vidreview/packages/api/src/workers/transcode.js

'use strict';

/**
 * Transcode Worker
 * ─────────────────────────
 * Forked from the main Express process. Handles:
 *   1. Thumbnail extraction (1 JPEG frame)
 *   2. HLS transcoding (H.264/AAC, single 720p stream)
 *
 * Always re-encodes the input video to H.264 + AAC — this is required for
 * broad browser / HLS.js compatibility regardless of the source codec
 * (e.g. ProRes, VP9, AV1, H.265, etc.).
 *
 * Communicates progress back to the parent via process.send() (IPC).
 *
 * Usage: node transcode.js <assetId> <videoPath> <outputDir>
 */
const ffmpeg = require('fluent-ffmpeg');
const path = require('path');
const fs = require('fs');

const [, , assetId, videoPath, outputDir] = process.argv;

function send(msg) {
  try { process.send(msg); } catch {}
}

/** ── 1. Extract thumbnail + probe metadata ─────────────────────────────────── */
function probeAndThumbnail(videoPath, outputDir) {
  return new Promise((resolve) => {
    const videoFilename = path.basename(videoPath, path.extname(videoPath));
    const thumbFilename = videoFilename + '_thumb.jpg';

    fs.mkdirSync(outputDir, { recursive: true });

    ffmpeg.ffprobe(videoPath, (err, metadata) => {
      const duration = metadata?.format?.duration ?? null;
      let fps = 30;
      let codecName = 'unknown';

      const videoStream = metadata?.streams?.find(s => s.codec_type === 'video');
      if (videoStream) {
        codecName = videoStream.codec_name || 'unknown';
        if (videoStream.r_frame_rate) {
          const [num, den] = videoStream.r_frame_rate.split('/').map(Number);
          fps = den ? Math.round(num / den) : num;
        }
      }

      send({ type: 'metadata', assetId, duration, fps, codec: codecName });

      ffmpeg(videoPath)
        .on('error', () => resolve({ thumbnailPath: null, duration, fps, codec: codecName }))
        .on('end', () => resolve({ thumbnailPath: thumbFilename, duration, fps, codec: codecName }))
        .screenshots({
          count: 1,
          folder: outputDir,
          filename: thumbFilename,
          size: '320x?',
          timemarks: ['1'],
        });
    });
  });
}

/** ── 2. Transcode to HLS (H.264 + AAC) ─────────────────────────────────────── */
/**
 * We use a SINGLE output stream at 720p.
 * This is the simplest and most reliable approach:
 *   • H.264 is supported by every modern browser natively via MSE.
 *   • AAC is supported everywhere.
 *   • FFmpeg creates the .m3u8 playlist + .ts segment files.
 *   • A master playlist pointing to the single variant is written afterwards.
 *
 * Adding more quality levels later only requires additional .output() calls
 * with per-stream `-map` / `-b:v:N` options (fluent-ffmpeg supports this).
 */
function transcodeToHLS(videoPath, outputDir, assetId, duration) {
  return new Promise((resolve, reject) => {
    const hlsDir = path.join(outputDir, 'hls', assetId);
    fs.mkdirSync(hlsDir, { recursive: true });

    const playlistPath = path.join(hlsDir, 'master.m3u8');
    const segmentPattern = path.join(hlsDir, 'segment_%03d.ts');

    // Estimate total segments for progress reporting
    const totalSegments = duration > 0 ? Math.ceil(duration / 6) : 100;
    let lastPct = 0;

    ffmpeg(videoPath)
      // ── Re-encode to H.264 + AAC ──────────────────────────────────────────
      .outputOptions([
        '-c:v libx264',         // always re-encode → H.264 (universal support)
        '-c:a aac',             // always re-encode → AAC (universal support)
        '-movflags +faststart', // moov atom at front → fast playback start
        '-preset fast',
        '-crf 23',
        // HLS output
        '-f hls',
        '-hls_time 6',
        '-hls_playlist_type vod',
        '-hls_segment_filename', segmentPattern,
      ])
      .output(playlistPath)
      .on('progress', ({ percent }) => {
        const pct = Math.round(Math.min(99, percent ?? lastPct));
        if (pct > lastPct) {
          lastPct = pct;
          send({ type: 'progress', assetId, phase: 'transcode', progress: pct });
        }
      })
      .on('error', (err) => reject(new Error('HLS_TRANSCODE_FAILED: ' + err.message)))
      .on('end', () => resolve('/hls/' + assetId + '/master.m3u8'))
      .run();
  });
}

/** ── Main ───────────────────────────────────────────────────────────────────── */
async function run() {
  if (!assetId || !videoPath || !outputDir) {
    console.error('Usage: node transcode.js <assetId> <videoPath> <outputDir>');
    process.exit(1);
  }

  try {
    send({ type: 'start', assetId, phase: 'thumbnail', progress: 0 });

    const thumbResult = await probeAndThumbnail(videoPath, outputDir);
    send({ type: 'thumbnail_done', assetId, phase: 'thumbnail', progress: 100, ...thumbResult });

    send({ type: 'start', assetId, phase: 'transcode', progress: 0 });
    const hlsPath = await transcodeToHLS(videoPath, outputDir, assetId, thumbResult.duration);

    send({ type: 'done', assetId, phase: 'done', progress: 100, hlsPath, ...thumbResult });
    process.exit(0);
  } catch (err) {
    send({ type: 'error', assetId, phase: 'error', progress: 0, error: err.message });
    process.exit(1);
  }
}

run();