From d5362e20ad78278ba076b39ca20d436b17a5400c Mon Sep 17 00:00:00 2001
From: Vanilagy <1696106+Vanilagy@users.noreply.github.com>
Date: Thu, 30 Jul 2026 17:45:33 +0200
Subject: [PATCH] Fix incorrect video depth being written in ISOBMFF for video
 tracks with an alpha channel (fixes #450)

---
 dev/convert.html             | 16 ++++++++--------
 src/isobmff/isobmff-boxes.ts |  8 ++++++--
 src/isobmff/isobmff-muxer.ts |  5 +++++
 3 files changed, 19 insertions(+), 10 deletions(-)

diff --git a/dev/convert.html b/dev/convert.html
index 13dce45..f8dfb49 100644
--- a/dev/convert.html
+++ b/dev/convert.html
@@ -25,7 +25,7 @@
 			chunked: true,
 			chunkSize: 2**20
 		});
-		const outputFormat = new Mediabunny.Mp4OutputFormat();
+		const outputFormat = new Mediabunny.MovOutputFormat();
 
 		const p = document.createElement('p');
 		p.textContent = 'Capturing...';
@@ -129,11 +129,11 @@
 			},
 			*/
 			video: {
-				codec: 'avc',
-				bitrate: new Mediabunny.Quality({
-					quality: Infinity,
-				}),
-				forceTranscode: true,
+				//codec: 'avc',
+				//bitrate: new Mediabunny.Quality({
+				//	quality: Infinity,
+				//}),
+				//forceTranscode: true,
 				//process: (sample) => {
 				//	sample.draw(ctx, 0, 0, 1280, 720);
 				//	return new Mediabunny.VideoSample(canvas, { timestamp: sample.timestamp, duration: sample.duration });
@@ -157,7 +157,7 @@
 				}
 			},
 			trim: {
-				end: 20,
+				end: 0.5,
 				//start: -2,
 				//end: 10,
 				//start: 300.14984567374756 - 100,
@@ -197,7 +197,7 @@
 		document.body.append(video);
 		video.play();
 
-		//download(new Blob([target.buffer]), 'converted' + outputFormat.fileExtension);
+		download(new Blob([target.buffer]), 'converted' + outputFormat.fileExtension);
 
 		function download(blob, filename) {
 			const url = URL.createObjectURL(blob);
diff --git a/src/isobmff/isobmff-boxes.ts b/src/isobmff/isobmff-boxes.ts
index e070e6e..485ae57 100644
--- a/src/isobmff/isobmff-boxes.ts
+++ b/src/isobmff/isobmff-boxes.ts
@@ -718,8 +718,12 @@ export const videoSampleDescription = (
 	u32(0x00480000), // Vertical resolution
 	u32(0), // Reserved
 	u16(1), // Frame count
-	Array(32).fill(0), // Compressor name
-	u16(0x0018), // Depth
+
+	// Compressor name
+	ascii('Mediabunny'),
+	Array(32 - 'Mediabunny'.length).fill(0),
+
+	u16(trackData.info.hasAlphaChannel ? 32 : 24), // Depth
 	i16(0xffff), // Pre-defined
 ], [
 	VIDEO_CODEC_TO_CONFIGURATION_BOX[trackData.track.source._codec]?.(trackData) ?? null,
diff --git a/src/isobmff/isobmff-muxer.ts b/src/isobmff/isobmff-muxer.ts
index 76105fd..60f8e97 100644
--- a/src/isobmff/isobmff-muxer.ts
+++ b/src/isobmff/isobmff-muxer.ts
@@ -108,6 +108,7 @@ export type IsobmffTrackData = {
 		 * https://stackoverflow.com/questions/24884827
 		 */
 		requiresAnnexBTransformation: boolean;
+		hasAlphaChannel: boolean;
 	};
 } | {
 	track: OutputAudioTrack;
@@ -413,6 +414,9 @@ export class IsobmffMuxer extends Muxer {
 					den: displayAspectHeight * decoderConfig.codedWidth,
 				});
 
+		// Kicks for ProRes 4444.
+		const hasAlphaChannel = decoderConfig.codec === 'ap4h' || decoderConfig.codec === 'ap4x';
+
 		const newTrackData: IsobmffVideoTrackData = {
 			muxer: this,
 			track,
@@ -423,6 +427,7 @@ export class IsobmffMuxer extends Muxer {
 				pixelAspectRatio,
 				decoderConfig: decoderConfig,
 				requiresAnnexBTransformation,
+				hasAlphaChannel,
 			},
 			timescale,
 			samples: [],