Add metadata read/write support for Matroska

src/matroska/ebml.ts

@@ -129,8 +129,27 @@ export enum EBMLId {
 	ProjectionType = 0x7671,
 	ProjectionPoseRoll = 0x7675,
 	Attachments = 0x1941a469,
+	AttachedFile = 0x61a7,
+	FileDescription = 0x467e,
+	FileName = 0x466e,
+	FileMediaType = 0x4660,
+	FileData = 0x465c,
+	FileUID = 0x46ae,
 	Chapters = 0x1043a770,
 	Tags = 0x1254c367,
+	Tag = 0x7373,
+	Targets = 0x63c0,
+	TargetTypeValue = 0x68ca,
+	TargetType = 0x63ca,
+	TagTrackUID = 0x63c5,
+	TagEditionUID = 0x63c9,
+	TagChapterUID = 0x63c4,
+	TagAttachmentUID = 0x63c6,
+	SimpleTag = 0x67c8,
+	TagName = 0x45a3,
+	TagLanguage = 0x447a,
+	TagString = 0x4487,
+	TagBinary = 0x4485,
 }
 
 export const LEVEL_0_EBML_IDS: EBMLId[] = [

src/matroska/matroska-demuxer.ts

@@ -30,6 +30,7 @@ import {
 	InputVideoTrack,
 	InputVideoTrackBacking,
 } from '../input-track';
+import { MediaMetadata } from '../metadata';
 import { PacketRetrievalOptions } from '../media-sink';
 import {
 	assert,
@@ -76,6 +77,8 @@ type Segment = {
 	infoSeen: boolean;
 	tracksSeen: boolean;
 	cuesSeen: boolean;
+	attachmentsSeen: boolean;
+	tagsSeen: boolean;
 
 	timestampScale: number;
 	timestampFactor: number;
@@ -90,6 +93,9 @@ type Segment = {
 
 	clusters: Cluster[];
 	clusterLookupMutex: AsyncMutex;
+
+	metadata: MediaMetadata;
+	metadataCollected: boolean;
 };
 
 type SeekEntry = {
@@ -198,6 +204,14 @@ export class MatroskaDemuxer extends Demuxer {
 	currentCluster: Cluster | null = null;
 	currentBlock: ClusterBlock | null = null;
 	currentCueTime: number | null = null;
+	currentTagTargetIsMovie: boolean = true;
+	currentSimpleTagName: string | null = null;
+	currentAttachedFile: {
+		fileName: string | null;
+		fileMediaType: string | null;
+		fileData: Uint8Array | null;
+		fileDescription: string | null;
+	} | null = null;
 
 	isWebM = false;
 
@@ -232,6 +246,27 @@ export class MatroskaDemuxer extends Demuxer {
 		});
 	}
 
+	async getMetadata() {
+		await this.readMetadata();
+
+		// Load metadata from each segment lazily (only once)
+		for (const segment of this.segments) {
+			if (!segment.metadataCollected) {
+				await this.loadSegmentMetadata(segment);
+				segment.metadataCollected = true;
+			}
+		}
+
+		// This is kinda handwavy, and how we handle multiple segments isn't suuuuper well-defined anyway; so we just
+		// shallow-merge metadata from all (usually just one) segments.
+		let metadata: MediaMetadata = {};
+		for (const segment of this.segments) {
+			metadata = { ...metadata, ...segment.metadata };
+		}
+
+		return metadata;
+	}
+
 	readMetadata() {
 		return this.readMetadataPromise ??= (async () => {
 			let currentPos = 0;
@@ -311,6 +346,8 @@ export class MatroskaDemuxer extends Demuxer {
 			infoSeen: false,
 			tracksSeen: false,
 			cuesSeen: false,
+			tagsSeen: false,
+			attachmentsSeen: false,
 
 			timestampScale: -1,
 			timestampFactor: -1,
@@ -327,6 +364,9 @@ export class MatroskaDemuxer extends Demuxer {
 
 			clusters: [],
 			clusterLookupMutex: new AsyncMutex(),
+
+			metadata: {},
+			metadataCollected: false,
 		};
 		this.segments.push(this.currentSegment);
 
@@ -371,6 +411,22 @@ export class MatroskaDemuxer extends Demuxer {
 				let slice = this.reader.requestSlice(dataStartPos, size);
 				if (slice instanceof Promise) slice = await slice;
 
+				if (slice) {
+					this.readContiguousElements(slice);
+				}
+			} else if (id === EBMLId.Tags || id === EBMLId.Attachments) {
+				// Metadata found at the beginning of the segment, great, let's parse it
+				if (id === EBMLId.Tags) {
+					this.currentSegment.tagsSeen = true;
+				} else {
+					this.currentSegment.attachmentsSeen = true;
+				}
+
+				assertDefinedSize(size);
+
+				let slice = this.reader.requestSlice(dataStartPos, size);
+				if (slice instanceof Promise) slice = await slice;
+
 				if (slice) {
 					this.readContiguousElements(slice);
 				}
@@ -386,10 +442,10 @@ export class MatroskaDemuxer extends Demuxer {
 			}
 		}
 
-		if (this.reader.fileSize !== null) {
-			// Sort the seek entries by file position so reading them exhibits a sequential pattern
-			this.currentSegment.seekEntries.sort((a, b) => a.segmentPosition - b.segmentPosition);
+		// Sort the seek entries by file position so reading them exhibits a sequential pattern
+		this.currentSegment.seekEntries.sort((a, b) => a.segmentPosition - b.segmentPosition);
 
+		if (this.reader.fileSize !== null) {
 			// Use the seek head to read missing metadata elements
 			for (const seekEntry of this.currentSegment.seekEntries) {
 				const target = METADATA_ELEMENTS.find(x => x.id === seekEntry.id);
@@ -745,6 +801,50 @@ export class MatroskaDemuxer extends Demuxer {
 		}
 	}
 
+	async loadSegmentMetadata(segment: Segment) {
+		for (const seekEntry of segment.seekEntries) {
+			if (seekEntry.id === EBMLId.Tags && !segment.tagsSeen) {
+				// We need to load the tags
+			} else if (seekEntry.id === EBMLId.Attachments && !segment.attachmentsSeen) {
+				// We need to load the attachments
+			} else {
+				continue;
+			}
+
+			let slice = this.reader.requestSliceRange(
+				segment.dataStartPos + seekEntry.segmentPosition,
+				MIN_HEADER_SIZE,
+				MAX_HEADER_SIZE,
+			);
+			if (slice instanceof Promise) slice = await slice;
+			if (!slice) continue;
+
+			const header = readElementHeader(slice);
+			if (!header || header.id !== seekEntry.id) continue;
+
+			const { size } = header;
+			assertDefinedSize(size);
+
+			assert(!this.currentSegment);
+			this.currentSegment = segment;
+
+			let dataSlice = this.reader.requestSlice(slice.filePos, size);
+			if (dataSlice instanceof Promise) dataSlice = await dataSlice;
+			if (dataSlice) {
+				this.readContiguousElements(dataSlice);
+			}
+
+			this.currentSegment = null;
+
+			// Mark as seen
+			if (seekEntry.id === EBMLId.Tags) {
+				segment.tagsSeen = true;
+			} else if (seekEntry.id === EBMLId.Attachments) {
+				segment.attachmentsSeen = true;
+			}
+		}
+	}
+
 	readContiguousElements(slice: FileSlice) {
 		const startIndex = slice.filePos;
 
@@ -1251,11 +1351,201 @@ export class MatroskaDemuxer extends Demuxer {
 				// We'll offset this by the block's timestamp later
 				this.currentBlock.referencedTimestamps.push(relativeTimestamp);
 			}; break;
+
+			case EBMLId.Tag: {
+				this.currentTagTargetIsMovie = true;
+				this.readContiguousElements(slice.slice(dataStartPos, size));
+			}; break;
+
+			case EBMLId.Targets: {
+				this.readContiguousElements(slice.slice(dataStartPos, size));
+			}; break;
+
+			case EBMLId.TargetTypeValue: {
+				const targetTypeValue = readUnsignedInt(slice, size);
+				if (targetTypeValue !== 50) {
+					this.currentTagTargetIsMovie = false;
+				}
+			}; break;
+
+			case EBMLId.TagTrackUID:
+			case EBMLId.TagEditionUID:
+			case EBMLId.TagChapterUID:
+			case EBMLId.TagAttachmentUID: {
+				this.currentTagTargetIsMovie = false;
+			}; break;
+
+			case EBMLId.SimpleTag: {
+				if (!this.currentTagTargetIsMovie) break;
+
+				this.currentSimpleTagName = null;
+				this.readContiguousElements(slice.slice(dataStartPos, size));
+			}; break;
+
+			case EBMLId.TagName: {
+				this.currentSimpleTagName = readUnicodeString(slice, size);
+			}; break;
+
+			case EBMLId.TagString: {
+				if (!this.currentSimpleTagName) break;
+
+				const value = readUnicodeString(slice, size);
+				this.processTagValue(this.currentSimpleTagName, value);
+			}; break;
+
+			case EBMLId.TagBinary: {
+				if (!this.currentSimpleTagName) break;
+
+				const value = readBytes(slice, size);
+				this.processTagValue(this.currentSimpleTagName, value);
+			}; break;
+
+			case EBMLId.AttachedFile: {
+				if (!this.currentSegment) break;
+
+				this.currentAttachedFile = {
+					fileName: null,
+					fileMediaType: null,
+					fileData: null,
+					fileDescription: null,
+				};
+
+				this.readContiguousElements(slice.slice(dataStartPos, size));
+
+				// Only process image attachments
+				if (this.currentAttachedFile.fileMediaType?.startsWith('image/') && this.currentAttachedFile.fileData) {
+					const fileName = this.currentAttachedFile.fileName;
+					let kind: 'coverFront' | 'coverBack' | 'unknown' = 'unknown';
+
+					if (fileName) {
+						const lowerName = fileName.toLowerCase();
+						if (lowerName.startsWith('cover.')) {
+							kind = 'coverFront';
+						} else if (lowerName.startsWith('back.')) {
+							kind = 'coverBack';
+						}
+					}
+
+					this.currentSegment.metadata.images ??= [];
+					this.currentSegment.metadata.images.push({
+						data: this.currentAttachedFile.fileData,
+						mimeType: this.currentAttachedFile.fileMediaType,
+						kind,
+						name: this.currentAttachedFile.fileName ?? undefined,
+						description: this.currentAttachedFile.fileDescription ?? undefined,
+					});
+				}
+
+				this.currentAttachedFile = null;
+			}; break;
+
+			case EBMLId.FileName: {
+				if (!this.currentAttachedFile) break;
+
+				this.currentAttachedFile.fileName = readUnicodeString(slice, size);
+			}; break;
+
+			case EBMLId.FileMediaType: {
+				if (!this.currentAttachedFile) break;
+
+				this.currentAttachedFile.fileMediaType = readAsciiString(slice, size);
+			}; break;
+
+			case EBMLId.FileData: {
+				if (!this.currentAttachedFile) break;
+
+				this.currentAttachedFile.fileData = readBytes(slice, size);
+			}; break;
+
+			case EBMLId.FileDescription: {
+				if (!this.currentAttachedFile) break;
+
+				this.currentAttachedFile.fileDescription = readUnicodeString(slice, size);
+			}; break;
 		}
 
 		slice.filePos = dataStartPos + size;
 		return true;
 	}
+
+	processTagValue(name: string, value: string | Uint8Array) {
+		if (!this.currentSegment?.metadata) return;
+
+		const metadata = this.currentSegment.metadata;
+		metadata.raw ??= {};
+		metadata.raw[name] = value;
+
+		if (typeof value === 'string') {
+			switch (name.toLowerCase()) {
+				case 'title': {
+					metadata.title ??= value;
+				}; break;
+
+				case 'description': {
+					metadata.description ??= value;
+				}; break;
+
+				case 'artist': {
+					metadata.artist ??= value;
+				}; break;
+
+				case 'album': {
+					metadata.album ??= value;
+				}; break;
+
+				case 'album_artist': {
+					metadata.albumArtist ??= value;
+				}; break;
+
+				case 'genre': {
+					metadata.genre ??= value;
+				}; break;
+
+				case 'comment': {
+					metadata.comment ??= value;
+				}; break;
+
+				case 'lyrics': {
+					metadata.lyrics ??= value;
+				}; break;
+
+				case 'date': {
+					const date = new Date(value);
+					if (!Number.isNaN(date.getTime())) {
+						metadata.date ??= date;
+					}
+				}; break;
+
+				case 'track_number':
+				case 'part_number': {
+					const parts = value.split('/');
+					const trackNum = Number.parseInt(parts[0]!, 10);
+					const trackNumMax = parts[1] && Number.parseInt(parts[1], 10);
+
+					if (Number.isInteger(trackNum) && trackNum > 0) {
+						metadata.trackNumber ??= trackNum;
+					}
+					if (trackNumMax && Number.isInteger(trackNumMax) && trackNumMax > 0) {
+						metadata.trackNumberMax ??= trackNumMax;
+					}
+				}; break;
+
+				case 'disc_number':
+				case 'disc': {
+					const discParts = value.split('/');
+					const discNum = Number.parseInt(discParts[0]!, 10);
+					const discNumMax = discParts[1] && Number.parseInt(discParts[1], 10);
+
+					if (Number.isInteger(discNum) && discNum > 0) {
+						metadata.discNumber ??= discNum;
+					}
+					if (discNumMax && Number.isInteger(discNumMax) && discNumMax > 0) {
+						metadata.discNumberMax ??= discNumMax;
+					}
+				}; break;
+			}
+		}
+	}
 }
 
 abstract class MatroskaTrackBacking implements InputTrackBacking {

src/matroska/matroska-muxer.ts

@@ -13,7 +13,10 @@ import {
 	TRANSFER_CHARACTERISTICS_MAP,
 	UNDETERMINED_LANGUAGE,
 	assert,
+	assertNever,
 	colorSpaceIsComplete,
+	imageMimeTypeToExtension,
+	keyValueIterator,
 	normalizeRotation,
 	promiseWithResolvers,
 	roundToMultiple,
@@ -74,22 +77,6 @@ type InternalMediaChunk = {
 	additions: Uint8Array | null;
 };
 
-type SeekHead = {
-	id: number;
-	data: {
-		id: number;
-		data: ({
-			id: number;
-			data: Uint8Array;
-			size?: undefined;
-		} | {
-			id: number;
-			size: number;
-			data: number;
-		})[];
-	}[];
-};
-
 type MatroskaTrackData = {
 	chunkQueue: InternalMediaChunk[];
 	lastWrittenMsTimestamp: number | null;
@@ -137,8 +124,10 @@ export class MatroskaMuxer extends Muxer {
 
 	private segment: EBMLElement | null = null;
 	private segmentInfo: EBMLElement | null = null;
-	private seekHead: SeekHead | null = null;
+	private seekHead: EBMLElement | null = null;
 	private tracksElement: EBMLElement | null = null;
+	private tagsElement: EBMLElement | null = null;
+	private attachmentsElement: EBMLElement | null = null;
 	private segmentDuration: EBMLElement | null = null;
 	private cues: EBMLElement | null = null;
 
@@ -169,10 +158,6 @@ export class MatroskaMuxer extends Muxer {
 
 		this.writeEBMLHeader();
 
-		if (!this.format._options.appendOnly) {
-			this.createSeekHead();
-		}
-
 		this.createSegmentInfo();
 		this.createCues();
 
@@ -207,24 +192,72 @@ export class MatroskaMuxer extends Muxer {
 	 * Creates a SeekHead element which is positioned near the start of the file and allows the media player to seek to
 	 * relevant sections more easily. Since we don't know the positions of those sections yet, we'll set them later.
 	 */
-	private createSeekHead() {
+	private maybeCreateSeekHead(writeOffsets: boolean) {
+		if (this.format._options.appendOnly) {
+			return;
+		}
+
 		const kaxCues = new Uint8Array([0x1c, 0x53, 0xbb, 0x6b]);
 		const kaxInfo = new Uint8Array([0x15, 0x49, 0xa9, 0x66]);
 		const kaxTracks = new Uint8Array([0x16, 0x54, 0xae, 0x6b]);
+		const kaxAttachments = new Uint8Array([0x19, 0x41, 0xa4, 0x69]);
+		const kaxTags = new Uint8Array([0x12, 0x54, 0xc3, 0x67]);
 
 		const seekHead = { id: EBMLId.SeekHead, data: [
 			{ id: EBMLId.Seek, data: [
 				{ id: EBMLId.SeekID, data: kaxCues },
-				{ id: EBMLId.SeekPosition, size: 5, data: 0 },
+				{
+					id: EBMLId.SeekPosition,
+					size: 5,
+					data: writeOffsets
+						? this.ebmlWriter.offsets.get(this.cues!)! - this.segmentDataOffset
+						: 0,
+				},
 			] },
 			{ id: EBMLId.Seek, data: [
 				{ id: EBMLId.SeekID, data: kaxInfo },
-				{ id: EBMLId.SeekPosition, size: 5, data: 0 },
+				{
+					id: EBMLId.SeekPosition,
+					size: 5,
+					data: writeOffsets
+						? this.ebmlWriter.offsets.get(this.segmentInfo!)! - this.segmentDataOffset
+						: 0,
+				},
 			] },
 			{ id: EBMLId.Seek, data: [
 				{ id: EBMLId.SeekID, data: kaxTracks },
-				{ id: EBMLId.SeekPosition, size: 5, data: 0 },
+				{
+					id: EBMLId.SeekPosition,
+					size: 5,
+					data: writeOffsets
+						? this.ebmlWriter.offsets.get(this.tracksElement!)! - this.segmentDataOffset
+						: 0,
+				},
 			] },
+			this.attachmentsElement
+				? { id: EBMLId.Seek, data: [
+						{ id: EBMLId.SeekID, data: kaxAttachments },
+						{
+							id: EBMLId.SeekPosition,
+							size: 5,
+							data: writeOffsets
+								? this.ebmlWriter.offsets.get(this.attachmentsElement)! - this.segmentDataOffset
+								: 0,
+						},
+					] }
+				: null,
+			this.tagsElement
+				? { id: EBMLId.Seek, data: [
+						{ id: EBMLId.SeekID, data: kaxTags },
+						{
+							id: EBMLId.SeekPosition,
+							size: 5,
+							data: writeOffsets
+								? this.ebmlWriter.offsets.get(this.tagsElement)! - this.segmentDataOffset
+								: 0,
+						},
+					] }
+				: null,
 		] };
 		this.seekHead = seekHead;
 	}
@@ -379,14 +412,187 @@ export class MatroskaMuxer extends Muxer {
 		];
 	}
 
+	private maybeCreateTags() {
+		const simpleTags: EBMLElement[] = [];
+
+		const addSimpleTag = (key: string, value: string | Uint8Array) => {
+			simpleTags.push({ id: EBMLId.SimpleTag, data: [
+				{ id: EBMLId.TagName, data: new EBMLUnicodeString(key) },
+				typeof value === 'string'
+					? { id: EBMLId.TagString, data: new EBMLUnicodeString(value) }
+					: { id: EBMLId.TagBinary, data: value },
+			] });
+		};
+
+		const metadata = this.output._metadata;
+		const writtenTags = new Set<string>();
+
+		for (const { key, value } of keyValueIterator(metadata)) {
+			switch (key) {
+				case 'title': {
+					addSimpleTag('TITLE', value);
+					writtenTags.add('TITLE');
+				}; break;
+
+				case 'description': {
+					addSimpleTag('DESCRIPTION', value);
+					writtenTags.add('DESCRIPTION');
+				}; break;
+
+				case 'artist': {
+					addSimpleTag('ARTIST', value);
+					writtenTags.add('ARTIST');
+				}; break;
+
+				case 'album': {
+					addSimpleTag('ALBUM', value);
+					writtenTags.add('ALBUM');
+				}; break;
+
+				case 'albumArtist': {
+					addSimpleTag('ALBUM_ARTIST', value);
+					writtenTags.add('ALBUM_ARTIST');
+				}; break;
+
+				case 'genre': {
+					addSimpleTag('GENRE', value);
+					writtenTags.add('GENRE');
+				}; break;
+
+				case 'comment': {
+					addSimpleTag('COMMENT', value);
+					writtenTags.add('COMMENT');
+				}; break;
+
+				case 'lyrics': {
+					addSimpleTag('LYRICS', value);
+					writtenTags.add('LYRICS');
+				}; break;
+
+				case 'date': {
+					addSimpleTag('DATE', value.toISOString().split('T')[0]!);
+					writtenTags.add('DATE');
+				}; break;
+
+				case 'trackNumber': {
+					const string = metadata.trackNumberMax !== undefined
+						? `${value}/${metadata.trackNumberMax}`
+						: value.toString();
+
+					addSimpleTag('PART_NUMBER', string);
+					writtenTags.add('PART_NUMBER');
+				}; break;
+
+				case 'discNumber': {
+					const string = metadata.discNumberMax !== undefined
+						? `${value}/${metadata.discNumberMax}`
+						: value.toString();
+
+					addSimpleTag('DISC', string);
+					writtenTags.add('DISC');
+				}; break;
+
+				case 'trackNumberMax':
+				case 'discNumberMax': {
+					// Handled with trackNumber and discNumber respectively
+				}; break;
+
+				case 'images':
+				case 'raw': {
+					// Handled elsewhere
+				}; break;
+
+				default: assertNever(key);
+			}
+		}
+
+		if (metadata.raw) {
+			for (const key in metadata.raw) {
+				const value = metadata.raw[key]!;
+				if (value == null || writtenTags.has(key)) {
+					continue;
+				}
+
+				if (typeof value === 'string' || value instanceof Uint8Array) {
+					addSimpleTag(key, value);
+				}
+			}
+		}
+
+		if (simpleTags.length === 0) {
+			return;
+		}
+
+		this.tagsElement = {
+			id: EBMLId.Tags,
+			data: [{ id: EBMLId.Tag, data: [
+				{ id: EBMLId.Targets, data: [
+					{ id: EBMLId.TargetTypeValue, data: 50 },
+					{ id: EBMLId.TargetType, data: 'MOVIE' },
+				] },
+				...simpleTags,
+			] }],
+		};
+	}
+
+	private maybeCreateAttachments() {
+		const metadata = this.output._metadata;
+		if (!metadata.images || metadata.images.length === 0) {
+			return;
+		}
+
+		const existingFileUids = new Set<number>();
+
+		this.attachmentsElement = { id: EBMLId.Attachments, data: metadata.images.map((image): EBMLElement => {
+			let imageName = image.name;
+			if (imageName === undefined) {
+				const baseName = image.kind === 'coverFront' ? 'cover' : image.kind === 'coverBack' ? 'back' : 'image';
+				imageName = baseName + (imageMimeTypeToExtension(image.mimeType) ?? '');
+			}
+
+			let fileUid: number;
+			while (true) {
+				fileUid = Math.floor(Math.random() * Number.MAX_SAFE_INTEGER);
+
+				if (fileUid !== 0 && !existingFileUids.has(fileUid)) {
+					break;
+				}
+			}
+
+			existingFileUids.add(fileUid);
+
+			return {
+				id: EBMLId.AttachedFile,
+				data: [
+					image.description !== undefined
+						? { id: EBMLId.FileDescription, data: new EBMLUnicodeString(image.description) }
+						: null,
+					{ id: EBMLId.FileName, data: new EBMLUnicodeString(imageName) },
+					{ id: EBMLId.FileMediaType, data: image.mimeType },
+					{ id: EBMLId.FileData, data: image.data },
+					{ id: EBMLId.FileUID, data: fileUid },
+				],
+			};
+		}) };
+	}
+
 	private createSegment() {
+		this.createTracks();
+		this.maybeCreateTags();
+		this.maybeCreateAttachments();
+		this.maybeCreateSeekHead(false);
+
 		const segment: EBML = {
 			id: EBMLId.Segment,
 			size: this.format._options.appendOnly ? -1 : SEGMENT_SIZE_BYTES,
 			data: [
-				!this.format._options.appendOnly ? this.seekHead as EBML : null,
+				this.seekHead, // null if append-only
 				this.segmentInfo,
 				this.tracksElement,
+				// Matroska spec says put this at the end of the file, but I think placing it before the first cluster
+				// makes more sense, and FFmpeg agrees (argumentum ad ffmpegum fallacy)
+				this.attachmentsElement,
+				this.tagsElement,
 			],
 		};
 		this.segment = segment;
@@ -754,7 +960,6 @@ export class MatroskaMuxer extends Muxer {
 	private writeBlock(trackData: MatroskaTrackData, chunk: InternalMediaChunk) {
 		// Due to the interlacing algorithm, this code will be run once we've seen one chunk from every media track.
 		if (!this.segment) {
-			this.createTracks();
 			this.createSegment();
 		}
 
@@ -956,7 +1161,6 @@ export class MatroskaMuxer extends Muxer {
 		this.allTracksKnown.resolve();
 
 		if (!this.segment) {
-			this.createTracks();
 			this.createSegment();
 		}
 
@@ -984,14 +1188,9 @@ export class MatroskaMuxer extends Muxer {
 			this.ebmlWriter.writeEBML(this.segmentDuration);
 
 			// Fill in SeekHead position data and write it again
-			this.seekHead!.data[0]!.data[1]!.data
-				= this.ebmlWriter.offsets.get(this.cues)! - this.segmentDataOffset;
-			this.seekHead!.data[1]!.data[1]!.data
-				= this.ebmlWriter.offsets.get(this.segmentInfo!)! - this.segmentDataOffset;
-			this.seekHead!.data[2]!.data[1]!.data
-				= this.ebmlWriter.offsets.get(this.tracksElement!)! - this.segmentDataOffset;
-
-			this.writer.seek(this.ebmlWriter.offsets.get(this.seekHead!)!);
+			assert(this.seekHead);
+			this.writer.seek(this.ebmlWriter.offsets.get(this.seekHead)!);
+			this.maybeCreateSeekHead(true);
 			this.ebmlWriter.writeEBML(this.seekHead);
 
 			this.writer.seek(endPos);