feat(align): warped MIDI rendering and peaks

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/align/peaks.ts

@@ -0,0 +1,14 @@
+/** Per-bucket min/max of a waveform, for drawing. Pure. */
+export function peaks(x: Float32Array, buckets: number): { min: Float32Array; max: Float32Array } {
+  const min = new Float32Array(buckets), max = new Float32Array(buckets);
+  const n = x.length;
+  for (let b = 0; b < buckets; b++) {
+    const from = Math.floor((b * n) / buckets);
+    const to = Math.max(from + 1, Math.floor(((b + 1) * n) / buckets));
+    let lo = Infinity, hi = -Infinity;
+    for (let i = from; i < to && i < n; i++) { const v = x[i]!; if (v < lo) lo = v; if (v > hi) hi = v; }
+    min[b] = lo === Infinity ? 0 : lo;
+    max[b] = hi === -Infinity ? 0 : hi;
+  }
+  return { min, max };
+}

src/align/render.ts

@@ -0,0 +1,48 @@
+import * as at from "@coderline/alphatab";
+import type { Audio } from "../core/types.ts";
+import { renderMidiFile } from "../audio/render.ts";
+import type { RefTrack } from "./smf.ts";
+import type { Warp } from "./warp.ts";
+
+const TPS = 1920; // division 960 at a fixed 120 bpm
+const PEAK = Math.pow(10, -1 / 20);
+const MELODIC_CHANNELS = [0, 1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15];
+
+/**
+ * Render reference tracks to audio with `warp` applied (MIDI time -> audio time). Notes are placed on a fixed-tempo
+ * tick grid so alphaSynth output time equals audio time. Peak-normalised to -1 dBFS; `durationSec` pads/trims.
+ * Pure: no I/O. Notes landing before audio t=0 are clamped to 0 (dropped if they end before it).
+ */
+export function renderMidi(
+  tracks: RefTrack[], warp: Warp, sf2: Uint8Array,
+  opts: { sampleRate?: number; mono?: boolean; durationSec?: number } = {},
+): Audio {
+  const sampleRate = opts.sampleRate ?? 44100;
+  const midi = new at.midi.MidiFile();
+  midi.division = 960;
+  midi.addEvent(new at.midi.TempoChangeEvent(0, 500000));
+  const seen = new Set<number>();
+  let next = 0;
+  for (const t of tracks) {
+    const ch = t.isDrums ? 9 : MELODIC_CHANNELS[next++ % MELODIC_CHANNELS.length]!;
+    if (!seen.has(ch)) { seen.add(ch); midi.addEvent(new at.midi.ProgramChangeEvent(0, 0, ch, t.isDrums ? 0 : t.program)); }
+    for (const n of t.notes) {
+      const on = Math.round(Math.max(0, warp.toAudio(n.onset)) * TPS);
+      const off = Math.max(on + 1, Math.round(warp.toAudio(n.offset) * TPS));
+      if (warp.toAudio(n.offset) <= 0) continue;
+      midi.addEvent(new at.midi.NoteOnEvent(0, on, ch, n.pitch, n.velocity));
+      midi.addEvent(new at.midi.NoteOffEvent(0, off, ch, n.pitch, 0));
+    }
+  }
+  const frames = opts.durationSec === undefined ? undefined : Math.round(opts.durationSec * sampleRate);
+  const out = renderMidiFile(midi, sf2, sampleRate, { frames });
+  let chans = out.channels;
+  if (opts.mono) {
+    const [L, R] = chans as [Float32Array, Float32Array];
+    chans = [L.map((v, i) => (v + R[i]!) / 2)];
+  }
+  let peak = 0;
+  for (const c of chans) for (let i = 0; i < c.length; i++) { const a = Math.abs(c[i]!); if (a > peak) peak = a; }
+  if (peak > 0) { const g = PEAK / peak; for (const c of chans) for (let i = 0; i < c.length; i++) c[i]! *= g; }
+  return { sampleRate, channels: chans };
+}

src/audio/render.ts

@@ -5,7 +5,7 @@ import { songToScore } from "../score/gp.ts";
 const TAIL_SECONDS = 2;
 
 /** Offline-render sink: AlphaSynth requires an ISynthOutput but exportAudio never uses it. */
-function dummyOutput(sampleRate: number): at.synth.ISynthOutput {
+export function dummyOutput(sampleRate: number): at.synth.ISynthOutput {
   const ev = { on() {}, off() {} };
   return {
     sampleRate, open() {}, play() {}, destroy() {}, pause() {}, addSamples() {}, resetSamples() {}, activate() {},
@@ -18,6 +18,39 @@ function songSeconds(song: Song): number {
   return (song.bars * song.timeSig[0] * (4 / song.timeSig[1]) * 60) / song.bpm;
 }
 
+/** Render a MidiFile offline. With `frames`, output is exactly that long (zero-padded or truncated); else full length. */
+export function renderMidiFile(
+  midi: at.midi.MidiFile, sf2: Uint8Array, sampleRate: number,
+  opts: { trackVolume?: Map<number, number>; frames?: number } = {},
+): Audio {
+  const synth = new at.synth.AlphaSynth(dummyOutput(sampleRate), 500);
+  const eo = new at.synth.AudioExportOptions();
+  eo.soundFonts = [sf2];
+  eo.sampleRate = sampleRate;
+  eo.useSyncPoints = false;
+  if (opts.trackVolume) eo.trackVolume = opts.trackVolume;
+  const exporter = synth.exportAudio(eo, midi, [], new Map());
+  const cap = opts.frames ?? Infinity;
+  const Ls: Float32Array[] = [], Rs: Float32Array[] = [];
+  let pos = 0;
+  while (pos < cap) {
+    const chunk = exporter.render(500);
+    if (!chunk) break;
+    const s = chunk.samples;
+    const frames = Math.min(s.length >> 1, cap - pos);
+    const l = new Float32Array(frames), r = new Float32Array(frames);
+    for (let i = 0; i < frames; i++) { l[i] = s[2 * i]!; r[i] = s[2 * i + 1]!; }
+    Ls.push(l); Rs.push(r);
+    pos += frames;
+  }
+  synth.destroy();
+  const n = opts.frames ?? pos;
+  const L = new Float32Array(n), R = new Float32Array(n);
+  let o = 0;
+  for (let k = 0; k < Ls.length; k++) { L.set(Ls[k]!, o); R.set(Rs[k]!, o); o += Ls[k]!.length; }
+  return { sampleRate, channels: [L, R] };
+}
+
 /**
  * Render each track in isolation with alphaSynth. Isolation: `AudioExportOptions.trackVolume` is keyed by MIDI
  * channel (despite the doc's "track index"), so every channel not owned by the target track is muted (volume 0).
@@ -36,27 +69,8 @@ export function renderStems(song: Song, sf2: Uint8Array, sampleRate = 44100): Ma
     const trackVolume = new Map<number, number>();
     for (let ch = 0; ch < 16; ch++) if (!own.has(ch)) trackVolume.set(ch, 0);
 
-    const synth = new at.synth.AlphaSynth(dummyOutput(sampleRate), 500);
-    const opts = new at.synth.AudioExportOptions();
-    opts.soundFonts = [sf2];
-    opts.sampleRate = sampleRate;
-    opts.useSyncPoints = false;
-    opts.trackVolume = trackVolume;
-    const exporter = synth.exportAudio(opts, midi, [], new Map());
-
-    const L = new Float32Array(total), R = new Float32Array(total);
-    let pos = 0;
-    for (;;) {
-      const chunk = exporter.render(500);
-      if (!chunk) break;
-      const s = chunk.samples;
-      const frames = Math.min(s.length >> 1, total - pos);
-      for (let i = 0; i < frames; i++) { L[pos + i] = s[2 * i]!; R[pos + i] = s[2 * i + 1]!; }
-      pos += frames;
-      if (pos >= total) break;
-    }
-    synth.destroy();
-    stems.set(track.name, { sampleRate, channels: [L, R] });
+    const audio = renderMidiFile(midi, sf2, sampleRate, { trackVolume, frames: total });
+    stems.set(track.name, audio);
   }
   return stems;
 }