subwave-nowplaying/README.md

# subwave-nowplaying

Tronbyt/Tidbyt app showing the current track on a [SUB/WAVE](https://github.com/perminder-klair/subwave)
internet radio station: album art with a slow zoom/pan reveal, and four
scrolling lines — track title/artist, a thin separator, the current show
and host, and local time/temperature — plus a track progress bar pinned to
the bottom of the 32px-wide text column.

Not tied to any specific station — set your own station's base URL in the
app's config when adding it to a device.

## Configuration

- **Station URL** — the base URL of your SUB/WAVE instance, e.g.
  `https://radio.example.com`. No trailing slash needed (stripped
  automatically). No auth required — SUB/WAVE's now-playing and cover-art
  endpoints are public.
- **Color scheme** — matches SUB/WAVE's own web UI theme picker (Blueprint,
  Classic Dark, Classic Light, Cyberpunk, Flare, Recon, Signal, Vinyl), so
  the device can mirror your station's own site theme. Defaults to
  Cyberpunk. Each theme sets a background fill, the main text color
  (title/artist/show), an accent (host/DJ name + progress bar fill), and a
  muted color for the time/temp line.
- **Album art zoom** — how far the album art's breathing zoom effect goes
  in: Off, Subtle, Moderate (default), or Strong.

## How it works

- `GET {station_url}/api/now-playing` — track title, artist, `subsonic_id`,
  DJ/station name, and stream online status. Cached 20s.
- `GET {station_url}/api/cover/{subsonic_id}` — album art JPEG, proxied by
  SUB/WAVE itself (no Navidrome credentials needed). Cached 300s.
- If the station reports `streamOnline: false`, shows "<station> offline"
  instead of stale track info.
- If a track has no `subsonic_id`, or the cover fetch fails, falls back to a
  full-width layout with no art rather than a broken image.
- Album art gets a slow "Ken Burns" breathing effect: starts and ends on
  the full frame, easing in to a zoomed-in crop at the midpoint, on a
  continuous ~10s round-trip loop (not a one-shot reveal — see below). How
  far it zooms in is configurable (Off/Subtle/Moderate/Strong — see
  Configuration). Rendered oversized (36x36 vs. the 32x32 display slot,
  just enough headroom for `animation.Scale` to have a visible effect)
  and scaled *up only*, from 1.0 to the configured zoom amount, never
  below 1.0 — scaling down was tried (to make the rest state reveal the
  literal whole image, or to re-center an off-center crop) and both
  attempts produced a real, on-device black border; see the fuller
  history below.

  `animation.Scale`'s pivot defaults to the *center* of the widget per
  pixlet's own docs, so Scale alone is already close to centered — just
  slightly skewed toward the top-left as the zoom amount increases.
  Paired with a computed `animation.Translate` to cancel out that residual
  skew. Combining Scale with Translate at all was tried once before and
  reverted after it produced a periodic rendering "pop" roughly every
  0.7-1s — confirmed with both WebP and GIF output, independent of zoom
  amplitude. Root-caused to pixlet's own source
  (`render/animation/translate.go`): a Translate's x/y are rounded to the
  nearest whole pixel every frame by default, while Scale is never
  rounded, so a continuously-changing Scale combined with a rounded
  Translate holds still for several frames and then jumps a whole pixel
  at once. Passing `rounding = "none"` on the `Transformation` disables
  that snapping and fixes it — confirmed via frame-by-frame pixel-delta
  analysis (the amount of change is now even from frame to frame, instead
  of long still stretches punctuated by sudden jumps). "strong" is capped
  at 1.45 (not pushed further): above ~1.46 the centered crop starts
  revealing a real black border in the bottom-right corner, verified with
  the calibration tool — this is the safe ceiling, not an arbitrary
  choice. Verified payload stays well under the 460KB device limit at
  every zoom level, measured against the live station (though with
  `rounding = "none"`, WebP can no longer dedupe as many near-identical
  frames, so payloads run larger than the "pop" version did — ~370KB
  vs. ~220-320KB — still comfortably within budget).

  **Why a loop, not a one-shot reveal, and why it's short:** a one-way
  version looked jumpy in practice — since the app re-renders on
  `NOWPLAYING_TTL` (and the server appears to refresh pushed apps well
  under a minute regardless), a fresh render restarts its own animation
  from tick 0, so whatever the *previous* pushed image happened to be
  showing gets abruptly replaced. A loop that starts and ends in the same
  state doesn't eliminate that restart, but makes it invisible — and
  keeping the round-trip short (~10s) makes it far more likely a full
  cycle plays before the next refresh cuts it off; a 60s version tried
  earlier only ever showed its outbound half. Considered using genuinely
  random per-frame content (like `ormakers-tronbyt`'s LoRa segment, which
  pre-crops 60 frames directly from a high-resolution *static* source
  photo via offline Pillow processing) for a sharper zoom, but that
  technique fundamentally doesn't transfer here: album art is fetched live
  per-track at render time, and pixlet's Starlark sandbox has no
  image-cropping/pixel-analysis capability at all (confirmed by checking
  its full module list) — so there's no way to pre-bake or crop-at-render-
  time from a source we don't have until the request hits the live
  station.
- Shows the current show name (`context.activeShow.name`) and host name
  (`dj.name`) below the artist, separated by a thin divider line, and the
  local time/temperature (`context.clock`/`context.weather`) below that —
  all as independently scrolling Marquee lines, confined to the 32px-wide
  text column next to the album art (no overlay/overlap with the art).

**Known quirk — occasional Marquee glitch:** pixlet's `render.Marquee` runs
its own internal scroll-cycle animation, and stacking several Marquees of
different text lengths in one Root can occasionally desync them — a line
may briefly blank or show a garbled frame while another Marquee is
mid-cycle. This is a real, currently-unfixed pixlet bug (there's an
open `FIXME: Behaviour when children themselves are animated is a bit
weird` in pixlet's own widget docs), not something introduced by this app.
It's also not unique to this app: `tronbyt/apps/apps/goosefm/goose_fm.star`
(an accepted official community app) stacks 3 Marquees for the same kind of
now-playing info and exhibits the identical occasional glitch on close
frame-by-frame inspection. In practice it's subtle enough to be easy to
miss on a real, continuously-scrolling display. A custom hand-rolled scroll
(via `animation.Transformation` + `Translate`, avoiding `Marquee` entirely)
was prototyped and could fully eliminate this, but only by consolidating
down to 2 scrolling lines total (title alone, everything else combined into
one line) — full independent lines for every field was preferred instead,
accepting the same minor quirk real Tidbyt/Tronbyt apps already ship with.

**Progress bar caveat:** the now-playing API reports track `duration` but
not elapsed/remaining time, so the app tracks playback position itself —
the first time it sees a given `subsonic_id`, it stamps the current
wall-clock time in pixlet's cache, then diffs against that stamp on later
renders to compute elapsed seconds. This only works correctly on a
long-lived server process with a persistent cache (true for
`tronbyt/server`'s in-memory cache) — a one-shot `pixlet render` CLI
invocation always starts a fresh process with an empty cache, so the bar
will always show 0% when testing locally. This is expected, not a bug.

**Note on JSON nulls:** SUB/WAVE sends some fields as explicit JSON `null`
(e.g. `context.activeShow` between scheduled shows) rather than omitting the
key. Starlark's `dict.get(key, default)` only falls back to `default` when
the key is *absent* — an explicit `null` (decoded to Starlark `None`) passes
straight through, so this app also does `... or {}` on those lookups before
chaining another `.get()` off them, to avoid a `NoneType has no .get` crash.

**Note on unreachable hosts:** if the station URL is wrong or the host is
completely unreachable (DNS failure, connection refused), `http.get` raises
a hard error and the render fails for that cycle — this is normal pixlet/
Starlark behavior (no try/except in the language), not a bug. Only
HTTP-level errors (404, 500, etc.) are something the app can catch and
display a message for, which it does.

## Dev/test

```
pixlet render subwave_nowplaying.star station_url=https://your-station.example.com
pixlet serve subwave_nowplaying.star
```

`dev/calibration_test.star` renders a synthetic test grid (`dev/calibration.png`
— concentric color rings, corner markers, a crosshair) through the exact same
zoom logic as the real `_art_widget`, to visually check cropping without
depending on what a live station happens to be playing:

```
pixlet render dev/calibration_test.star -o calib.gif
```

At rest you should see a thin, *even* sliver of magenta on all four edges;
as it zooms in, each corner marker should get clipped by the same amount.
If one side loses noticeably more than the others, the zoom isn't
centered. Keep its copy of the zoom math in sync with `_art_widget` if that
ever changes.

## Deploy

Upload `subwave_nowplaying.star` directly through your Tronbyt server's
"Upload app" button on a device's Add App page (single file, no bundled
assets — no need to zip). Configure the Station URL field when adding it to
your device's schedule.