feedBack/plugins/highway_3d/CLAUDE.md

3D Highway Plugin — AI Maintainer Guide

This guide tells future AI assistants where each visual element lives in screen.js, what controls it, and the gotchas to watch for. The goal is for small polishes (color tweaks, sizing, animation timing, add/remove a label) to land in the right place on the first try without grep spelunking.

The whole renderer is one file — screen.js, wrapped in an IIFE, registered as window.feedBackViz_highway_3d (a feedBack#36 setRenderer factory). No imports beyond Three.js loaded from the vendored /static/vendor/three/three.module.min.js (pinned r170; swapped from CDN when bundled into core).

Styling (feedBack styles capability). This plugin owns its Tailwind CSS: it ships assets/plugin.css and declares "styles": "assets/plugin.css" in plugin.json, so core's prebuilt static/tailwind.min.css no longer scans it (it's excluded from core's content globs). The frontend injects assets/plugin.css as a <link> when the renderer activates. This is the one maintainer-time build step: after you add/change a Tailwind class in screen.js or settings.html, run bash build-tailwind.sh (pinned tailwindcss@3.4.19, corePlugins.preflight=false — utilities only) and bump the version in plugin.json so the injected <link>'s ?v= cache-buster fetches the fresh file. The generated assets/plugin.css is committed; end users never build. See docs/plugin-styles.md.

Navigation note: This guide references functions by name and uses the existing banner comments (/* ── Scene initialisation ─ */, etc.) as section anchors. Line numbers are deliberately avoided so this stays correct as the file evolves. Use Grep for the function name or banner text to jump to a section.

File structure at a glance

The file is laid out top-to-bottom as:

1. Constants block — palette (S_COL), scale (SCALE, K), fret/string counts, geometry sizes, camera, fog
2. Pure helpers — fretX, fretMid, dZ, computeBPM
3. Three.js loader — loadThree() (loads vendored /static/vendor/three/three.module.min.js, memoized)
4. Splitscreen helpers — _ssActive, _ssIsCanvasFocused (read window.feedBackSplitscreen)
5. createFactory() — the rest of the file is one big closure
   • Per-instance state (Three.js refs, pools, camera state, lifecycle flags)
   • txtMat() text-sprite cache, pool() factory
   • drawChordDiagram() — 2D canvas chord diagram (top-left overlay)
   • drawLyrics() — 2D canvas lyrics renderer (top centre)
   • initScene() — one-time WebGL setup: scene, camera, lights, materials, pools
   • buildBoard() — static fretboard geometry: strings, fret wires, fret dots, board plane
   • updateStringHighlights() — per-frame string emissive glow + opacity
   • update(bundle) — the big per-frame function: notes, chords, beats, lane, fret labels
   • drawNote() — single note: outline, body, sustain, drop line, technique labels, projection
   • camUpdate() — smooth camera lerp + self-correcting NDC look-at
   • applySize() — DPR + canvas size + aspect clamping
   • teardown() — dispose all GPU resources + reset state
   • canvasSize() — resilient canvas-dimension lookup
   • Returned API — init / draw / resize / destroy (setRenderer contract)

Coordinate system

• +X runs along the fretboard (low frets → high frets, fretX(f) and fretMid(f)).
• +Y is up (string Y is sY(s), low strings have lower Y when not inverted).
• +Z is toward the camera. Notes spawn at negative Z and approach Z=0 (the hit line). Past notes would be at positive Z, but noteZ is clamped via Math.min(0, dZ(dt)) in drawNote() so they stop at the string plane.
• Camera sits at roughly (curX + 20*K, h*0.95, dist*0.75) — positive Z, slightly above and behind the play line, looking toward (curX, curLookY, -FOCUS_D * 0.35).

dZ(dt) = -dt * TS — the closer to "now," the closer to Z=0. TS = 200*K is the world-units-per-second scroll rate.

The K scale and why everything is multiplied by it

SCALE = 2.25, K = SCALE / 300 ≈ 0.0075. Almost every world-space dimension is expressed as N * K so the whole scene scales as one unit. Tweaking SCALE alone resizes the entire highway. If you change a literal world dimension, write it as N * K to keep it consistent — naked numeric literals in Three.js geometry creation calls (e.g. inside BoxGeometry) are an obvious smell.

Concrete sizes (search the constants block for the names):

Const	Value (world units)	Meaning
STR_THICK	0.25 * K	String thickness
S_BASE / S_GAP	3 * K / 4 * K	Lowest-string Y / inter-string gap
NW, NH, ND	5 * K, 3 * K, 0.5 * K	Note width / height / depth
TS	200 * K	Scroll speed (world units per second)
AHEAD / BEHIND	3.0 / 0.5	Seconds visible ahead / behind hit line
CAM_DIST_BASE / CAM_H_BASE	240 * K / 150 * K	Reference camera distance / height
FOG_START / FOG_END	200 * K / 670 * K	Fog kicks in past hit line, swallows by the horizon

"I want to change X" — quick lookup

Each entry names the function or banner you should grep for, plus key sub-blocks (also marked with banner comments inside the function).

Strings

• String colors → S_COL array in the top-level constants block. Eight-element vibrant palette; index s is the string (0 = high E for guitar). MAX_RENDER_STRINGS keys off S_COL.length.
• String count for the active arrangement → resolveStringCount(bundle) (top-level helper). Reads bundle.stringCount (feedBack#93) with a bass-name fallback. Don't reintroduce tuning.length — see Pitfall #4.
• String thickness / gap / base Y → STR_THICK, S_BASE, S_GAP constants.
• String-to-Y mapping (respects invert) → the sY(s) arrow function inside createFactory(). Single source of truth for "where on Y is string s."
• Static string mesh creation → buildBoard(), the // Thin Line strings (glow layer) and // BoxGeometry strings — emissive glow ... comment blocks. Two layers: low-opacity Line for soft glow, BoxGeometry mesh per string with its own material clone (kept in stringLines[] for live emissive updates).
• Live string glow / pulse → updateStringHighlights(noteState). Tunables: BASE_GLOW, MAX_GLOW, IDLE_OP. Driven by noteState.stringSustain and noteState.stringAnticipation.

Fretboard

• Fret count → NFRETS constant. Increasing requires nothing else.
• Fret X positioning → fretX(f) and fretMid(f) (top-level helpers). Logarithmic guitar-fret spacing within SCALE.
• Fretboard plane / fret wires / fret dots → buildBoard(), separate banner-style comment blocks (// Fret wires, // Fret dots). The dark background plane is the first thing built; main fret wires use 0xbbbbff / opacity 0.8, minor wires 0x666688 / opacity 0.4. Single/double dots: DOTS array + DDOTS set in the constants block.
• Fret-row label colors / sizing (the heat-coloured row of fret numbers below the board) → update(), // ── Dynamic fret number row ── block. Active = #ffe84d, inactive = #9ab8cc, opacity / scale driven by noteState.fretHeat[f]. Text rendering (font, outline, shadow) is governed by the 'fretRow' preset in TXT_STYLES — see "Tweaking text-sprite styling".
• Active-fret cooldown → FRET_COOLDOWN constant. How long after the last note in a fret it stays in the active set.

Notes

• Single-note rendering → drawNote(). Handles outline, core body, open-string variant, sustain trail, lane drop line, all technique labels, fret connector label, and the board projection. Each visual block has its own banner comment (// ── Outline ──, // ── Core (filled note body) ──, // ── Sustain trail ──, // ── Lane drop line ──, // ── Technique labels ──, // ── Per-note fret connector label ──, // ── Board projection ──).
• Note geometry / size → gNote = new T.BoxGeometry(NW, NH, ND) in initScene(). Per-note scale tweaks happen inside drawNote().
• Note approach rotation (vertical → horizontal) → search approachRot inside drawNote(). Maps dt / AHEAD to [0, π/2]. Open strings skip the rotation.
• Note color → mStr[s] (idle) / mGlow[s] (hit), built in initScene(). Hit material is white-with-emissive, idle is dim emissive of the string color.
• Sustain trail → // ── Sustain trail ── block in drawNote(). Geometry: scaled gSus (BoxGeometry(1,1,1)). Width NW * 0.85, height NH * 0.12. Outline mesh + colored core mesh.
• Lane drop line → // ── Lane drop line ── block in drawNote(). Vertical line from each upcoming note down to the fretboard plane in the string's color.
• Per-note fret connector label → // ── Per-note fret connector label ── block in drawNote(). Number below the board with a thin line up to the note. Be careful with replace_all on the 0.5 and 0.4 floats in the alpha formula — they're separate constants. Uses the 'noteFret' preset in TXT_STYLES (also applied to the on-body fret number when showFretOnNote is enabled).
• Technique markers (bend, slide, hammer/pull/tap, accent, tremolo, palm-mute, pinch harmonic) → // ── Technique labels ── block in drawNote(). Most are small if-blocks using txtMat(text, color, wide, style) (cached sprite material; 'technique' preset in TXT_STYLES). Exceptions: a bend draws a string-coloured chevron strength stack (bendChevronMat, one chevron per half-step), and hammer-on / pull-off draw a white ▲/▼ triangle with a string-coloured border (triMat) — both pinned to the gem; the bend ribbon's up→hold→down contour is driven by bendSemisAtTime.
• Open-string note → special-cased throughout drawNote(): n.f === 0. Wider/flatter geometry, "0" label sprite, uses openX (the chord's open-string centroid) when supplied.
• Board projection ("ghost" preview) → // ── Board projection ── block in drawNote(). Two meshes per string (projMeshArr, projGlowArr), one visible per frame for the next note. Linger window PROJ_WIN. Gated on the projectionVisible setting (BG_DEFAULTS / h3dBgSetProjectionVisible / the "Show note preview on the fretboard" checkbox in settings.html) — when off, the block is skipped and update()'s per-frame m.visible = false reset leaves the ghost hidden. The glow has renderOrder = -1 which fights the strings — see Pitfall #6.
• Note-hit "sizzle" (feedBack#254) → drawNotedetectSizzle() (called from the lyricsCtx block in draw(), just before drawNotedetectLabels()). For each confirmed hit/active note (_ndGood in drawNote() pushes {x, y, z, s, alpha, color} onto the per-frame _ndSizzle array — alpha is the provider's clamped fade, color an optional palette override), it projects the note's world point through the up-to-date cam, sizes the burst from a fretboard-X-axis offset projection (reliable even when the note's rotated flat at the line), and twinkles a few short crackling ellipse-arc segments + tiny dots hugging the note's rectangle — re-randomised every frame, contained to ≲1.4× the note, half white / half the string colour (or the provider's color when given). Every dot/arc's globalAlpha and shadowBlur are scaled by the entry's alpha, and the per-element "off-this-frame" probability rises as alpha decays, so a struck-note glow visibly thins and fades. Also: _ndGood swaps the note's outline to mGlow[s] (bright string-tinted, not green). Knobs are inline: arc/dot count, base on-probability, line widths, shadowBlur, spread radii. Lives entirely on the 2D overlay layer — no Three.js geometry/disposal.

Chords

• Chord rendering loop → update(), // ── Chords ── block. Iterates bundle.chords, calls drawNote() per chord-note, then draws the frame box, name label, and barre indicator.
• Chord linger after hit → the 0.55-second value passed as the linger arg to drawNote() from inside the chord loop, and used in the chord-frame Z clamp + opacity formulas.
• Chord frame-box (rectangle around frets in the chord) → inside the chord loop, search for the drawEdge helper. Four edges + a low-opacity fill. isRepeat halves the height + dims it.
• Chord name label (gold) → in the same chord loop, search chordName. Cached via txtMat(chordName, '#e8d080', true). Anchored above the chord box.
• Barre indicator (white vertical line at the barre fret during linger) → in the chord loop, gated on /barre/i.test(chordName) && chDt <= 0. Position is fretMid(bFret) where bFret is the lowest fretted string.
• Repeat-chord detection → prevChordSig / prevChordTime inside the chord loop. Same shape within 0.5 s → isRepeat = true (suppresses note bodies, dims frame).
• Chord diagram (top-left 2D overlay) → drawChordDiagram(), called from the lyricsCtx block at the bottom of the returned draw(). The chord-to-display is selected in update() under // ── Chord diagram: track most recently hit chord ── and stashed in _diagChord (most recently hit named chord within the 0.55 s linger window).

Camera

• Reference values → CAM_H_BASE, CAM_DIST_BASE, REF_ASPECT, FOCUS_D, CAM_LERP_BASE in the constants block.
• Smooth lerp + look-at → camUpdate(). BPM-scaled lerp speed (CAM_LERP_BASE * bpm/120).
• Self-correcting framing → bottom half of camUpdate(). Projects the fretboard mid-Y to NDC, nudges tgtLookY until that point sits at NDC Y ≈ DESIRED_NDC_Y (lower third of frame). This is what lets the camera adapt automatically to ultra-wide split-screen panels.
• Aspect compensation → aspectScale = Math.max(1, REF_ASPECT / Math.max(cam.aspect, 0.5)) in applySize(). Clamped to ≥ 1 so wide panels keep baseline depth (don't dolly in flat). Removing the Math.max(1, …) is the bug we already fixed; don't reintroduce it.

Beats and sections

• Beat lines (downbeats highlighted) → update(), // ── Beat lines ── block. mBeatM (full opacity 0.25) for measure starts, mBeatQ (0.07) for other beats.
• Section labels → update(), // ── Section labels ── block. Cyan (#00cccc) sprite at fret 12, above the highest string.

Scene colors (two independent axes: Background + Highway)

• Scene-color themes → BG_THEMES table near the top of createFactory(). One combined table is the single source of truth, but it drives two independent axes that share the same id-set:
  • Background axis — setting key bgTheme, setter window.h3dBgSetBgTheme, state bgThemeId. Owns clear (WebGL clear color) + fog.
  • Highway axis — setting key hwTheme, setter window.h3dBgSetHwTheme, state hwThemeId. Owns board (fretboard/highway-surface plane) + optional lane/laneDim (the lit lane strip). Any background id can mix with any highway id; picking the same id in both gives the original "matched" look. Per-axis accessors are _bgBackgroundColors(id) / _bgHighwayColors(id) (both alias _bgThemeColors). Both axes default to 'default' (byte-identical to the original look).
• Applying a theme → _applyBgTheme(). Background half sets clear+fog from bgThemeId (skipped under the venue scene); highway half sets the board plane + lane materials (mLaneOdd/mLaneEven) from hwThemeId. Re-run on init, buildBoard(), and the settings listener (which fires for both bgTheme and hwTheme), so changing either dropdown retints only its half live.
• Backward-compat migration → _bgLoadSettings(): the first time it loads with no stored hwTheme (_bgHasStored false), it seeds hwThemeId from bgThemeId and persists hwTheme once (a one-time backfill, written without _bgEmitChange). So a pre-split single-bgTheme pick is byte-identical right after the upgrade, and from then on the two axes are fully independent — changing the Background dropdown never drags the Highway surface, and the settings UI's Highway value can't disagree with what's rendered. settings.html shows the same first-load value via storedHwTheme == null ? bgTheme : coerceHwTheme(...).
• Adding/removing a theme → edit BG_THEMES (the colors) AND settings.html's SCENE_THEMES array (the {id,label} list — the single source the two dropdowns' <option>s and the VALID_BG_THEMES validator are both generated from). Keep the two id-sets aligned.

Highway lane (the highlighted strip under active frets)

• Lane drawing → update(), // ── Dynamic highway lane ── block. pLane is a single quad on the fretboard plane; pLaneDivider is thin vertical lines at each fret inside the lane. Width keys off the active-fret range; min width ≈ 4 frets.
• Lane intensity → highwayIntensity accumulated from upcoming notes (further notes dim it, near notes light it).
• Lane color → the lit quad color is mLaneOdd.color (stock HWY_LANE_STRIPE_ODD_HEX = 0x103B5C), the dimmer alternating row mLaneEven.color (HWY_LANE_STRIPE_EVEN_HEX = 0x08283C). These are now theme-aware: _applyBgTheme() recolors them from the active HIGHWAY theme's optional lane/laneDim fields, falling back to the stock hexes when a highway theme omits them. (_laneTargetColor, set in initScene(), is kept in sync with the lit color but has no live consumer today.)

Lyrics & overlays

• Lyrics overlay → drawLyrics(). 2D canvas, top centre, semi-transparent rounded background, syllable-level highlighting (current syllable in white, played in muted, upcoming in dim).
• Chord diagram overlay → drawChordDiagram() (see "Chords" above). 2D canvas, top-left, fades over the 0.55 s linger window. Respects inverted (column 0 is high-e when inverted, low-E otherwise).
• The lyricsCanvas is created in initScene() with z-index:1, appended to wrap after ren.domElement — this is the empirically-correct stacking order for all browsers/contexts (including splitscreen panels with position:relative; overflow:hidden). Don't reorder; see Pitfall #5.

Splitscreen

• Focus dim → _isFocused flag, manipulated by _updateFocusState(). Fades ambient + directional light intensity in non-focused panels.
• Per-panel resize fallback → search _lastHwW in the returned draw(). The renderer self-detects when the highway canvas backing-store dimensions change and re-runs applySize(). Needed because the splitscreen plugin overrides hw.resize and never calls renderer.resize().
• Reduced DPR in split → applySize() clamps DPR to 1.25 when splitscreen is active vs 2 otherwise (search baseDPR). Keeps four-panel quad layout from melting GPUs.

Splitscreen panel controls/settings

• Per-panel background overrides use localStorage keys shaped as h3d_bg_panel<N>_<key>. When present, they override the global h3d_bg_<key> value for panel N; when absent, the global value still applies.
• Keep per-panel keys to BG_DEFAULTS entries that _bgLoadSettings() reads. Do not add panel-only keys outside that load path.
• panelControls is a static, host-readable, curated descriptor list for controls a host can expose per panel. It documents the supported per-panel surface; the renderer still loads values through _bgLoadSettings().
• Asset/background image keys remain global-only. Do not make uploaded or selected asset references panel-scoped unless that contract is explicitly widened.
• Host refresh nudges that call toggle setters must pass real booleans, not strings such as 'false', so setters can distinguish true from false.

The bundle object

Every per-frame renderer call receives a bundle from feedBack core. Fields used by this plugin:

• currentTime — playback time in seconds (drives dt for everything)
• notes, chords, beats, sections — chart arrays (already difficulty-filtered by core)
• chordTemplates — array indexed by ch.id; each { name, frets: [N] }
• lyrics — syllable array [{ w, t, d }, …]
• inverted — display flag honored via sY(s) (low-string-on-top vs the default low-string-on-bottom)
• lyricsVisible — gate for lyrics overlay
• renderScale — pixel-ratio multiplier from the user's quality setting
• songInfo.arrangement — only field of songInfo this plugin reads, used as the bass-name fallback in resolveStringCount()
• stringCount — feedBack#93; always prefer this over deriving from tuning/arrangement
• lefty — display flag consumed by this renderer from bundle.lefty. Captured into _leftyCached before each frame so xFret(), xFretMid(), boardSpanX(), board geometry, note placement, and the camera shoulder offset mirror the fret axis for left-handed mode. A runtime lefty flip rebuilds board state and mirrors curX/tgtX plus the lookahead camera X cache so the camera does not drift across the neck.
• getNoteState(note, chartTime) — feedBack#254; per-note judgment from a scorer (note_detect). Captured each frame into _ndGetNoteState at the top of update() and consulted in drawNote() AFTER the event-driven _ndHitMarks/_ndMissMarks lookup AND over the proximity-based hit heuristic, both of which it overrides when it has a verdict: 'hit'/'active' → mGlow[s] outline (bright string-tinted, not green) + mGlow[s] body + mGlow[s] sustain trail + a queue entry for drawNotedetectSizzle (so a held sustain keeps glowing/sparkling as long as the provider keeps returning 'active'); 'miss' → mMissOutline and _showHit = false (suppresses the bright body even if the note is near the line). Called with the note's chart time (n.t), which is how note_detect keys its noteResults map — not now. Returns null on cores without the API or songs with no scorer — then the event path / hit heuristic drive feedback for older note_detect builds. notedetect ≥1.13 object verdicts additionally carry { points, mult, popKey } (game-scoring layer): points is the note's awarded score, mult the multiplier tier it landed at, and popKey a dedup key — chord members all return the chord-level judgment's key so a chord pops once, not once per gem. Consumed by the score-pop spawn in drawNote() (see Score FX below); all three are absent on older notedetect builds, so guard with !== undefined.

tuning and capo aren't consumed by this plugin.

Core reuses the bundle OBJECT across frames (mutated in place); never cache it or compare its identity between frames — field values are only valid for the current draw call. Field-identity caches on ARRAY fields (this plugin's _mergeCacheChordsRef === bundle.chords etc.) remain valid: arrays still swap reference when chart data changes. Core also exposes bundle.lowerBoundT(arr, time) (lower-bound on .t, notes/chords) and bundle.lowerBoundTime(arr, time) (on .time, beats/anchors/sections) — prefer these over the local lowerBoundT helper when a downlevel-host fallback isn't needed.

Score FX (notedetect game-scoring layer)

• "+N" score pops → _fxSpawnPop() from drawNote() (just after the provider verdict-override block), drawn by drawScoreFx() (called from the lyricsCtx block in draw(), right after drawNotedetectLabels()). Fixed 24-slot pool (_fxPops), deduped per popKey via the TTL'd _fxSeen map (pruned in drawScoreFx). Pops rise/fade over 700 ms; font size scales with the multiplier tier.
• Session FX → notedetect:fx events ({ fxType: 'multiplier'|'milestone'|'streakBreak', ... }). notedetect dispatches each detail object twice in the same task: on window (unscoped, first) and as a bubbling CustomEvent from its per-panel instanceRoot (scoped, second). The listener (_fxOnFx, bound with the other notedetect listeners) treats element-targeted copies as authoritative — accepted only when their root lives in this panel's container — and defers the window copy by a task (setTimeout 0): if the element copy (same detail reference) arrived meanwhile it's dropped as a duplicate, otherwise it's the compat fallback for a detector whose root isn't in the DOM. This keeps splitscreen panels from rendering each other's FX even for the first event of a session. Effects: milestone → particle burst from a 4-slot Float32Array pool (_fxBursts), multiplier tier-up → expanding ring pulse at the strike-line centre, streak break → brief red wash.
• Skin palette → _fxResolvePalette() reads localStorage['feedBack_notedetect_skin'] (neon/esports/metal → _FX_PALETTES) at listener-bind time and on the notedetect:skin bus event. The display fonts are document-loaded by notedetect's stylesheet, so the overlay canvas can reference the family names directly.
• Everything lives on the 2D overlay layer — no Three.js geometry, no txtMat() cache traffic, nothing to dispose; teardown() deactivates the pools and removes both listeners.
• This block is the reference implementation for other renderer plugins (drum highway, piano, custom highways) that want score pops / session FX: copy the _fxOnFx dedup+scoping listener, the popKey-keyed seen-map (cleared on backward seek), and the _FX_PALETTES skin mapping. The full consumer contract (events, payloads, provider verdict fields, theming variables) is documented in feedBack-plugin-notedetect's CLAUDE.md.

If you need a bundle field that isn't here yet, check _makeBundle() in static/highway.js in the feedBack core repo — this is the plugin repo, static/highway.js is not here. The full path in the parent feedBack checkout is feedBack/static/highway.js.

Per-string state arrays

Several frame-local arrays are sized to nStr:

const noteState = {
    stringSustain:    new Array(nStr).fill(false),
    stringAnticipation: new Array(nStr).fill(0),
    fretHeat:         new Array(NFRETS + 1).fill(0),
    strGlow:          new Array(nStr).fill(0.5),
};

Anything that indexes a per-string array MUST be guarded by validString(s). The function checks that s is an integer in [0, nStr) (returning false otherwise so the caller can skip), warns once when an out-of-range index is seen, and keeps the mStr / mGlow / mSus / projMeshArr lookups safe. It does NOT clamp — out-of-range strings are dropped, not silently mapped to a valid one. filterValidNotes(notes) is the chord-note equivalent (allocates only when something would actually be dropped).

Object pools

Pools live as closure refs (pNote, pSus, pLbl, pBeat, pSec, pFretLbl, pLane, pLaneDivider, pChordBox, pChordLbl, pBarreLine, pNoteFretLabel, pConnectorLine, pDropLine, pSusOutline).

The pool factory pool(parent, mk) returns { get(), reset() }. Every pool MUST be .reset()-ed at the top of update() — otherwise objects from the previous frame stay visible. When you add a new pool, add the reset call too. Search for the existing block of .reset() calls at the top of update() to find where to add yours.

If a pool's mesh has per-instance state (its own material clone, its own texture map), set those fields each get() call so a recycled instance picks up the right values. The "first context wins" trap is real — recycled sprites that retain a stale material.map from a previous frame won't repaint. The chord-name label loops on this (search lbl.material.map !== mat.map) by checking before swapping.

Key gotchas / pitfalls

1. Adding a new pool? Reset it. The reset block at the top of update() is easy to miss when adding a new pool elsewhere.
2. txtMat() is cache-keyed by (style, text, color, wide). Calling it with a numeric text works (it's coerced via String(...)), but new label content creates a new texture forever. Don't generate dynamic per-frame text (e.g. interpolated values) through txtMat() or you'll leak GPU memory. For static labels that change occasionally (chord names, fret numbers), the cache is fine. The style arg picks a preset from the TXT_STYLES table — see "Tweaking text-sprite styling" below.
3. Disposal in teardown() matters. Three.js doesn't garbage-collect GPU resources. Every material.dispose(), geometry.dispose(), map.dispose(), and ren.dispose() call there is load-bearing. teardown() is called from init() (when re-initing), destroy() (setRenderer swap or highway.stop()), and on init failure.
4. Don't use tuning.length for string count. bundle.tuning (and arr.tuning server-side) is always 6 elements even for bass — feedBack pre-fills the array with zeros for unused strings. Use bundle.stringCount (feedBack#93), with /bass/i.test(arrangement) as the only acceptable fallback. There's a comment in resolveStringCount() documenting this.
5. lyricsCanvas DOM order. The 2D overlay canvas is appended to wrap AFTER ren.domElement and given z-index:1. This is the empirically-correct order — earlier versions had it before the WebGL canvas, which broke in splitscreen panels with position:relative; overflow:hidden. Don't reorder without testing both modes.
6. Projection glow renderOrder = -1 in initScene(). This is a known-suboptimal setting — it forces the glow to draw before the strings in the transparent queue, so the string visibly cuts through the preview. Removing the line lets natural Z-sort layer it correctly. Plus the projection's world-Y matches the string Y, which after perspective projection puts the preview slightly screen-lower than the string; bumping projY = y + NH * 0.4 recenters it. (Both fixes live on the fix/preview-stacking branch.)
7. renderOrder on transparent objects is sticky. Three.js sorts the transparent queue by renderOrder first, then back-to-front. A stray m.renderOrder = -1 on something will pull it under everything regardless of Z. When in doubt, leave renderOrder at the default 0 and rely on Z position.
   • Corollary: depthTest: false alone does NOT make a sprite "always on top." It removes the sprite from depth-buffer comparison, but draw order in the transparent queue is still determined by renderOrder then Z. Anything rendered after a depthTest: false sprite will still overdraw it. For HUD-style overlays that must always be visible (fret-row labels — issue #35, technique callouts), set renderOrder = 1000 AND keep depthTest: false. Both knobs together is the contract; either alone leaves the door open to occlusion.
8. ch.id may be missing. Some chord events lack an id (or it doesn't index into chordTemplates). Always optional-chain: bundle.chordTemplates?.[ch.id]?.name. The chord diagram + name label both gate on a non-empty result.
9. The aspectScale clamp (Math.max(1, …)). Without it, ultra-wide split-screen panels (top/bottom layout, ~5:1 aspect) yield aspectScale ≈ 0.33, which dollies the camera way in and kills highway depth. The clamp keeps wide panels at baseline depth and only allows narrow panels to dolly the camera back.
10. The _oobStringWarned flag is reset on nStr change in the returned draw() — switching from guitar (6) to bass (4) re-arms the warning so a malformed bass chart still gets logged.
11. renderOrder values for the lane and dividers are explicit in update() (lane.renderOrder = 1, div.renderOrder = 2). The lane plane needs to draw above the static fretboard plane (which has no renderOrder), and dividers need to draw above the lane.

Tweaking colors safely

The eight-color palette S_COL is the single source of truth for per-string color. Don't hardcode hex values inside drawNote() or update() — every per-string color reference is either an entry in S_COL or one of the per-string material arrays (mStr, mGlow, mSus, mProj, mProjGlow) built from it.

If a planned color-palette feature lands (issue #10), expect it to swap the palette source array but keep this single-array indirection. Anything that hardcodes color today will break that swap; flag it during review.

Non-string colors (the stock lane hexes HWY_LANE_STRIPE_ODD_HEX/_EVEN_HEX — now overridable per Highway theme, see "Scene colors" above; fret-row label colors #ffe84d / #9ab8cc, fret-dot color 0x556677, lyrics box rgba, chord-name gold #e8d080, etc.) are scattered as literals — that's intentional for now, since they're scene-wide accents rather than per-string. Pulling them into named constants is fine if you're already in that area.

Tweaking text-sprite styling

Every text label in the 3D scene is rasterised by txtMat(text, color, wide, style) and the look (font, outline, drop-shadow, source-canvas resolution) is driven by a preset in the TXT_STYLES table at the top of createFactory(). Do not edit the body of txtMat() to change a single label class — change the relevant preset entry instead, so the rest stay unaffected.

Current presets and their callers:

Preset	Used by	Default look
fretRow	Fret-number row under the board (update(), fret-row block)	Arial Black 900, 256px source canvas, 18px dark outline + soft drop-shadow — designed to pop against any background
noteFret	Per-note connector numbers + on-body fret label (drawNote())	Same heavy treatment as fretRow
chord	3D chord-name labels above chord boxes	bold sans, 128px source, 6px outline (lighter so the gold reads)
section	Section banners ("Verse", "Chorus") at fret 12	bold sans, 128px source, 6px outline
technique	Bend / slide / H / P / T / PH / PM / accent / tremolo / open-string overlay	bold sans, 128px source, 6px outline
open	The "0" label on open-string note bodies	bold sans, 128px source, 6px outline

Style fields:

• font / wideFont — full CSS font shorthand (weight + size + family); wideFont is used when wide=true (long-aspect labels: chord names, section names, "↑1/2", "~~~"). Keep both in sync if you change weight or family.
• srcH — source-canvas height in px. Wide labels use srcH * 4 for width. Larger srcH keeps glyph strokes crisp after bilinear downsampling onto small sprites — bumping it from 128 → 256 was the difference between thin-and-blurry and crisp on the fret-number presets. Keep srcH power-of-two (128, 256, 512, …): WebGL1 and Three.js silently disable mipmap generation on NPOT textures and fall back to a non-mipmap min-filter, which causes shimmer/aliasing on labels far down the highway. The 4× width derivation preserves POT-ness too (e.g. 256 → 1024 wide).
• stroke / strokeW — outline color and line-width in source-canvas px. Set stroke: null or strokeW: 0 to skip the outline (faster cache rasterisation, no contrast halo).
• shadow — { color, blur, dx, dy } or null. Drawn via canvas 2D shadowColor / shadowBlur / shadowOffsetX/Y before the stroke and fill, so it haloes the whole glyph.

Cache key includes the preset name (style|wide|text|color), so two presets with otherwise-identical text produce two distinct cached materials. Adding a new preset is safe — just add the entry to TXT_STYLES and pass its name as the 4th arg at the call site. Forgetting to pass style falls back to 'technique' (the broadest, most generic preset) and is the right default for a brand-new label class.

Don't generate per-frame distinct text through txtMat() (e.g. interpolated values, tick counters). The cache is unbounded and will leak GPU memory across the session — see Pitfall #2.

Lifecycle (setRenderer contract)

Per feedBack#36, the factory returns { init, draw, resize, destroy }:

• init(canvas, bundle) tears down any prior state, sets highwayCanvas, lazily loads Three.js, runs initScene(), calls applySize() (with a retrySize rAF loop fallback if the canvas isn't laid out yet).
• draw(bundle) is gated on _isReady. Re-resolves nStr / inverted / renderScale, then update(bundle) → camUpdate(bundle) → ren.render → 2D overlays. The _lastHwW/_lastHwH check at the top auto-resizes when the splitscreen plugin bypasses resize().
• resize(w, h) is gated on _isReady. Just calls applySize().
• destroy() is idempotent. Sets flags, runs teardown(), drops highwayCanvas. Tolerates being called on an instance that's been destroyed and re-init'd already (resets _lastHwW/H, _diagChord, etc.).

The factory returns a fresh instance per call, so splitscreen's per-panel setRenderer(feedBackViz_highway_3d()) gets independent state per panel — important because the chord diagram, projection meshes, etc. are all per-instance.

Branching / PR conventions

• Feature branches off main, descriptive name (e.g. fix/preview-stacking, feat/palette-picker).
• PR target: target the contributor's own fork by default unless they ask otherwise; confirm before opening a PR upstream. Run git remote -v in this directory to see the remotes that are configured locally.
• Commit messages: short imperative subject, optional body explaining why. Don't summarize the diff — the diff already does that.
• This plugin is bundled in-tree at plugins/highway_3d/ inside the got-feedback/feedBack repository (not a gitlink/submodule). It ships with the default container image. Changes go through the normal feedBack PR process — no separate upstream repo to sync.

When in doubt

• screen.js is one file — Grep for the function name or banner text before guessing.
• The constants block at the top is intentionally exhaustive; scan it before introducing a new magic number.
• If a "polish" feels like it should be one or two lines but stretches into restructuring, double-check whether a per-frame state field, pool reset, or validString() guard already covers your case.