From 445911edd091fd9662546c24df029c39f0187222 Mon Sep 17 00:00:00 2001
From: Vanilagy <1696106+Vanilagy@users.noreply.github.com>
Date: Mon, 3 Aug 2026 14:50:36 +0200
Subject: [PATCH] Fix ISOBMFF muxer track alternate groups (fixes #454), fixed
 mutex race condition in Output

---
 dev/mux.html                 | 14 +++++++-------
 src/isobmff/isobmff-boxes.ts | 13 ++++++++++++-
 src/output.ts                |  5 +++--
 3 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/dev/mux.html b/dev/mux.html
index e619bb9..1c39112 100644
--- a/dev/mux.html
+++ b/dev/mux.html
@@ -47,7 +47,7 @@
 	format = new Mediabunny.MkvOutputFormat();
 	format = new Mediabunny.MovOutputFormat();
 	format = new Mediabunny.Mp4OutputFormat({ fastStart: 'reserve' });
-	format = new Mediabunny.WebMOutputFormat();
+	format = new Mediabunny.Mp4OutputFormat();
 	let target = new Mediabunny.BufferTarget();
 
 	/*
@@ -118,21 +118,21 @@
 	*/
 
 	let videoSource = new Mediabunny.CanvasSource(canvas, {
-		codec: 'vp9',
+		codec: 'avc',
 		//fullCodecString: 'avc1.42001f',
 		bitrate: 1e6,
 		alpha: 'keep',
 		onEncoderConfig: console.log,
 	});
 	let audioSource = new Mediabunny.AudioBufferSource({
-		codec: 'opus',
+		codec: 'aac',
 		bitrate: 128e3,
 	});
 	let subtitleSource = new Mediabunny.TextSubtitleSource('webvtt');
 
 	output.addVideoTrack(videoSource, { languageCode: 'eng', name: 'Mononoké', maximumPacketCount: 100 });
 	output.addAudioTrack(audioSource, { name: 'Yooo', maximumPacketCount: 1000 });
-	//output.addSubtitleTrack(subtitleSource);
+	output.addSubtitleTrack(subtitleSource);
 
 	output.start();
 
@@ -191,8 +191,8 @@ Testing... <00:17.350>One... <00:18.125>Two...
 9. <b>justify (bottom, right)</b>.
 `;
 
-	//subtitleSource.add(simpleWebvttFile);
-	//subtitleSource.close();
+	subtitleSource.add(simpleWebvttFile);
+	subtitleSource.close();
 	
 	const p = document.createElement('p');
 	document.body.append(p);
@@ -216,5 +216,5 @@ Testing... <00:17.350>One... <00:18.125>Two...
 	await output.finalize();
 
 	console.log(target);
-	//download(new Blob([target.buffer]), 'test' + format.fileExtension);
+	download(new Blob([target.buffer]), 'test' + format.fileExtension);
 </script>
\ No newline at end of file
diff --git a/src/isobmff/isobmff-boxes.ts b/src/isobmff/isobmff-boxes.ts
index ebbc3a9..57fea8d 100644
--- a/src/isobmff/isobmff-boxes.ts
+++ b/src/isobmff/isobmff-boxes.ts
@@ -493,6 +493,17 @@ export const tkhd = (
 		flags |= 0x1; // Track enabled
 	}
 
+	// Set the alternate group based on the track type; this mirror's how FFmpeg does it. A more advanced version would
+	// determine the alternate groups based on the actual track pairability graph. Note that it appears important that
+	// video get assigned to group 0, see https://github.com/Vanilagy/mediabunny/issues/454.
+	const alternateGroup = trackData.type === 'video'
+		? 0
+		: trackData.type === 'audio'
+			? 1
+			: trackData.type === 'subtitle'
+				? 2
+				: assertNever(trackData);
+
 	return fullBox('tkhd', +needsU64, flags, [
 		u32OrU64(creationTime), // Creation time
 		u32OrU64(creationTime), // Modification time
@@ -501,7 +512,7 @@ export const tkhd = (
 		u32OrU64(durationInGlobalTimescale), // Duration
 		Array(8).fill(0), // Reserved
 		u16(0), // Layer
-		u16(trackData.track.id), // Alternate group
+		u16(alternateGroup), // Alternate group
 		fixed_8_8(trackData.type === 'audio' ? 1 : 0), // Volume
 		u16(0), // Reserved
 		matrixToBytes(matrix), // Matrix
diff --git a/src/output.ts b/src/output.ts
index 505e732..44f1a49 100644
--- a/src/output.ts
+++ b/src/output.ts
@@ -830,7 +830,8 @@ export class Output<
 		return this._startPromise = (async () => {
 			this.state = 'started';
 
-			const release = await this._mutex.acquire();
+			// We want to call muxer.start immediately, so we avoid using an await here
+			const releasePromise = this._mutex.acquire();
 
 			try {
 				await this._muxer.start();
@@ -838,7 +839,7 @@ export class Output<
 				const promises = this.tracks.map(track => track.source._start());
 				await Promise.all(promises);
 			} finally {
-				release();
+				(await releasePromise)();
 			}
 		})();
 	}