Fix incorrect video depth being written in ISOBMFF for video tracks with an alpha channel (fixes #450)

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,

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: [],