Add configurable minimum MP4 fragment duration and Matroska cluster duration

src/isobmff/isobmff-muxer.ts

@@ -118,6 +118,7 @@ export class IsobmffMuxer extends Muxer {
 	private nextFragmentNumber = 1;
 	// Only relevant for fragmented files, to make sure new fragments start with the highest timestamp seen so far
 	private maxWrittenTimestamp = -Infinity;
+	private minimumFragmentDuration: number;
 
 	constructor(output: Output, format: IsobmffOutputFormat) {
 		super(output);
@@ -136,6 +137,8 @@ export class IsobmffMuxer extends Muxer {
 		if (this.fastStart === 'in-memory' || this.isFragmented) {
 			this.writer.ensureMonotonicity = true;
 		}
+
+		this.minimumFragmentDuration = format._options.minimumFragmentDuration ?? 1;
 	}
 
 	async start() {
@@ -683,7 +686,7 @@ export class IsobmffMuxer extends Muxer {
 				});
 
 				if (
-					currentChunkDuration >= 1.0
+					currentChunkDuration >= this.minimumFragmentDuration
 					&& keyFrameQueuedEverywhere
 					&& sample.timestamp > this.maxWrittenTimestamp
 				) {

src/matroska/matroska-muxer.ts

@@ -703,7 +703,7 @@ export class MatroskaMuxer extends Muxer {
 				// CURRENT chunk, meaning that starting the next cluster at the same timestamp is forbidden (since
 				// the already-written block would belong into it instead).
 				&& msTimestamp > this.currentClusterMaxMsTimestamp
-				&& relativeTimestamp >= 1000
+				&& relativeTimestamp >= 1000 * (this.format._options.minimumClusterDuration ?? 1)
 			)
 			// The cluster would exceed its maximum allowed length. This puts us in an unfortunate position and forces
 			// us to begin the next cluster with a delta frame. Although this is undesirable, it is not forbidden by the

src/output-format.ts

@@ -110,6 +110,12 @@ export type IsobmffOutputFormatOptions = {
 	 * the type of output target used.
 	 */
 	fastStart?: false | 'in-memory' | 'fragmented';
+
+	/**
+	 * When using `fastStart: 'fragmented'`, this field controls the minimum duration of each fragment, in seconds.
+	 * New fragments will only be created when the current fragment is longer than this value. Defaults to 1 second.
+	 */
+	minimumFragmentDuration?: number;
 };
 
 /**
@@ -127,6 +133,12 @@ export abstract class IsobmffOutputFormat extends OutputFormat {
 		if (options.fastStart !== undefined && ![false, 'in-memory', 'fragmented'].includes(options.fastStart)) {
 			throw new TypeError('options.fastStart, when provided, must be false, "in-memory", or "fragmented".');
 		}
+		if (
+			options.minimumFragmentDuration !== undefined
+			&& (!Number.isFinite(options.minimumFragmentDuration) || options.minimumFragmentDuration < 0)
+		) {
+			throw new TypeError('options.minimumFragmentDuration, when provided, must be a non-negative number.');
+		}
 
 		super();
 
@@ -226,6 +238,12 @@ export type MkvOutputFormatOptions = {
 	 * option when you want to write out a file for later use.
 	 */
 	streamable?: boolean;
+
+	/**
+	 * This field controls the minimum duration of each Matroska cluster, in seconds. New clusters will only be created
+	 * when the current cluster is longer than this value. Defaults to 1 second.
+	 */
+	minimumClusterDuration?: number;
 };
 
 /**
@@ -243,6 +261,12 @@ export class MkvOutputFormat extends OutputFormat {
 		if (options.streamable !== undefined && typeof options.streamable !== 'boolean') {
 			throw new TypeError('options.streamable, when provided, must be a boolean.');
 		}
+		if (
+			options.minimumClusterDuration !== undefined
+			&& (!Number.isFinite(options.minimumClusterDuration) || options.minimumClusterDuration < 0)
+		) {
+			throw new TypeError('options.minimumClusterDuration, when provided, must be a non-negative number.');
+		}
 
 		super();