of the screen and nowhere else, a square ball runs the diagonals
  off the walls, and where the ball lands on the frame's face sets the angle it
  leaves at. The frame is solid on all four sides, so a ball that comes at its
  flank or up under it is turned away too. It plays the ball's predicted
  landing spot, but aims a width wide
  once in a while, and every ball that gets past it puts one on the
  seven-segment counter in the top right.
- **trail** — the frame glides along a curving path, steering from waypoint to
  waypoint, and drops a screenshot of itself every frame-width or so. The stills
  stay where they were taken and fade over a few seconds, leaving a comet tail of
  frozen video behind the live one.
- **wash** — the frame parks in the middle and the colour changes instead: a
  gradient creeps round the colour wheel behind the poster, taking a minute and
  a half per lap, and the frame's glow travels with it. The one still animation,
  which is what makes it worth having between the moving ones.

### Speed

Every speed in `config.js` is in pixels per second, so the same numbers cross a
small screen in less time than a big one — on 1280x800 the frame covers the
width in two thirds of the time it takes on 1920x1080, and the whole player
reads as hurried. The queue page's **Speed** switch fixes that: **Low-res
screen** multiplies every animation clock by `LOW_RES_SPEED` (0.65, the
1280/1920 ratio), which slows the drift, the glides, the pong rally, the drag
beats, the wash lap, the screenshot intervals and the fades together.

It is one factor read per frame rather than a set of slower constants, so
flipping the switch takes effect on the next frame — no animation restarts, and
one part-way through a beat keeps its place. The audio crossfade is deliberately
left out: it is tied to the track that is ending. The setting is saved with the
queue, since it describes the screen the player is on.

The queue page's **Animation** buttons pin the player to one of them, which is
how you look at a single animation without waiting for the cycle to reach it.
**Auto** (the default) hands selection back to the shuffle bag. Switching takes
effect immediately; while pinned, track changes leave the animation running.

**The corners and trail animations photograph the frame.** The decks are
cross-origin YouTube iframes, so nothing on the page can read their pixels —
a screenshot is the only way to capture what is playing.

The server takes it. It runs on the same machine as the screen, so it can grab
the region itself with [`grim`](https://sr.ht/~emersion/grim) and hand back the
finished crop, which the player drops straight into an `<img>`. No picker, no
permission prompt, and no "Sharing this tab" bar parked over the party. This
needs a wlroots compositor — Hyprland, Sway, river — and `grim` on `PATH`.

`server/screen.js` also asks `hyprctl` where the player's window is and how big.
That is what makes the crop reliable: the page measures in CSS pixels, which are
device pixels divided by `devicePixelRatio`, and that ratio moves with browser
zoom and fractional display scaling. Comparing the window's real size against the
viewport the page reports gives the scale outright, and the vertical room left
over is the tab and address bars. **So the server path needs no fullscreen and no
particular zoom** — it works in an ordinary window. **Start** still asks for
fullscreen because that is what you want on a projector, but nothing breaks if
you refuse.

The window is found by matching the page's `document.title` as a prefix, so the
player must be the **active tab** in its window — which it is when it is the
thing on screen. If the server cannot find it, the player says so in the corner.

Without `grim`, the player falls back to `getDisplayMedia` and asks to share
your screen on the first click of **Start**. That path costs a picker and a
sharing indicator, and what you choose in the picker matters:

- **Chrome/Chromium** preselects *This tab* — take it, and the mapping is exact.
- **Firefox** has no tab capture; the picker offers only windows and screens.
  Choose the **entire screen**.
- **A single window** cannot be used: it includes the tab and URL bars, and
  Wayland gives the page no window coordinates to subtract.

**The fallback needs the page fullscreen**, which is the one case it can map: a
CSS pixel lines up with a screen pixel only there, and only at 100% zoom, since
`innerWidth` scales with zoom and `screen.width` does not. Leave fullscreen and
the screenshots pause until you press <kbd>F11</kbd>. The exception is Chrome's
tab capture, which is the viewport by definition and lines up in a window too.
The server path has none of these limits.

Going without is fine: the frame still parks in its corner and still glides,
just unphotographed. That is what happens when you open the player from another
machine over the network — the server would be screenshotting its own screen
rather than yours, so the player does not ask it to, and a plain-HTTP origin has
no `getDisplayMedia` either. Whenever screenshots are off or misaligned, the
player says why in the bottom-left corner and logs it to the console.

## Adding animations

`#frameMover` carries only the motion transform (see `public/player/motion.js`).
`#frameSkin` inside it is the cosmetic layer, and the two video decks sit inside
that. New effects should nest a fresh wrapper rather than sharing a transform
with the bounce loop.

An animation is `start(context) => stop()`, where `context` is
`{ element, capture }` — `capture` may be null, and is read late, so an
animation must cope with it being absent. `stop()` has to undo everything:
timers, listeners, and any DOM it added, because the next animation takes over
the same element immediately. Register it under a new name in the `ANIMATIONS`
map in `motion.js` **and** in the one in `config.js` (which supplies the label
for the queue page's button), and it joins the rotation.

Anything time-driven should take its seconds from `tickSeconds(now, last)`
rather than from the raw timestamps, and any timer should divide its interval by
`speed`. That is what puts a new animation under the **Speed** switch — and the
clamp inside it is also what stops a hidden tab from teleporting the frame when
it comes back.

Selection uses a shuffle bag (`createPicker`): every animation comes up exactly
once per pass, so they stay evenly used, and the same one never runs twice in a
row — including across the seam between two passes.