Recording: 60fps capture, remux when no rescale, CRF 16 slow otherwise

captureStream at 60fps with an 80 Mbps cap. When the requested frame equals
the source and the first pass is H.264, the MP4 is a pure remux (-c copy):
the hardware realtime encode is the only compression in the final file. The
re-encode path (vertical reels, VP9 fallback) moves to CRF 16 preset slow at
constant 60fps.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XVMQ2JhsJ77pGKqcvvFHqc

main.js

@@ -309,13 +309,22 @@ function transcodeToMp4(input, output, opts) {
   const h = (opts && opts.h) || 1080;
   // Cover the target frame then centre-crop, so the chosen aspect is filled
   // without distortion.
-  const vf = `scale=${w}:${h}:force_original_aspect_ratio=increase,crop=${w}:${h}`;
-  // Post-processing, not realtime: spend encode time on quality. CRF 18 +
-  // medium is visually transparent for VJ motion; -r 30 turns the recorder's
-  // variable frame timing into constant 30fps (players and socials want CFR).
-  const args = ['-y', '-i', input, '-vf', vf, '-r', '30',
-    '-c:v', 'libx264', '-preset', 'medium', '-crf', '18', '-pix_fmt', 'yuv420p',
-    '-c:a', 'aac', '-b:a', '256k', '-movflags', '+faststart', output];
+  const args = ['-y', '-i', input];
+  if (opts && opts.srcH264 && opts.srcW === w && opts.srcH === h) {
+    // Source already matches the requested frame and is H.264: REMUX without
+    // re-encoding — the hardware realtime pass stays the only encode in the
+    // final file (zero generational loss).
+    args.push('-c', 'copy');
+  } else {
+    // Rescale/crop needed (e.g. vertical reels) or non-H264 fallback source.
+    // Post-processing, not realtime: spend encode time on quality. -r 60
+    // turns the recorder's variable frame timing into constant fps.
+    const vf = `scale=${w}:${h}:force_original_aspect_ratio=increase,crop=${w}:${h}`;
+    args.push('-vf', vf, '-r', '60',
+      '-c:v', 'libx264', '-preset', 'slow', '-crf', '16', '-pix_fmt', 'yuv420p',
+      '-c:a', 'aac', '-b:a', '256k');
+  }
+  args.push('-movflags', '+faststart', output);
   return new Promise((resolve, reject) => {
     execFile(ffmpegPath(), args, { maxBuffer: 1 << 24 }, (err, stdout, stderr) => {
       if (err) reject(new Error('ffmpeg: ' + String(stderr || err.message).slice(-500)));