openutau-render.js

#!/usr/bin/env node
import {readFileSync} from "fs";
import fs from "fs/promises";
import path from "path";
import { spawn } from "child_process";
import readline from "readline";
import YAML from "yaml";

function parseUSTX(ustxPath) {
  const raw = readFileSync(ustxPath, "utf-8");
  const data = YAML.parse(raw);
  console.log(data)

  if (!Array.isArray(data.tracks)) {
    throw new Error("Invalid USTX: missing tracks array");
  }

  return data.tracks
    .filter(track => !track.mute)
    .map(track => ({
      name: track.track_name,
      volume: track.volume ?? 0,
      pan: (track.pan ?? 0) / 100
    }));
}

function prompt(question) {
  const rl = readline.createInterface({
    input: process.stdin,
    output: process.stdout
  });
  return new Promise(resolve =>
    rl.question(question, answer => {
      rl.close();
      resolve(answer);
    })
  );
}

function sanitizeFFmpegLabel(name) {
  return name.replace(/[^a-zA-Z0-9_]/g, "_");
}

const targetDir = process.argv[2];
if (!targetDir) {
  console.error("Usage: node openutau-render <directory>");
  process.exit(1);
}

const absDir = path.resolve(targetDir);

const files = await fs.readdir(absDir);
const ustxFiles = files.filter(f => f.toLowerCase().endsWith(".ustx"));

if (ustxFiles.length === 0) {
  console.error("No USTX files found.");
  process.exit(1);
}

let ustxFile;
if (ustxFiles.length === 1) {
  ustxFile = ustxFiles[0];
} else {
  console.log("Multiple USTX files found:");
  ustxFiles.forEach((f, i) => console.log(`  [${i}] ${f}`));
  const idx = Number(await prompt("Select one: "));
  ustxFile = ustxFiles[idx];
}

const ustxPath = path.join(absDir, ustxFile);
const baseName = path.parse(ustxFile).name;

const tracks = parseUSTX(ustxPath);

if (!tracks.length) {
  console.error("No tracks found in USTX.");
  process.exit(1);
}

const resolvedTracks = [];

for (const track of tracks) {
  const wav = path.join(absDir, `${baseName}_${track.name}.wav`);
  const mp3 = path.join(absDir, `${baseName}_${track.name}.mp3`);

  let filePath;
  try {
    await fs.access(wav);
    filePath = wav;
  } catch {
    try {
      await fs.access(mp3);
      filePath = mp3;
    } catch {
      console.error(`Missing audio for track: ${track.name}; Make sure each track has a unique name, and then export all tracks via File -> Export Audio -> Export Wav Files To..., and choose the same directory as your project files`);
      process.exit(1);
    }
  }

  resolvedTracks.push({ ...track, filePath });
}

const ffmpegArgs = [];
const filterParts = [];
const mixInputs = [];

resolvedTracks.forEach((track, i) => {
  ffmpegArgs.push("-i", track.filePath);

  const label = `a${i}`;
  const pan = track.pan;
  const volume = track.volume;

  const left = (1 - pan) / 2;
  const right = (1 + pan) / 2;

  filterParts.push(
    `[${i}:a]` +
    `volume=${volume}dB,` +
    `pan=stereo|c0=${left}*c0+${left}*c1|c1=${right}*c0+${right}*c1` +
    `[${label}]`
  );

  mixInputs.push(`[${label}]`);
});

filterParts.push(
  `${mixInputs.join("")}amix=inputs=${mixInputs.length}:normalize=0`
);

const outputPath = path.join(absDir, `${baseName}_mixdown.wav`);

ffmpegArgs.push(
  "-filter_complex", filterParts.join(";"),
  outputPath
);

console.log("Running ffmpeg...");
console.log(ffmpegArgs)
const ffmpeg = spawn("ffmpeg", ffmpegArgs, { stdio: "inherit" });

ffmpeg.on("exit", code => {
  if (code === 0) {
    console.log("Mixdown complete:", outputPath);
  } else {
    console.error("ffmpeg failed.");
  }
});