And another one

Custom fps convertion WIP

.github/workflows/release.yml

@@ -15,6 +15,16 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Merge main into release branch
+        run: |
+          git config user.name "github-actions[bot]"
+          git config user.email "github-actions[bot]@users.noreply.github.com"
+          git checkout release
+          git merge origin/main --no-ff -m "Merge main into release for tag ${{ github.event.release.tag_name }}"
+          git push origin release
 
       - name: Set up Node.js
         uses: actions/setup-node@v4

README.md

@@ -22,9 +22,13 @@ Mediabunny is a JavaScript library for reading, writing, and converting media fi
   <a href="https://diffusion.studio/" target="_blank">
     <img src="./docs/public/sponsors/diffusionstudio.png" width="60" height="60" alt="Diffusion Studio">
   </a>
+  &nbsp;&nbsp;&nbsp;&nbsp;
+  <a href="https://kino.ai/" target="_blank">
+    <img src="./docs/public/sponsors/kino.jpg" width="60" height="60" alt="Kino">
+  </a>
 </div>
 
-[Get featured](https://github.com/sponsors/Vanilagy)
+[Sponsor Mediabunny's development](https://github.com/sponsors/Vanilagy)
 
 ## Features
 
@@ -49,6 +53,8 @@ Core features include:
 npm install mediabunny
 ```
 
+Requires any JavaScript environment that can run ECMAScript 2021 or later. Mediabunny is expected to be run in modern browsers. For types, TypeScript 5.7 or later is required.
+
 ### Read file metadata
 
 ```js

dev/convert.html

@@ -70,8 +70,10 @@
 			},
 			*/
 			video: {
-				forceTranscode: true,
-				codec: 'av1',
+				frameRate: 27.123,
+				//width: 320,
+				//forceTranscode: true,
+				//codec: 'av1',
 				//discard: true,
 				//width: 1280,
 				//discard: true,

dev/demux.html

@@ -16,6 +16,28 @@
 			source
 		});
 
+		const audioTrack = await input.getPrimaryAudioTrack();
+		const sink = new Mediabunny.EncodedPacketSink(audioTrack);
+
+		let thing = await sink.getFirstPacket();
+		while (thing) {
+			console.log(thing);
+			if (thing.timestamp >= 2.4) break;
+
+			thing = await sink.getNextPacket(thing);
+
+		}
+		console.log("done") 
+
+		/*
+		for await (const packet of sink.packets()) {
+			console.log(packet);
+
+			if (packet.timestamp >= 2.4) break;
+		}
+		*/
+
+		/*
 		const videoTrack = await input.getPrimaryVideoTrack();
 		const packetSink = new Mediabunny.EncodedPacketSink(videoTrack);
 		const sampleSink = new Mediabunny.VideoSampleSink(videoTrack);
@@ -29,6 +51,7 @@
 			}
 		}
 		console.log("don")
+		*/
 
 		/*
 		console.time()

dev/live.html

@@ -23,21 +23,29 @@
             format: new Mediabunny.Mp4OutputFormat(),
         });
         if (videoTrack) {
-            output.addVideoTrack(new Mediabunny.MediaStreamVideoTrackSource(videoTrack, {
+            const source = new Mediabunny.MediaStreamVideoTrackSource(videoTrack, {
                 codec: 'avc',
                 bitrate: Mediabunny.QUALITY_MEDIUM
-            }));
+            });
+
+            source.errorPromise.catch((d) => console.log("Hello?????", d));
+
+            output.addVideoTrack(source);
         }
         if (audioTrack) {
-            output.addAudioTrack(new Mediabunny.MediaStreamAudioTrackSource(audioTrack, {
+            const source = new Mediabunny.MediaStreamAudioTrackSource(audioTrack, {
                 codec: 'aac',
                 bitrate: Mediabunny.QUALITY_MEDIUM
-            }));
+            });
+
+            source.errorPromise.catch((d) => console.log("Hello!!???", d));
+
+            output.addAudioTrack(source);
         }
 
         await output.start();
 
-        await new Promise(resolve => setTimeout(resolve, 3000));
+        await new Promise(resolve => setTimeout(resolve, 5000));
 
         await output.finalize();
 

docs/guide/converting-media-files.md

@@ -11,6 +11,7 @@ It has the following features:
 - Trimming
 - Video resizing & fitting
 - Video rotation
+- Video frame rate adjustment
 - Audio resampling
 - Audio up/downmixing
 
@@ -101,6 +102,7 @@ type ConversionOptions = {
 		height?: number;
 		fit?: 'fill' | 'contain' | 'cover';
 		rotate?: 0 | 90 | 180 | 270;
+		frameRate?: number;
 		codec?: VideoCodec;
 		bitrate?: number | Quality;
 		forceTranscode?: boolean;
@@ -141,6 +143,10 @@ The `width`, `height` and `fit` properties control how the video is resized. If
 
 If `width` or `height` is used in conjunction with `rotation`, they control the post-rotation dimensions.
 
+### Adjusting frame rate
+
+The `frameRate` property can be used to set the frame rate of the output video in Hz. If not specified, the original input frame rate will be used (which may be variable).
+
 ### Transcoding video
 
 Use the `codec` property to control the codec of the output track. This should be set to a [codec](./supported-formats-and-codecs#video-codecs) supported by the output file, or else the track will be [discarded](#discarded-tracks).

docs/guide/installation.md

@@ -17,6 +17,10 @@ bun add mediabunny
 ```
 :::
 
+::: info
+Requires any JavaScript environment that can run ECMAScript 2021 or later. Mediabunny is expected to be run in modern browsers. For types, TypeScript 5.7 or later is required.
+:::
+
 Then, simply import it like this:
 ```ts
 import { ... } from 'mediabunny'; // ESM

docs/guide/media-sources.md

@@ -163,6 +163,9 @@ const videoTrackSource = new MediaStreamVideoTrackSource(videoTrack, {
 	codec: 'vp9',
 	bitrate: 1e7,
 });
+
+// Make sure to allow any internal errors to properly bubble up
+videoTrackSource.errorPromise.catch((error) => ...);
 ```
 
 This source requires no additional method calls; data will automatically be captured and piped to the output file as soon as `start()` is called on the `Output`. Make sure to `stop()` on `videoTrack` after finalizing the `Output` if you don't need the user's media anymore.
@@ -171,6 +174,10 @@ This source requires no additional method calls; data will automatically be capt
 If this source is the only MediaStreamTrack source in the `Output`, then the first video sample added by it starts at timestamp 0. If there are multiple, then the earliest media sample across all tracks starts at timestamp 0, and all tracks will be perfectly synchronized with each other.
 :::
 
+::: warning
+`MediaStreamVideoTrackSource`'s internals are detached from the typical code flow but can still throw, so make sure to utilize `errorPromise` to deal with any errors and to stop the `Output`.
+:::
+
 ### `EncodedVideoPacketSource`
 
 The most barebones of all video sources, this source can be used to directly pipe [encoded packets](./packets-and-samples#encodedpacket) of video data to the output. This source requires that you take care of the encoding process yourself, which enables you to use the WebCodecs API manually or to plug in your own encoding stack. Alternatively, you may retrieve the encoded packets directly by reading them from another media file, allowing you to skip decoding and reencoding video data.
@@ -312,6 +319,9 @@ const audioTrackSource = new MediaStreamAudioTrackSource(audioTrack, {
 	codec: 'opus',
 	bitrate: 128e3,
 });
+
+// Make sure to allow any internal errors to properly bubble up
+audioTrackSource.errorPromise.catch((error) => ...);
 ```
 
 This source requires no additional method calls; data will automatically be captured and piped to the output file as soon as `start()` is called on the `Output`. Make sure to `stop()` on `audioTrack` after finalizing the `Output` if you don't need the user's media anymore.
@@ -320,6 +330,10 @@ This source requires no additional method calls; data will automatically be capt
 If this source is the only MediaStreamTrack source in the `Output`, then the first audio sample added by it starts at timestamp 0. If there are multiple, then the earliest media sample across all tracks starts at timestamp 0, and all tracks will be perfectly synchronized with each other.
 :::
 
+::: warning
+`MediaStreamAudioTrackSource`'s internals are detached from the typical code flow but can still throw, so make sure to utilize `errorPromise` to deal with any errors and to stop the `Output`.
+:::
+
 ### `EncodedAudioPacketSource`
 
 The most barebones of all audio sources, this source can be used to directly pipe [encoded packets](./packets-and-samples#encodedpacket) of audio data to the output. This source requires that you take care of the encoding process yourself, which enables you to use the WebCodecs API manually or to plug in your own encoding stack. Alternatively, you may retrieve the encoded packets directly by reading them from another media file, allowing you to skip decoding and reencoding audio data.

docs/guide/packets-and-samples.md

@@ -401,7 +401,7 @@ An audio sample represents a section of audio data. It can be created directly f
 
 ### Creating audio samples
 
-Audio samples can be constructed either from an `AudioData` instance or an initialization object:
+Audio samples can be constructed either from an `AudioData` instance, an initialization object, or an `AudioBuffer`:
 
 ```ts
 import { AudioSample } from 'mediabunny';
@@ -417,6 +417,11 @@ const sample = new AudioSample({
     sampleRate: 44100, // in Hz
     timestamp: 0, // in seconds
 });
+
+// From AudioBuffer:
+const timestamp = 0; // in seconds
+const samples = AudioSample.fromAudioBuffer(audioBuffer, timestamp);
+// => Returns multiple AudioSamples if the AudioBuffer is very long
 ```
 
 The following audio sample formats are supported:

docs/guide/quick-start.md

@@ -312,6 +312,7 @@ const output = new Output({
 const uploadComplete = fetch('https://example.com/upload', {
 	method: 'POST',
 	body: readable,
+	duplex: 'half',
 	headers: {
 		'Content-Type': output.format.mimeType,
 	},

docs/guide/reading-media-files.md

@@ -286,6 +286,18 @@ See [Media sinks](./media-sinks) for a full list of sinks.
 
 ### Examples
 
+Loop over all raw encoded packets of a track:
+```ts
+import { EncodedPacketSink } from 'mediabunny';
+
+const videoTrack = await input.getPrimaryVideoTrack();
+const sink = new EncodedPacketSink(videoTrack);
+
+for await (const packet of sink.packets()) {
+	console.log(packet.timestamp);
+}
+```
+
 Here we iterate over all samples (frames) of a video track:
 ```ts
 import { VideoSampleSink } from 'mediabunny';

docs/index.md

@@ -90,12 +90,15 @@ const sponsors = {
 	gold: [
 		{ image: '/sponsors/gling.svg', name: 'Gling AI', url: 'https://www.gling.ai/' },
 		{ image: '/sponsors/diffusionstudio.png', name: 'Diffusion Studio', url: 'https://diffusion.studio/' },
+		{ image: '/sponsors/kino.jpg', name: 'Kino', url: 'https://kino.ai/' },
 	],
 	individual: [
 		{ image: 'https://avatars.githubusercontent.com/u/84167135', name: 'Memenome', url: 'https://github.com/memenome' },
+		{ image: 'https://avatars.githubusercontent.com/u/9549394', name: 'studnitz', url: 'https://github.com/studnitz' },
 		{ image: 'https://avatars.githubusercontent.com/u/30229596', name: 'Pablo Bonilla', url: 'https://github.com/devPablo' },
 		{ image: 'https://avatars.githubusercontent.com/u/58149663', name: 'H7GhosT', url: 'https://github.com/H7GhosT' },
 		{ image: 'https://avatars.githubusercontent.com/u/91711202', name: 'ihasq', url: 'https://github.com/ihasq' },
+		{ image: 'https://avatars.githubusercontent.com/u/61233224', name: 'Allwhy', url: 'https://github.com/Allwhy' },
 	],
 };
 </script>

examples/file-compression/file-compression.ts

@@ -91,13 +91,17 @@ const compressFile = async (file: File) => {
 
 		// Display the final media file
 		videoElement.style.display = '';
-		videoElement.src = URL.createObjectURL(new Blob([output.target.buffer!]));
+		videoElement.src = URL.createObjectURL(new Blob([output.target.buffer!], { type: output.format.mimeType }));
 		void videoElement.play();
 
 		compressionFacts.style.display = '';
 		compressionFacts.textContent
 			= `${(output.target.buffer!.byteLength / file.size * 100).toPrecision(3)}% of original size`;
 	} catch (error) {
+		console.error(error);
+
+		await currentConversion?.cancel();
+
 		errorElement.textContent = String(error);
 		clearInterval(currentIntervalId);
 

examples/live-recording/index.html

@@ -22,6 +22,7 @@
 		<hr class="w-full max-w-96 my-4 border-zinc-300 dark:border-zinc-700" style="display: none;">
 
 		<p id="error-element" class="text-red-500"></p>
+		<p id="warning-element" class="text-amber-500"></p>
 
 		<div class="flex gap-4" id="main-container" style="display: none;">
 			<div class="flex flex-col items-center">

examples/live-recording/live-recording.ts

@@ -1,4 +1,5 @@
 import {
+	canEncodeAudio,
 	CanvasSource,
 	MediaStreamAudioTrackSource,
 	Mp4OutputFormat,
@@ -13,6 +14,7 @@ const mainContainer = document.querySelector('#main-container') as HTMLDivElemen
 const videoElement = document.querySelector('video') as HTMLVideoElement;
 const downloadButton = document.querySelector('#download-button') as HTMLAnchorElement;
 const errorElement = document.querySelector('#error-element') as HTMLParagraphElement;
+const warningElement = document.querySelector('#warning-element') as HTMLParagraphElement;
 
 const canvas = document.querySelector('canvas') as HTMLCanvasElement;
 const context = canvas.getContext('2d', { alpha: false, desynchronized: true })!;
@@ -39,19 +41,29 @@ const startRecording = async () => {
 		videoElement.src = '';
 		downloadButton.style.display = 'none';
 		errorElement.textContent = '';
+		warningElement.textContent = '';
 
 		// Paint a white background to the canvas
 		context.fillStyle = 'white';
 		context.fillRect(0, 0, canvas.width, canvas.height);
 
-		// Get user microphone
-		mediaStream = await navigator.mediaDevices.getUserMedia({ audio: true });
+		const audioIsEncodable = await canEncodeAudio('opus', {
+			bitrate: QUALITY_MEDIUM,
+		});
+
+		let audioTrack: MediaStreamAudioTrack | null = null;
+		if (audioIsEncodable) {
+			// Get user microphone
+			mediaStream = await navigator.mediaDevices.getUserMedia({ audio: true });
+			audioTrack = mediaStream.getAudioTracks()[0] ?? null;
+		} else {
+			warningElement.textContent
+				= 'Audio is not yet encodable by your browser, so the audio track has been omitted.';
+		}
 
 		horizontalRule.style.display = '';
 		mainContainer.style.display = '';
 
-		const audioTrack = mediaStream.getAudioTracks()[0];
-
 		// Create a new output file
 		output = new Output({
 			// We're using fragmented MP4 here; streamable WebM would also work
@@ -99,6 +111,8 @@ const startRecording = async () => {
 				codec: 'opus',
 				bitrate: QUALITY_MEDIUM,
 			});
+			audioSource.errorPromise.catch(cancelRecording); // Make sure errors are bubbled up
+
 			output.addAudioTrack(audioSource);
 		}
 
@@ -108,9 +122,9 @@ const startRecording = async () => {
 		readyForMoreFrames = true;
 		lastFrameNumber = -1;
 
-		// Start the video frame capture loop
-		void addVideoFrame();
-		videoCaptureInterval = window.setInterval(() => void addVideoFrame(), 1000 / frameRate);
+		// Start the video frame capture loop, making sure errors are caught
+		void addVideoFrame().catch(cancelRecording);
+		videoCaptureInterval = window.setInterval(() => void addVideoFrame().catch(cancelRecording), 1000 / frameRate);
 
 		const mimeType = await output.getMimeType();
 		sourceBuffer = mediaSource.addSourceBuffer(mimeType);
@@ -121,21 +135,35 @@ const startRecording = async () => {
 		toggleRecordingButton.textContent = 'Stop recording';
 		toggleRecordingButton.disabled = false;
 	} catch (error) {
-		errorElement.textContent = String(error);
-
-		mainContainer.style.display = 'none';
-		toggleRecordingButton.textContent = 'Start recording';
-		toggleRecordingButton.disabled = false;
-		recording = false;
+		await cancelRecording(error);
 	}
 };
 
+const cancelRecording = async (error: unknown) => {
+	if (!recording) {
+		return; // Already canceled
+	}
+
+	console.error(error);
+
+	errorElement.textContent = String(error);
+
+	clearInterval(videoCaptureInterval);
+	mainContainer.style.display = 'none';
+	toggleRecordingButton.textContent = 'Start recording';
+	toggleRecordingButton.disabled = false;
+	recording = false;
+	await output?.cancel();
+
+	mediaStream?.getTracks().forEach(track => track.stop());
+};
+
 const stopRecording = async () => {
 	toggleRecordingButton.textContent = 'Stopping...';
 	toggleRecordingButton.disabled = true;
 
 	clearInterval(videoCaptureInterval);
-	mediaStream.getTracks().forEach(track => track.stop());
+	mediaStream?.getTracks().forEach(track => track.stop());
 
 	await output.finalize();
 

examples/media-player/media-player.ts

@@ -175,8 +175,10 @@ const initMediaPlayer = async (file: File) => {
 			controlsElement.style.opacity = '1';
 			playerContainer.style.cursor = '';
 		}
-	} catch (e) {
-		errorElement.textContent = String(e);
+	} catch (error) {
+		console.error(error);
+
+		errorElement.textContent = String(error);
 		playerContainer.style.display = 'none';
 	}
 };

examples/metadata-extraction/metadata-extraction.ts

@@ -129,6 +129,8 @@ const renderObject = (object: Record<string, unknown>) => {
 				listItem.removeChild(loadingSpan);
 				listItem.appendChild(renderValue(resolvedValue));
 			}).catch((error) => {
+				console.error(error);
+
 				// Show the promise error
 				listItem.removeChild(loadingSpan);
 				const errorSpan = document.createElement('span');

examples/procedural-generation/procedural-generation.ts

@@ -7,6 +7,7 @@ import {
 	QUALITY_HIGH,
 	getFirstEncodableAudioCodec,
 	getFirstEncodableVideoCodec,
+	OutputFormat,
 } from 'mediabunny';
 
 const durationSlider = document.querySelector('#duration-slider') as HTMLInputElement;
@@ -57,6 +58,8 @@ let currentScaleIndex = 0;
 let collisionCount = 0;
 let collisionsPerScale = 0;
 
+let output: Output<OutputFormat, BufferTarget>;
+
 /** === MAIN VIDEO FILE GENERATION LOGIC === */
 
 const generateVideo = async () => {
@@ -82,7 +85,7 @@ const generateVideo = async () => {
 		initScene(duration);
 
 		// Create a new output file
-		const output = new Output({
+		output = new Output({
 			target: new BufferTarget(), // Stored in memory
 			format: new Mp4OutputFormat(),
 		});
@@ -177,13 +180,17 @@ const generateVideo = async () => {
 		videoInfo.style.display = '';
 
 		// Display and play the resulting media file
-		const videoBlob = new Blob([output.target.buffer!], { type: 'video/mp4' });
+		const videoBlob = new Blob([output.target.buffer!], { type: output.format.mimeType });
 		resultVideo.src = URL.createObjectURL(videoBlob);
 		void resultVideo.play();
 
 		const fileSizeMiB = (videoBlob.size / (1024 * 1024)).toPrecision(3);
 		videoInfo.textContent = `File size: ${fileSizeMiB} MiB`;
 	} catch (error) {
+		console.error(error);
+
+		await output?.cancel();
+
 		clearInterval(progressInterval);
 		errorElement.textContent = String(error);
 		progressBarContainer.style.display = 'none';

examples/thumbnail-generation/thumbnail-generation.ts

@@ -96,8 +96,10 @@ const generateThumbnails = async (file: File) => {
 
 			i++;
 		}
-	} catch (e) {
-		errorElement.textContent = String(e);
+	} catch (error) {
+		console.error(error);
+
+		errorElement.textContent = String(error);
 		thumbnailContainer.innerHTML = '';
 	}
 };

package-lock.json

@@ -1,16 +1,16 @@
 {
   "name": "mediabunny",
-  "version": "1.3.1",
+  "version": "1.4.4",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "mediabunny",
-      "version": "1.3.1",
+      "version": "1.4.4",
       "license": "MPL-2.0",
       "dependencies": {
         "@types/dom-mediacapture-transform": "^0.1.11",
-        "@types/dom-webcodecs": "^0.1.15"
+        "@types/dom-webcodecs": "0.1.13"
       },
       "devDependencies": {
         "@eslint/js": "^9.22.0",
@@ -2443,9 +2443,9 @@
       }
     },
     "node_modules/@types/dom-webcodecs": {
-      "version": "0.1.15",
-      "resolved": "https://registry.npmjs.org/@types/dom-webcodecs/-/dom-webcodecs-0.1.15.tgz",
-      "integrity": "sha512-omOlCPvTWyPm4ZE5bZUhlSvnHM2ZWM2U+1cPiYFL/e8aV5O9MouELp+L4dMKNTON0nTeHqEg+KWDfFQMY5Wkaw==",
+      "version": "0.1.13",
+      "resolved": "https://registry.npmjs.org/@types/dom-webcodecs/-/dom-webcodecs-0.1.13.tgz",
+      "integrity": "sha512-O5hkiFIcjjszPIYyUSyvScyvrBoV3NOEEZx/pMlsu44TKzWNkLVBBxnxJz42in5n3QIolYOcBYFCPZZ0h8SkwQ==",
       "license": "MIT"
     },
     "node_modules/@types/estree": {

package.json

@@ -1,7 +1,7 @@
 {
   "name": "mediabunny",
   "author": "Vanilagy",
-  "version": "1.3.1",
+  "version": "1.5.0",
   "description": "Pure TypeScript media toolkit for reading, writing, and converting media files, directly in the browser.",
   "type": "module",
   "main": "./dist/bundles/mediabunny.cjs",
@@ -31,7 +31,7 @@
     "docs:preview": "vitepress preview docs",
     "dev": "vite",
     "examples:build": "vite build",
-    "fix-build-import-paths": "find dist -name \"*.js\" -type f -exec sed -i '' \"s/ from '\\([^']*\\)';/ from '\\1.js';/g\" {} \\;",
+    "fix-build-import-paths": "tsx scripts/add-import-extensions.ts",
     "append-namespace": "echo 'export as namespace Mediabunny;' >> dist/mediabunny.d.ts"
   },
   "license": "MPL-2.0",
@@ -49,7 +49,7 @@
   },
   "dependencies": {
     "@types/dom-mediacapture-transform": "^0.1.11",
-    "@types/dom-webcodecs": "^0.1.15"
+    "@types/dom-webcodecs": "0.1.13"
   },
   "devDependencies": {
     "@eslint/js": "^9.22.0",

scripts/add-import-extensions.ts

@@ -0,0 +1,37 @@
+import * as fs from 'fs';
+import * as path from 'path';
+
+// .js extensions are technically required in compliant ECMAScript, and Webpack needs them, so we add them here.
+
+const walkDir = (dir: string) => {
+	const files: string[] = [];
+	const items = fs.readdirSync(dir);
+
+	for (const item of items) {
+		const fullPath = path.join(dir, item);
+		const stat = fs.statSync(fullPath);
+
+		if (stat.isDirectory()) {
+			files.push(...walkDir(fullPath));
+		} else if (item.endsWith('.js')) {
+			files.push(fullPath);
+		}
+	}
+
+	return files;
+};
+
+const fixFile = (filePath: string) => {
+	const content = fs.readFileSync(filePath, 'utf8');
+	const fixed = content.replace(
+		/(\s+from\s+['"])([^'"]*)(['"])/g,
+		'$1$2.js$3',
+	);
+
+	if (content !== fixed) {
+		fs.writeFileSync(filePath, fixed);
+	}
+};
+
+const jsFiles = walkDir('dist');
+jsFiles.forEach(fixFile);

scripts/check-docblocks.ts

@@ -1,5 +1,5 @@
 import ts from 'typescript';
-import * as fs from 'node:fs';
+import * as fs from 'fs';
 
 const checkDocblocks = (filePath: string) => {
 	const program = ts.createProgram([filePath], {});
@@ -17,6 +17,8 @@ const checkDocblocks = (filePath: string) => {
 			ts.isInterfaceDeclaration(node)
 			|| ts.isClassDeclaration(node)
 			|| ts.isMethodDeclaration(node)
+			|| ts.isGetAccessorDeclaration(node)
+			|| ts.isSetAccessorDeclaration(node)
 			|| ts.isPropertyDeclaration(node)
 			|| ts.isFunctionDeclaration(node)
 			|| ts.isTypeAliasDeclaration(node)

src/conversion.ts

@@ -77,6 +77,11 @@ export type ConversionOptions = {
 		 * rotation is _in addition to_ the natural rotation of the input video as specified in input file's metadata.
 		 */
 		rotate?: Rotation;
+		/**
+		 * The desired frame rate of the output video, in hertz. If not specified, the original input frame rate will
+		 * be used (which may be variable).
+		 */
+		frameRate?: number;
 		/** The desired output video codec. */
 		codec?: VideoCodec;
 		/** The desired bitrate of the output video. */
@@ -261,8 +266,14 @@ export class Conversion {
 		if (options.video?.rotate !== undefined && ![0, 90, 180, 270].includes(options.video.rotate)) {
 			throw new TypeError('options.video.rotate, when provided, must be 0, 90, 180 or 270.');
 		}
+		if (
+			options.video?.frameRate !== undefined
+			&& (!Number.isFinite(options.video.frameRate) || options.video.frameRate <= 0)
+		) {
+			throw new TypeError('options.video.frameRate, when provided, must be a finite positive number.');
+		}
 		if (options.audio !== undefined && (!options.audio || typeof options.audio !== 'object')) {
-			throw new TypeError('options.video, when provided, must be an object.');
+			throw new TypeError('options.audio, when provided, must be an object.');
 		}
 		if (options.audio?.discard !== undefined && typeof options.audio.discard !== 'boolean') {
 			throw new TypeError('options.audio.discard, when provided, must be a boolean.');
@@ -470,7 +481,10 @@ export class Conversion {
 		}
 
 		const firstTimestamp = await track.getFirstTimestamp();
-		const needsTranscode = !!this._options.video?.forceTranscode || this._startTimestamp > 0 || firstTimestamp < 0;
+		const needsTranscode = !!this._options.video?.forceTranscode
+			|| this._startTimestamp > 0
+			|| firstTimestamp < 0
+			|| !!this._options.video?.frameRate;
 		const needsRerender = width !== originalWidth
 			|| height !== originalHeight
 			|| (totalRotation !== 0 && !outputSupportsRotation);
@@ -547,10 +561,10 @@ export class Conversion {
 				onEncodedPacket: sample => this._reportProgress(track.id, sample.timestamp + sample.duration),
 			};
 
-			if (needsRerender) {
-				const source = new VideoSampleSource(encodingConfig);
-				videoSource = source;
+			const source = new VideoSampleSource(encodingConfig);
+			videoSource = source;
 
+			if (needsRerender) {
 				this._trackPromises.push((async () => {
 					await this._started;
 
@@ -562,6 +576,27 @@ export class Conversion {
 						poolSize: 1,
 					});
 					const iterator = sink.canvases(this._startTimestamp, this._endTimestamp);
+					const frameRate = this._options.video?.frameRate;
+
+					let lastCanvas: HTMLCanvasElement | OffscreenCanvas | null = null;
+					let lastCanvasTimestamp: number | null = null;
+					let lastCanvasEndTimestamp: number | null = null;
+
+					/** Repeats the last sample to pad out the time until the specified timestamp. */
+					const padFrames = async (until: number) => {
+						assert(lastCanvas);
+						assert(frameRate !== undefined);
+
+						const frameDifference = Math.round((until - lastCanvasTimestamp!) * frameRate);
+
+						for (let i = 1; i < frameDifference; i++) {
+							const sample = new VideoSample(lastCanvas, {
+								timestamp: lastCanvasTimestamp! + i / frameRate,
+								duration: 1 / frameRate,
+							});
+							await source.add(sample);
+						}
+					};
 
 					for await (const { canvas, timestamp, duration } of iterator) {
 						if (this._synchronizer.shouldWait(track.id, timestamp)) {
@@ -572,37 +607,134 @@ export class Conversion {
 							return;
 						}
 
+						let adjustedSampleTimestamp = Math.max(timestamp - this._startTimestamp, 0);
+						lastCanvasEndTimestamp = timestamp + duration;
+
+						if (frameRate !== undefined) {
+							// Logic for skipping/repeating frames when a frame rate is set
+							const alignedTimestamp = Math.floor(adjustedSampleTimestamp * frameRate) / frameRate;
+
+							if (lastCanvas !== null) {
+								if (alignedTimestamp <= lastCanvasTimestamp!) {
+									lastCanvas = canvas;
+									lastCanvasTimestamp = alignedTimestamp;
+
+									// Skip this sample, since we already added one for this frame
+									continue;
+								} else {
+									// Check if we may need to repeat the previous frame
+									await padFrames(alignedTimestamp);
+								}
+							}
+
+							adjustedSampleTimestamp = alignedTimestamp;
+						}
+
 						const sample = new VideoSample(canvas, {
-							timestamp: Math.max(timestamp - this._startTimestamp, 0),
-							duration,
+							timestamp: adjustedSampleTimestamp,
+							duration: frameRate !== undefined ? 1 / frameRate : duration,
 						});
 
 						await source.add(sample);
-						sample.close();
+
+						if (frameRate !== undefined) {
+							lastCanvas = canvas;
+							lastCanvasTimestamp = adjustedSampleTimestamp;
+						} else {
+							sample.close();
+						}
 					}
+
+					if (lastCanvas) {
+						assert(lastCanvasEndTimestamp !== null);
+						assert(frameRate !== undefined);
+
+						// If necessary, pad until the end timestamp of the last sample
+						await padFrames(Math.floor(lastCanvasEndTimestamp * frameRate) / frameRate);
+					}
+
+					source.close();
+					this._synchronizer.closeTrack(track.id);
 				})());
 			} else {
-				const source = new VideoSampleSource(encodingConfig);
-				videoSource = source;
-
 				this._trackPromises.push((async () => {
 					await this._started;
 
 					const sink = new VideoSampleSink(track);
+					const frameRate = this._options.video?.frameRate;
+
+					let lastSample: VideoSample | null = null;
+					let lastSampleTimestamp: number | null = null;
+					let lastSampleEndTimestamp: number | null = null;
+
+					/** Repeats the last sample to pad out the time until the specified timestamp. */
+					const padFrames = async (until: number) => {
+						assert(lastSample);
+						assert(frameRate !== undefined);
+
+						const frameDifference = Math.round((until - lastSampleTimestamp!) * frameRate);
+
+						for (let i = 1; i < frameDifference; i++) {
+							lastSample.setTimestamp(lastSampleTimestamp! + i / frameRate);
+							lastSample.setDuration(1 / frameRate);
+							await source.add(lastSample);
+						}
+
+						lastSample.close();
+					};
 
 					for await (const sample of sink.samples(this._startTimestamp, this._endTimestamp)) {
 						if (this._synchronizer.shouldWait(track.id, sample.timestamp)) {
 							await this._synchronizer.wait(sample.timestamp);
 						}
 
-						sample.setTimestamp(Math.max(sample.timestamp - this._startTimestamp, 0));
-
 						if (this._canceled) {
+							lastSample?.close();
 							return;
 						}
 
+						let adjustedSampleTimestamp = Math.max(sample.timestamp - this._startTimestamp, 0);
+						lastSampleEndTimestamp = sample.timestamp + sample.duration;
+
+						if (frameRate !== undefined) {
+							// Logic for skipping/repeating frames when a frame rate is set
+							const alignedTimestamp = Math.floor(adjustedSampleTimestamp * frameRate) / frameRate;
+
+							if (lastSample !== null) {
+								if (alignedTimestamp <= lastSampleTimestamp!) {
+									lastSample.close();
+									lastSample = sample;
+									lastSampleTimestamp = alignedTimestamp;
+
+									// Skip this sample, since we already added one for this frame
+									continue;
+								} else {
+									// Check if we may need to repeat the previous frame
+									await padFrames(alignedTimestamp);
+								}
+							}
+
+							adjustedSampleTimestamp = alignedTimestamp;
+							sample.setDuration(1 / frameRate);
+						}
+
+						sample.setTimestamp(adjustedSampleTimestamp);
 						await source.add(sample);
-						sample.close();
+
+						if (frameRate !== undefined) {
+							lastSample = sample;
+							lastSampleTimestamp = adjustedSampleTimestamp;
+						} else {
+							sample.close();
+						}
+					}
+
+					if (lastSample) {
+						assert(lastSampleEndTimestamp !== null);
+						assert(frameRate !== undefined);
+
+						// If necessary, pad until the end timestamp of the last sample
+						await padFrames(Math.floor(lastSampleEndTimestamp * frameRate) / frameRate);
 					}
 
 					source.close();
@@ -612,6 +744,7 @@ export class Conversion {
 		}
 
 		this.output.addVideoTrack(videoSource, {
+			frameRate: this._options.video?.frameRate,
 			languageCode: track.languageCode,
 			rotation: needsRerender ? 0 : totalRotation, // Rerendering will bake the rotation into the output
 		});

src/isobmff/isobmff-demuxer.ts

@@ -1941,44 +1941,46 @@ abstract class IsobmffTrackBacking implements InputTrackBacking {
 	}
 
 	async getFirstPacket(options: PacketRetrievalOptions) {
-		if (this.internalTrack.demuxer.isFragmented) {
-			return this.performFragmentedLookup(
-				() => {
-					const startFragment = this.internalTrack.demuxer.fragments[0] ?? null;
-					if (startFragment?.isKnownToBeFirstFragment) {
-						// Walk from the very first fragment in the file until we find one with our track in it
-						let currentFragment: Fragment | null = startFragment;
-						while (currentFragment) {
-							const trackData = currentFragment.trackData.get(this.internalTrack.id);
-							if (trackData) {
-								return {
-									fragmentIndex: binarySearchExact(
-										this.internalTrack.fragments,
-										currentFragment.moofOffset,
-										x => x.moofOffset,
-									),
-									sampleIndex: 0,
-									correctSampleFound: true,
-								};
-							}
-
-							currentFragment = currentFragment.nextFragment;
-						}
-					}
-
-					return {
-						fragmentIndex: -1,
-						sampleIndex: -1,
-						correctSampleFound: false,
-					};
-				},
-				-Infinity, // Use -Infinity as a search timestamp to avoid using the lookup entries
-				Infinity,
-				options,
-			);
+		const regularPacket = await this.fetchPacketForSampleIndex(0, options);
+		if (regularPacket || !this.internalTrack.demuxer.isFragmented) {
+			// If there's a non-fragmented packet, always prefer that
+			return regularPacket;
 		}
 
-		return this.fetchPacketForSampleIndex(0, options);
+		return this.performFragmentedLookup(
+			() => {
+				const startFragment = this.internalTrack.demuxer.fragments[0] ?? null;
+				if (startFragment?.isKnownToBeFirstFragment) {
+					// Walk from the very first fragment in the file until we find one with our track in it
+					let currentFragment: Fragment | null = startFragment;
+					while (currentFragment) {
+						const trackData = currentFragment.trackData.get(this.internalTrack.id);
+						if (trackData) {
+							return {
+								fragmentIndex: binarySearchExact(
+									this.internalTrack.fragments,
+									currentFragment.moofOffset,
+									x => x.moofOffset,
+								),
+								sampleIndex: 0,
+								correctSampleFound: true,
+							};
+						}
+
+						currentFragment = currentFragment.nextFragment;
+					}
+				}
+
+				return {
+					fragmentIndex: -1,
+					sampleIndex: -1,
+					correctSampleFound: false,
+				};
+			},
+			-Infinity, // Use -Infinity as a search timestamp to avoid using the lookup entries
+			Infinity,
+			options,
+		);
 	}
 
 	private mapTimestampIntoTimescale(timestamp: number) {
@@ -1991,185 +1993,186 @@ abstract class IsobmffTrackBacking implements InputTrackBacking {
 	async getPacket(timestamp: number, options: PacketRetrievalOptions) {
 		const timestampInTimescale = this.mapTimestampIntoTimescale(timestamp);
 
-		if (this.internalTrack.demuxer.isFragmented) {
-			return this.performFragmentedLookup(
-				() => this.findSampleInFragmentsForTimestamp(timestampInTimescale),
-				timestampInTimescale,
-				timestampInTimescale,
-				options,
-			);
-		} else {
-			const sampleTable = this.internalTrack.demuxer.getSampleTableForTrack(this.internalTrack);
-			const sampleIndex = getSampleIndexForTimestamp(sampleTable, timestampInTimescale);
-			return this.fetchPacketForSampleIndex(sampleIndex, options);
+		const sampleTable = this.internalTrack.demuxer.getSampleTableForTrack(this.internalTrack);
+		const sampleIndex = getSampleIndexForTimestamp(sampleTable, timestampInTimescale);
+		const regularPacket = await this.fetchPacketForSampleIndex(sampleIndex, options);
+
+		if (!sampleTableIsEmpty(sampleTable) || !this.internalTrack.demuxer.isFragmented) {
+			// Prefer the non-fragmented packet
+			return regularPacket;
 		}
+
+		return this.performFragmentedLookup(
+			() => this.findSampleInFragmentsForTimestamp(timestampInTimescale),
+			timestampInTimescale,
+			timestampInTimescale,
+			options,
+		);
 	}
 
 	async getNextPacket(packet: EncodedPacket, options: PacketRetrievalOptions) {
-		if (this.internalTrack.demuxer.isFragmented) {
-			const locationInFragment = this.packetToFragmentLocation.get(packet);
-			if (locationInFragment === undefined) {
-				throw new Error('Packet was not created from this track.');
-			}
+		const regularSampleIndex = this.packetToSampleIndex.get(packet);
 
-			const trackData = locationInFragment.fragment.trackData.get(this.internalTrack.id)!;
-			const fragmentSample = trackData.samples[locationInFragment.sampleIndex]!;
-
-			const fragmentIndex = binarySearchExact(
-				this.internalTrack.fragments,
-				locationInFragment.fragment.moofOffset,
-				x => x.moofOffset,
-			);
-			assert(fragmentIndex !== -1);
-
-			return this.performFragmentedLookup(
-				() => {
-					if (locationInFragment.sampleIndex + 1 < trackData.samples.length) {
-						// We can simply take the next sample in the fragment
-						return {
-							fragmentIndex,
-							sampleIndex: locationInFragment.sampleIndex + 1,
-							correctSampleFound: true,
-						};
-					} else {
-						// Walk the list of fragments until we find the next fragment for this track
-						let currentFragment = locationInFragment.fragment;
-						while (currentFragment.nextFragment) {
-							currentFragment = currentFragment.nextFragment;
-
-							const trackData = currentFragment.trackData.get(this.internalTrack.id);
-							if (trackData) {
-								const fragmentIndex = binarySearchExact(
-									this.internalTrack.fragments,
-									currentFragment.moofOffset,
-									x => x.moofOffset,
-								);
-								assert(fragmentIndex !== -1);
-
-								return {
-									fragmentIndex,
-									sampleIndex: 0,
-									correctSampleFound: true,
-								};
-							}
-						}
-
-						return {
-							fragmentIndex,
-							sampleIndex: -1,
-							correctSampleFound: false,
-						};
-					}
-				},
-				fragmentSample.presentationTimestamp,
-				Infinity,
-				options,
-			);
+		if (regularSampleIndex !== undefined) {
+			// Prefer the non-fragmented packet
+			return this.fetchPacketForSampleIndex(regularSampleIndex + 1, options);
 		}
 
-		const sampleIndex = this.packetToSampleIndex.get(packet);
-		if (sampleIndex === undefined) {
+		const locationInFragment = this.packetToFragmentLocation.get(packet);
+		if (locationInFragment === undefined) {
 			throw new Error('Packet was not created from this track.');
 		}
-		return this.fetchPacketForSampleIndex(sampleIndex + 1, options);
+
+		const trackData = locationInFragment.fragment.trackData.get(this.internalTrack.id)!;
+
+		const fragmentIndex = binarySearchExact(
+			this.internalTrack.fragments,
+			locationInFragment.fragment.moofOffset,
+			x => x.moofOffset,
+		);
+		assert(fragmentIndex !== -1);
+
+		return this.performFragmentedLookup(
+			() => {
+				if (locationInFragment.sampleIndex + 1 < trackData.samples.length) {
+					// We can simply take the next sample in the fragment
+					return {
+						fragmentIndex,
+						sampleIndex: locationInFragment.sampleIndex + 1,
+						correctSampleFound: true,
+					};
+				} else {
+					// Walk the list of fragments until we find the next fragment for this track
+					let currentFragment = locationInFragment.fragment;
+					while (currentFragment.nextFragment) {
+						currentFragment = currentFragment.nextFragment;
+
+						const trackData = currentFragment.trackData.get(this.internalTrack.id);
+						if (trackData) {
+							const fragmentIndex = binarySearchExact(
+								this.internalTrack.fragments,
+								currentFragment.moofOffset,
+								x => x.moofOffset,
+							);
+							assert(fragmentIndex !== -1);
+
+							return {
+								fragmentIndex,
+								sampleIndex: 0,
+								correctSampleFound: true,
+							};
+						}
+					}
+
+					return {
+						fragmentIndex,
+						sampleIndex: -1,
+						correctSampleFound: false,
+					};
+				}
+			},
+			-Infinity, // Use -Infinity as a search timestamp to avoid using the lookup entries
+			Infinity,
+			options,
+		);
 	}
 
 	async getKeyPacket(timestamp: number, options: PacketRetrievalOptions) {
 		const timestampInTimescale = this.mapTimestampIntoTimescale(timestamp);
 
-		if (this.internalTrack.demuxer.isFragmented) {
-			return this.performFragmentedLookup(
-				() => this.findKeySampleInFragmentsForTimestamp(timestampInTimescale),
-				timestampInTimescale,
-				timestampInTimescale,
-				options,
-			);
-		}
-
 		const sampleTable = this.internalTrack.demuxer.getSampleTableForTrack(this.internalTrack);
 		const sampleIndex = getSampleIndexForTimestamp(sampleTable, timestampInTimescale);
 		const keyFrameSampleIndex = sampleIndex === -1
 			? -1
 			: getRelevantKeyframeIndexForSample(sampleTable, sampleIndex);
-		return this.fetchPacketForSampleIndex(keyFrameSampleIndex, options);
+		const regularPacket = await this.fetchPacketForSampleIndex(keyFrameSampleIndex, options);
+
+		if (!sampleTableIsEmpty(sampleTable) || !this.internalTrack.demuxer.isFragmented) {
+			// Prefer the non-fragmented packet
+			return regularPacket;
+		}
+
+		return this.performFragmentedLookup(
+			() => this.findKeySampleInFragmentsForTimestamp(timestampInTimescale),
+			timestampInTimescale,
+			timestampInTimescale,
+			options,
+		);
 	}
 
 	async getNextKeyPacket(packet: EncodedPacket, options: PacketRetrievalOptions) {
-		if (this.internalTrack.demuxer.isFragmented) {
-			const locationInFragment = this.packetToFragmentLocation.get(packet);
-			if (locationInFragment === undefined) {
-				throw new Error('Packet was not created from this track.');
-			}
-
-			const trackData = locationInFragment.fragment.trackData.get(this.internalTrack.id)!;
-			const fragmentSample = trackData.samples[locationInFragment.sampleIndex]!;
-
-			const fragmentIndex = binarySearchExact(
-				this.internalTrack.fragments,
-				locationInFragment.fragment.moofOffset,
-				x => x.moofOffset,
-			);
-			assert(fragmentIndex !== -1);
-
-			return this.performFragmentedLookup(
-				() => {
-					const nextKeyFrameIndex = trackData.samples.findIndex(
-						(x, i) => x.isKeyFrame && i > locationInFragment.sampleIndex,
-					);
-
-					if (nextKeyFrameIndex !== -1) {
-						// We can simply take the next key frame in the fragment
-						return {
-							fragmentIndex,
-							sampleIndex: nextKeyFrameIndex,
-							correctSampleFound: true,
-						};
-					} else {
-						// Walk the list of fragments until we find the next fragment for this track with a key frame
-						let currentFragment = locationInFragment.fragment;
-						while (currentFragment.nextFragment) {
-							currentFragment = currentFragment.nextFragment;
-
-							const trackData = currentFragment.trackData.get(this.internalTrack.id);
-							if (trackData && trackData.firstKeyFrameTimestamp !== null) {
-								const fragmentIndex = binarySearchExact(
-									this.internalTrack.fragments,
-									currentFragment.moofOffset,
-									x => x.moofOffset,
-								);
-								assert(fragmentIndex !== -1);
-
-								const keyFrameIndex = trackData.samples.findIndex(x => x.isKeyFrame);
-								assert(keyFrameIndex !== -1); // There must be one
-
-								return {
-									fragmentIndex,
-									sampleIndex: keyFrameIndex,
-									correctSampleFound: true,
-								};
-							}
-						}
-
-						return {
-							fragmentIndex,
-							sampleIndex: -1,
-							correctSampleFound: false,
-						};
-					}
-				},
-				fragmentSample.presentationTimestamp,
-				Infinity,
-				options,
-			);
+		const regularSampleIndex = this.packetToSampleIndex.get(packet);
+		if (regularSampleIndex !== undefined) {
+			// Prefer the non-fragmented packet
+			const sampleTable = this.internalTrack.demuxer.getSampleTableForTrack(this.internalTrack);
+			const nextKeyFrameSampleIndex = getNextKeyframeIndexForSample(sampleTable, regularSampleIndex);
+			return this.fetchPacketForSampleIndex(nextKeyFrameSampleIndex, options);
 		}
 
-		const sampleIndex = this.packetToSampleIndex.get(packet);
-		if (sampleIndex === undefined) {
+		const locationInFragment = this.packetToFragmentLocation.get(packet);
+		if (locationInFragment === undefined) {
 			throw new Error('Packet was not created from this track.');
 		}
-		const sampleTable = this.internalTrack.demuxer.getSampleTableForTrack(this.internalTrack);
-		const nextKeyFrameSampleIndex = getNextKeyframeIndexForSample(sampleTable, sampleIndex);
-		return this.fetchPacketForSampleIndex(nextKeyFrameSampleIndex, options);
+
+		const trackData = locationInFragment.fragment.trackData.get(this.internalTrack.id)!;
+
+		const fragmentIndex = binarySearchExact(
+			this.internalTrack.fragments,
+			locationInFragment.fragment.moofOffset,
+			x => x.moofOffset,
+		);
+		assert(fragmentIndex !== -1);
+
+		return this.performFragmentedLookup(
+			() => {
+				const nextKeyFrameIndex = trackData.samples.findIndex(
+					(x, i) => x.isKeyFrame && i > locationInFragment.sampleIndex,
+				);
+
+				if (nextKeyFrameIndex !== -1) {
+					// We can simply take the next key frame in the fragment
+					return {
+						fragmentIndex,
+						sampleIndex: nextKeyFrameIndex,
+						correctSampleFound: true,
+					};
+				} else {
+					// Walk the list of fragments until we find the next fragment for this track with a key frame
+					let currentFragment = locationInFragment.fragment;
+					while (currentFragment.nextFragment) {
+						currentFragment = currentFragment.nextFragment;
+
+						const trackData = currentFragment.trackData.get(this.internalTrack.id);
+						if (trackData && trackData.firstKeyFrameTimestamp !== null) {
+							const fragmentIndex = binarySearchExact(
+								this.internalTrack.fragments,
+								currentFragment.moofOffset,
+								x => x.moofOffset,
+							);
+							assert(fragmentIndex !== -1);
+
+							const keyFrameIndex = trackData.samples.findIndex(x => x.isKeyFrame);
+							assert(keyFrameIndex !== -1); // There must be one
+
+							return {
+								fragmentIndex,
+								sampleIndex: keyFrameIndex,
+								correctSampleFound: true,
+							};
+						}
+					}
+
+					return {
+						fragmentIndex,
+						sampleIndex: -1,
+						correctSampleFound: false,
+					};
+				}
+			},
+			-Infinity, // Use -Infinity as a search timestamp to avoid using the lookup entries
+			Infinity,
+			options,
+		);
 	}
 
 	private async fetchPacketForSampleIndex(sampleIndex: number, options: PacketRetrievalOptions) {
@@ -2701,3 +2704,7 @@ const extractRotationFromMatrix = (matrix: TransformationMatrix) => {
 	// Invert the rotation because matrices are post-multiplied in ISOBMFF
 	return -Math.atan2(sinTheta, cosTheta) * (180 / Math.PI);
 };
+
+const sampleTableIsEmpty = (sampleTable: SampleTable) => {
+	return sampleTable.sampleSizes.length === 0;
+};

src/isobmff/isobmff-muxer.ts

@@ -32,6 +32,7 @@ import {
 	transformAnnexBToLengthPrefixed,
 } from '../codec-data';
 import { buildIsobmffMimeType } from './isobmff-misc';
+import { MAX_BOX_HEADER_SIZE, MIN_BOX_HEADER_SIZE } from './isobmff-reader';
 
 export const GLOBAL_TIMESCALE = 1000;
 const TIMESTAMP_OFFSET = 2_082_844_800; // Seconds between Jan 1 1904 and Jan 1 1970
@@ -989,10 +990,7 @@ export class IsobmffMuxer extends Muxer {
 		const moofOffset = this.writer.getPos();
 		const mdatStartPos = moofOffset + this.boxWriter.measureBox(moofBox);
 
-		// Header with large size. We always reserve 16 bytes for it even if we don't end up using the large size.
-		const mdatHeaderSize = 16;
-
-		let currentPos = mdatStartPos + mdatHeaderSize;
+		let currentPos = mdatStartPos + MIN_BOX_HEADER_SIZE;
 		let fragmentStartTimestamp = Infinity;
 		for (const trackData of tracksInFragment) {
 			trackData.currentChunk!.offset = currentPos;
@@ -1006,6 +1004,15 @@ export class IsobmffMuxer extends Muxer {
 		}
 
 		const mdatSize = currentPos - mdatStartPos;
+		const needsLargeMdatSize = mdatSize >= 2 ** 32;
+
+		if (needsLargeMdatSize) {
+			// Shift all offsets by 8. Previously, all chunks were shifted assuming the large box size, but due to what
+			// I suspect is a bug in WebKit, it failed in Safari (when livestreaming with MSE, not for static playback).
+			for (const trackData of tracksInFragment) {
+				trackData.currentChunk!.offset! += MAX_BOX_HEADER_SIZE - MIN_BOX_HEADER_SIZE;
+			}
+		}
 
 		if (this.format._options.onMoof) {
 			this.writer.startTrackingWrites();
@@ -1025,11 +1032,11 @@ export class IsobmffMuxer extends Muxer {
 			this.writer.startTrackingWrites();
 		}
 
-		const mdatBox = mdat(mdatSize >= 2 ** 32);
+		const mdatBox = mdat(needsLargeMdatSize);
 		mdatBox.size = mdatSize;
 		this.boxWriter.writeBox(mdatBox);
 
-		this.writer.seek(mdatStartPos + mdatHeaderSize);
+		this.writer.seek(mdatStartPos + (needsLargeMdatSize ? MAX_BOX_HEADER_SIZE : MIN_BOX_HEADER_SIZE));
 
 		// Write sample data
 		for (const trackData of tracksInFragment) {

src/matroska/ebml.ts

@@ -499,7 +499,13 @@ export class EBMLReader {
 		const { view, offset } = this.reader.getViewAndOffset(this.pos, this.pos + length);
 		this.pos += length;
 
-		return String.fromCharCode(...new Uint8Array(view.buffer, offset, length));
+		// Actual string length might be shorter due to null terminators
+		let strLength = 0;
+		while (strLength < length && view.getUint8(offset + strLength) !== 0) {
+			strLength += 1;
+		}
+
+		return String.fromCharCode(...new Uint8Array(view.buffer, offset, strLength));
 	}
 
 	readElementId() {
@@ -588,6 +594,38 @@ export const CODEC_STRING_MAP: Partial<Record<MediaCodec, string>> = {
 	'webvtt': 'S_TEXT/WEBVTT',
 };
 
+export const readVarInt = (data: Uint8Array, offset: number) => {
+	if (offset >= data.length) {
+		throw new Error('Offset out of bounds.');
+	}
+
+	// Read the first byte to determine the width of the variable-length integer
+	const firstByte = data[offset]!;
+
+	// Find the position of VINT_MARKER, which determines the width
+	let width = 1;
+	let mask = 1 << 7;
+	while ((firstByte & mask) === 0 && width < 8) {
+		width++;
+		mask >>= 1;
+	}
+
+	if (offset + width > data.length) {
+		throw new Error('VarInt extends beyond data bounds.');
+	}
+
+	// First byte's value needs the marker bit cleared
+	let value = firstByte & (mask - 1);
+
+	// Read remaining bytes
+	for (let i = 1; i < width; i++) {
+		value *= 1 << 8;
+		value += data[offset + i]!;
+	}
+
+	return { value, width };
+};
+
 export function assertDefinedSize(size: number | null): asserts size is number {
 	if (size === null) {
 		throw new Error('Undefined element size is used in a place where it is not supported.');

src/matroska/matroska-demuxer.ts

@@ -57,6 +57,7 @@ import {
 	LEVEL_0_AND_1_EBML_IDS,
 	MAX_HEADER_SIZE,
 	MIN_HEADER_SIZE,
+	readVarInt,
 } from './ebml';
 import { buildMatroskaMimeType } from './matroska-misc';
 
@@ -107,12 +108,20 @@ type ClusterTrackData = {
 	}[];
 };
 
+enum BlockLacing {
+	None,
+	Xiph,
+	FixedSize,
+	Ebml,
+}
+
 type ClusterBlock = {
 	timestamp: number;
 	duration: number;
 	isKeyFrame: boolean;
 	referencedTimestamps: number[];
 	data: Uint8Array;
+	lacing: BlockLacing;
 };
 
 type CuePoint = {
@@ -133,6 +142,7 @@ type InternalTrack = {
 	inputTrack: InputTrack | null;
 	codecId: string | null;
 	codecPrivate: Uint8Array | null;
+	defaultDuration: number | null;
 	languageCode: string;
 	info:
 		| null
@@ -220,13 +230,16 @@ export class MatroskaDemuxer extends Demuxer {
 
 			const fileSize = await this.input.source.getSize();
 
+			// Loop over all top-level elements in the file
 			while (this.metadataReader.pos <= fileSize - MIN_HEADER_SIZE) {
 				await this.metadataReader.reader.loadRange(
 					this.metadataReader.pos,
 					this.metadataReader.pos + MAX_HEADER_SIZE,
 				);
 
-				const { id, size } = this.metadataReader.readElementHeader();
+				const header = this.metadataReader.readElementHeader();
+				const id = header.id;
+				let size = header.size;
 				const startPos = this.metadataReader.pos;
 
 				if (id === EBMLId.EBML) {
@@ -242,6 +255,26 @@ export class MatroskaDemuxer extends Demuxer {
 						// and only segment
 						break;
 					}
+				} else if (id === EBMLId.Cluster) {
+					// Clusters are not a top-level element in Matroska, but some files contain a Segment whose size
+					// doesn't contain any of the clusters that follow it. In the case, we apply the following logic: if
+					// we find a top-level cluster, attribute it to the previous segment.
+
+					if (size === null) {
+						// Just in case this is one of those weird sizeless clusters, let's do our best and still try to
+						// determine its size.
+						const nextElementPos = await this.clusterReader.searchForNextElementId(
+							LEVEL_0_AND_1_EBML_IDS,
+							fileSize,
+						);
+						size = (nextElementPos ?? fileSize) - startPos;
+					}
+
+					const lastSegment = last(this.segments);
+					if (lastSegment) {
+						// Extend the previous segment's size
+						lastSegment.elementEndPos = startPos + size;
+					}
 				}
 
 				assertDefinedSize(size);
@@ -368,6 +401,13 @@ export class MatroskaDemuxer extends Demuxer {
 			this.readContiguousElements(this.metadataReader, size);
 		}
 
+		if (this.currentSegment.timestampScale === -1) {
+			// TimestampScale element is missing. Technically an invalid file, but let's default to the typical value,
+			// which is 1e6.
+			this.currentSegment.timestampScale = 1e6;
+			this.currentSegment.timestampFactor = 1e9 / 1e6;
+		}
+
 		// Put default tracks first
 		this.currentSegment.tracks.sort((a, b) => Number(b.isDefault) - Number(a.isDefault));
 
@@ -464,16 +504,20 @@ export class MatroskaDemuxer extends Demuxer {
 		this.readContiguousElements(this.clusterReader, size);
 
 		for (const [trackId, trackData] of cluster.trackData) {
-			let blockReferencesExist = false;
+			const track = segment.tracks.find(x => x.id === trackId) ?? null;
 
 			// This must hold, as track datas only get created if a block for that track is encountered
 			assert(trackData.blocks.length > 0);
 
+			let blockReferencesExist = false;
+			let hasLacedBlocks = false;
+
 			for (let i = 0; i < trackData.blocks.length; i++) {
 				const block = trackData.blocks[i]!;
 				block.timestamp += cluster.timestamp;
 
 				blockReferencesExist ||= block.referencedTimestamps.length > 0;
+				hasLacedBlocks ||= block.lacing !== BlockLacing.None;
 			}
 
 			if (blockReferencesExist) {
@@ -496,16 +540,35 @@ export class MatroskaDemuxer extends Demuxer {
 					// Update block durations based on presentation order
 					const nextEntry = trackData.presentationTimestamps[i + 1]!;
 					currentBlock.duration = nextEntry.timestamp - currentBlock.timestamp;
+				} else if (currentBlock.duration === 0) {
+					if (track?.defaultDuration != null) {
+						if (currentBlock.lacing === BlockLacing.None) {
+							currentBlock.duration = track.defaultDuration;
+						} else {
+							// Handled by the lace resolution code
+						}
+					}
 				}
 			}
 
+			if (hasLacedBlocks) {
+				// Perform lace resolution. Here, we expand each laced block into multiple blocks where each contains
+				// one frame of the lace. We do this after determining block timestamps so we can properly distribute
+				// the block's duration across the laced frames.
+				this.expandLacedBlocks(trackData.blocks, track);
+
+				// Recompute since blocks have changed
+				trackData.presentationTimestamps = trackData.blocks
+					.map((block, i) => ({ timestamp: block.timestamp, blockIndex: i }))
+					.sort((a, b) => a.timestamp - b.timestamp);
+			}
+
 			const firstBlock = trackData.blocks[trackData.presentationTimestamps[0]!.blockIndex]!;
 			const lastBlock = trackData.blocks[last(trackData.presentationTimestamps)!.blockIndex]!;
 
 			trackData.startTimestamp = firstBlock.timestamp;
 			trackData.endTimestamp = lastBlock.timestamp + lastBlock.duration;
 
-			const track = segment.tracks.find(x => x.id === trackId);
 			if (track) {
 				const insertionIndex = binarySearchLessOrEqual(
 					track.clusters,
@@ -554,6 +617,120 @@ export class MatroskaDemuxer extends Demuxer {
 		return trackData;
 	}
 
+	expandLacedBlocks(blocks: ClusterBlock[], track: InternalTrack | null) {
+		// https://www.matroska.org/technical/notes.html#block-lacing
+
+		for (let blockIndex = 0; blockIndex < blocks.length; blockIndex++) {
+			const originalBlock = blocks[blockIndex]!;
+			if (originalBlock.lacing === BlockLacing.None) {
+				continue;
+			}
+
+			const data = originalBlock.data;
+			let pos = 0;
+
+			const frameSizes: number[] = [];
+			const frameCount = data[pos]! + 1;
+			pos++;
+
+			switch (originalBlock.lacing) {
+				case BlockLacing.Xiph: {
+					let totalUsedSize = 0;
+
+					// Xiph lacing, just like in Ogg
+					for (let i = 0; i < frameCount - 1; i++) {
+						let frameSize = 0;
+
+						while (pos < data.length) {
+							const value = data[pos]!;
+							frameSize += value;
+							pos++;
+
+							if (value < 255) {
+								frameSizes.push(frameSize);
+								totalUsedSize += frameSize;
+
+								break;
+							}
+						}
+					}
+
+					// Compute the last frame's size from whatever's left
+					frameSizes.push(data.length - (pos + totalUsedSize));
+				}; break;
+
+				case BlockLacing.FixedSize: {
+					// Fixed size lacing: all frames have same size
+					const totalDataSize = data.length - 1; // Minus the frame count byte
+					const frameSize = Math.floor(totalDataSize / frameCount);
+
+					for (let i = 0; i < frameCount; i++) {
+						frameSizes.push(frameSize);
+					}
+				}; break;
+
+				case BlockLacing.Ebml: {
+					// EBML lacing: first size absolute, subsequent ones are coded as signed differences from the last
+					const firstResult = readVarInt(data, pos);
+					let currentSize = firstResult.value;
+					frameSizes.push(currentSize);
+					pos += firstResult.width;
+
+					let totalUsedSize = currentSize;
+
+					for (let i = 1; i < frameCount - 1; i++) {
+						const diffResult = readVarInt(data, pos);
+						const unsignedDiff = diffResult.value;
+						const bias = (1 << (diffResult.width * 7 - 1)) - 1; // Typo-corrected version of 2^((7*n)-1)^-1
+						const diff = unsignedDiff - bias;
+
+						currentSize += diff;
+						frameSizes.push(currentSize);
+						pos += diffResult.width;
+
+						totalUsedSize += currentSize;
+					}
+
+					// Compute the last frame's size from whatever's left
+					frameSizes.push(data.length - (pos + totalUsedSize));
+				}; break;
+
+				default: assert(false);
+			}
+
+			assert(frameSizes.length === frameCount);
+
+			blocks.splice(blockIndex, 1); // Remove the original block
+			let dataOffset = pos;
+
+			// Now, let's insert each frame as its own block
+			for (let i = 0; i < frameCount; i++) {
+				const frameSize = frameSizes[i]!;
+				const frameData = data.subarray(dataOffset, dataOffset + frameSize);
+
+				const blockDuration = originalBlock.duration || (frameCount * (track?.defaultDuration ?? 0));
+
+				// Distribute timestamps evenly across the block duration
+				const frameTimestamp = originalBlock.timestamp + (blockDuration * i / frameCount);
+				const frameDuration = blockDuration / frameCount;
+
+				blocks.splice(blockIndex + i, 0, {
+					timestamp: frameTimestamp,
+					duration: frameDuration,
+					isKeyFrame: originalBlock.isKeyFrame,
+					referencedTimestamps: originalBlock.referencedTimestamps,
+					data: frameData,
+					lacing: BlockLacing.None,
+				});
+
+				dataOffset += frameSize;
+			}
+
+			blockIndex += frameCount; // Skip the blocks we just added
+			blockIndex--;
+		}
+	}
+
 	readContiguousElements(reader: EBMLReader, totalSize: number) {
 		const startIndex = reader.pos;
 
@@ -625,6 +802,7 @@ export class MatroskaDemuxer extends Demuxer {
 					inputTrack: null,
 					codecId: null,
 					codecPrivate: null,
+					defaultDuration: null,
 					languageCode: UNDETERMINED_LANGUAGE,
 					info: null,
 				};
@@ -786,6 +964,13 @@ export class MatroskaDemuxer extends Demuxer {
 				this.currentTrack.codecPrivate = reader.readBytes(size);
 			}; break;
 
+			case EBMLId.DefaultDuration: {
+				if (!this.currentTrack) break;
+
+				this.currentTrack.defaultDuration
+					= this.currentTrack.segment.timestampFactor * reader.readUnsignedInt(size) / 1e9;
+			}; break;
+
 			case EBMLId.Language: {
 				if (!this.currentTrack) break;
 
@@ -947,14 +1132,16 @@ export class MatroskaDemuxer extends Demuxer {
 
 				const flags = reader.readU8();
 				const isKeyFrame = !!(flags & 0x80);
+				const lacing = (flags >> 1) & 0x3 as BlockLacing; // If the block is laced, we'll expand it later
 
 				const trackData = this.getTrackDataInCluster(this.currentCluster, trackNumber);
 				trackData.blocks.push({
 					timestamp: relativeTimestamp, // We'll add the cluster's timestamp to this later
-					duration: 0,
+					duration: 0, // Will set later
 					isKeyFrame,
 					referencedTimestamps: [],
 					data: reader.readBytes(size - (reader.pos - dataStartPos)),
+					lacing,
 				});
 			}; break;
 
@@ -978,16 +1165,17 @@ export class MatroskaDemuxer extends Demuxer {
 				const trackNumber = reader.readVarInt();
 				const relativeTimestamp = reader.readS16();
 
-				// eslint-disable-next-line @typescript-eslint/no-unused-vars
 				const flags = reader.readU8();
+				const lacing = (flags >> 1) & 0x3 as BlockLacing; // If the block is laced, we'll expand it later
 
 				const trackData = this.getTrackDataInCluster(this.currentCluster, trackNumber);
 				this.currentBlock = {
 					timestamp: relativeTimestamp, // We'll add the cluster's timestamp to this later
-					duration: 0,
+					duration: 0, // Will set later
 					isKeyFrame: true,
 					referencedTimestamps: [],
 					data: reader.readBytes(size - (reader.pos - dataStartPos)),
+					lacing,
 				};
 				trackData.blocks.push(this.currentBlock);
 			}; break;
@@ -1110,7 +1298,6 @@ abstract class MatroskaTrackBacking implements InputTrackBacking {
 		}
 
 		const trackData = locationInCluster.cluster.trackData.get(this.internalTrack.id)!;
-		const block = trackData.blocks[locationInCluster.blockIndex]!;
 
 		const clusterIndex = binarySearchExact(
 			this.internalTrack.clusters,
@@ -1158,7 +1345,7 @@ abstract class MatroskaTrackBacking implements InputTrackBacking {
 					};
 				}
 			},
-			block.timestamp,
+			-Infinity, // Use -Infinity as a search timestamp to avoid using the cues
 			Infinity,
 			options,
 		);
@@ -1182,7 +1369,6 @@ abstract class MatroskaTrackBacking implements InputTrackBacking {
 		}
 
 		const trackData = locationInCluster.cluster.trackData.get(this.internalTrack.id)!;
-		const block = trackData.blocks[locationInCluster.blockIndex]!;
 
 		const clusterIndex = binarySearchExact(
 			this.internalTrack.clusters,
@@ -1237,7 +1423,7 @@ abstract class MatroskaTrackBacking implements InputTrackBacking {
 					};
 				}
 			},
-			block.timestamp,
+			-Infinity, // Use -Infinity as a search timestamp to avoid using the cues
 			Infinity,
 			options,
 		);

src/media-source.ts

@@ -24,7 +24,7 @@ import {
 	VideoCodec,
 } from './codec';
 import { OutputAudioTrack, OutputSubtitleTrack, OutputTrack, OutputVideoTrack } from './output';
-import { assert, assertNever, CallSerializer, clamp, setInt24, setUint24 } from './misc';
+import { assert, assertNever, CallSerializer, clamp, promiseWithResolvers, setInt24, setUint24 } from './misc';
 import { Muxer } from './muxer';
 import { SubtitleParser } from './subtitles';
 import { toAlaw, toUlaw } from './pcm';
@@ -78,7 +78,7 @@ export abstract class MediaSource {
 	}
 
 	/** @internal */
-	_start() {}
+	async _start() {}
 	/** @internal */
 	async _flushAndClose() {}
 
@@ -272,86 +272,94 @@ class VideoEncoderWrapper {
 	constructor(private source: VideoSource, private encodingConfig: VideoEncodingConfig) {}
 
 	async add(videoSample: VideoSample, shouldClose: boolean, encodeOptions?: VideoEncoderEncodeOptions) {
-		this.checkForEncoderError();
-		this.source._ensureValidAdd();
+		try {
+			this.checkForEncoderError();
+			this.source._ensureValidAdd();
 
-		// Ensure video sample size remains constant
-		if (this.lastWidth !== null && this.lastHeight !== null) {
-			if (videoSample.codedWidth !== this.lastWidth || videoSample.codedHeight !== this.lastHeight) {
-				throw new Error(
-					`Video sample size must remain constant. Expected ${this.lastWidth}x${this.lastHeight},`
-					+ ` got ${videoSample.codedWidth}x${videoSample.codedHeight}.`,
-				);
-			}
-		} else {
-			this.lastWidth = videoSample.codedWidth;
-			this.lastHeight = videoSample.codedHeight;
-		}
-
-		if (!this.encoderInitialized) {
-			if (!this.ensureEncoderPromise) {
-				void this.ensureEncoder(videoSample);
+			// Ensure video sample size remains constant
+			if (this.lastWidth !== null && this.lastHeight !== null) {
+				if (videoSample.codedWidth !== this.lastWidth || videoSample.codedHeight !== this.lastHeight) {
+					throw new Error(
+						`Video sample size must remain constant. Expected ${this.lastWidth}x${this.lastHeight},`
+						+ ` got ${videoSample.codedWidth}x${videoSample.codedHeight}.`,
+					);
+				}
+			} else {
+				this.lastWidth = videoSample.codedWidth;
+				this.lastHeight = videoSample.codedHeight;
 			}
 
-			// No, this "if" statement is not useless. Sometimes, the above call to `ensureEncoder` might have
-			// synchronously completed and the encoder is already initialized. In this case, we don't need to await the
-			// promise anymore. This also fixes nasty async race condition bugs when multiple code paths are calling
-			// this method: It's important that the call that initialized the encoder go through this code first.
 			if (!this.encoderInitialized) {
-				await this.ensureEncoderPromise;
+				if (!this.ensureEncoderPromise) {
+					void this.ensureEncoder(videoSample);
+				}
+
+				// No, this "if" statement is not useless. Sometimes, the above call to `ensureEncoder` might have
+				// synchronously completed and the encoder is already initialized. In this case, we don't need to await
+				// the promise anymore. This also fixes nasty async race condition bugs when multiple code paths are
+				// calling this method: It's important that the call that initialized the encoder go through this
+				// code first.
+				if (!this.encoderInitialized) {
+					await this.ensureEncoderPromise;
+				}
 			}
-		}
-		assert(this.encoderInitialized);
+			assert(this.encoderInitialized);
 
-		const keyFrameInterval = this.encodingConfig.keyFrameInterval ?? 5;
-		const multipleOfKeyFrameInterval = Math.floor(videoSample.timestamp / keyFrameInterval);
+			const keyFrameInterval = this.encodingConfig.keyFrameInterval ?? 5;
+			const multipleOfKeyFrameInterval = Math.floor(videoSample.timestamp / keyFrameInterval);
 
-		// Ensure a key frame every keyFrameInterval seconds. It is important that all video tracks follow the same
-		// "key frame" rhythm, because aligned key frames are required to start new fragments in ISOBMFF or clusters
-		// in Matroska (or at least desirable).
-		const finalEncodeOptions = {
-			...encodeOptions,
-			keyFrame: encodeOptions?.keyFrame
-				|| keyFrameInterval === 0
-				|| multipleOfKeyFrameInterval !== this.lastMultipleOfKeyFrameInterval,
-		};
-		this.lastMultipleOfKeyFrameInterval = multipleOfKeyFrameInterval;
+			// Ensure a key frame every keyFrameInterval seconds. It is important that all video tracks follow the same
+			// "key frame" rhythm, because aligned key frames are required to start new fragments in ISOBMFF or clusters
+			// in Matroska (or at least desirable).
+			const finalEncodeOptions = {
+				...encodeOptions,
+				keyFrame: encodeOptions?.keyFrame
+					|| keyFrameInterval === 0
+					|| multipleOfKeyFrameInterval !== this.lastMultipleOfKeyFrameInterval,
+			};
+			this.lastMultipleOfKeyFrameInterval = multipleOfKeyFrameInterval;
 
-		if (this.customEncoder) {
-			this.customEncoderQueueSize++;
-			const promise = this.customEncoderCallSerializer
-				.call(() => this.customEncoder!.encode(videoSample, finalEncodeOptions))
-				.then(() => {
-					this.customEncoderQueueSize--;
+			if (this.customEncoder) {
+				this.customEncoderQueueSize++;
+				const promise = this.customEncoderCallSerializer
+					.call(() => this.customEncoder!.encode(videoSample, finalEncodeOptions))
+					.then(() => {
+						this.customEncoderQueueSize--;
 
-					if (shouldClose) {
-						videoSample.close();
-					}
-				})
-				.catch((error: Error) => {
-					this.encoderError ??= error;
-				});
+						if (shouldClose) {
+							videoSample.close();
+						}
+					})
+					.catch((error: Error) => {
+						this.encoderError ??= error;
+					});
 
-			if (this.customEncoderQueueSize >= 4) {
-				await promise;
+				if (this.customEncoderQueueSize >= 4) {
+					await promise;
+				}
+			} else {
+				assert(this.encoder);
+				const videoFrame = videoSample.toVideoFrame();
+				this.encoder.encode(videoFrame, finalEncodeOptions);
+				videoFrame.close();
+
+				if (shouldClose) {
+					videoSample.close();
+				}
+
+				// We need to do this after sending the frame to the encoder as the frame otherwise might be closed
+				if (this.encoder.encodeQueueSize >= 4) {
+					await new Promise(resolve => this.encoder!.addEventListener('dequeue', resolve, { once: true }));
+				}
 			}
-		} else {
-			assert(this.encoder);
-			const videoFrame = videoSample.toVideoFrame();
-			this.encoder.encode(videoFrame, finalEncodeOptions);
-			videoFrame.close();
 
+			await this.muxer!.mutex.currentPromise; // Allow the writer to apply backpressure
+		} finally {
 			if (shouldClose) {
+				// Make sure it's always closed, even if there was an error
 				videoSample.close();
 			}
-
-			// We need to do this after sending the frame to the encoder as the frame otherwise might be closed
-			if (this.encoder.encodeQueueSize >= 4) {
-				await new Promise(resolve => this.encoder!.addEventListener('dequeue', resolve, { once: true }));
-			}
 		}
-
-		await this.muxer!.mutex.currentPromise; // Allow the writer to apply backpressure
 	}
 
 	private async ensureEncoder(videoSample: VideoSample) {
@@ -575,6 +583,20 @@ export class MediaStreamVideoTrackSource extends VideoSource {
 	private _abortController: AbortController | null = null;
 	/** @internal */
 	private _track: MediaStreamVideoTrack;
+	/** @internal */
+	private _workerTrackId: number | null = null;
+	/** @internal */
+	private _workerListener: ((event: MessageEvent) => void) | null = null;
+	/** @internal */
+	private _promiseWithResolvers = promiseWithResolvers();
+	/** @internal */
+	private _errorPromiseAccessed = false;
+
+	/** A promise that rejects upon any error within this source. This promise never resolves. */
+	get errorPromise() {
+		this._errorPromiseAccessed = true;
+		return this._promiseWithResolvers.promise;
+	}
 
 	constructor(track: MediaStreamVideoTrack, encodingConfig: VideoEncodingConfig) {
 		if (!(track instanceof MediaStreamTrack) || track.kind !== 'video') {
@@ -593,41 +615,102 @@ export class MediaStreamVideoTrackSource extends VideoSource {
 	}
 
 	/** @internal */
-	override _start() {
+	override async _start() {
+		if (!this._errorPromiseAccessed) {
+			console.warn(
+				'Make sure not to ignore the `errorPromise` field on MediaStreamVideoTrackSource, so that any internal'
+				+ ' errors get bubbled up properly.',
+			);
+		}
+
 		this._abortController = new AbortController();
 
-		let frameReceived = false;
+		let firstVideoFrameTimestamp: number | null = null;
+		let errored = false;
 
-		const processor = new MediaStreamTrackProcessor({ track: this._track });
-		const consumer = new WritableStream<VideoFrame>({
-			write: (videoFrame) => {
-				if (!frameReceived) {
-					setMediaStreamTimestampOffset(this, videoFrame);
-					frameReceived = true;
+		const onVideoFrame = (videoFrame: VideoFrame) => {
+			if (errored) {
+				videoFrame.close();
+				return;
+			}
+
+			if (firstVideoFrameTimestamp === null) {
+				firstVideoFrameTimestamp = videoFrame.timestamp / 1e6;
+
+				const muxer = this._connectedTrack!.output._muxer;
+				if (muxer.firstMediaStreamTimestamp === null) {
+					muxer.firstMediaStreamTimestamp = performance.now() / 1000;
+					this._timestampOffset = -firstVideoFrameTimestamp;
+				} else {
+					this._timestampOffset = (performance.now() / 1000 - muxer.firstMediaStreamTimestamp)
+						- firstVideoFrameTimestamp;
 				}
+			}
 
-				if (this._encoder.getQueueSize() >= 4) {
-					// Drop frames if the encoder is overloaded
-					videoFrame.close();
-					return;
-				}
+			if (this._encoder.getQueueSize() >= 4) {
+				// Drop frames if the encoder is overloaded
+				videoFrame.close();
+				return;
+			}
 
-				void this._encoder.add(new VideoSample(videoFrame), true)
-					.catch((error) => {
-						this._abortController?.abort();
-						throw error;
-					});
-			},
-		});
+			void this._encoder.add(new VideoSample(videoFrame), true)
+				.catch((error) => {
+					errored = true;
 
-		processor.readable.pipeTo(consumer, {
-			signal: this._abortController.signal,
-		}).catch((err) => {
-			// Handle abort error silently
-			if (err instanceof DOMException && err.name === 'AbortError') return;
-			// Handle other errors
-			console.error('Pipe error:', err);
-		});
+					this._abortController?.abort();
+					this._promiseWithResolvers.reject(error);
+
+					if (this._workerTrackId !== null) {
+						// Tell the worker to stop the track
+						sendMessageToMediaStreamTrackProcessorWorker({
+							type: 'stopTrack',
+							trackId: this._workerTrackId,
+						});
+					}
+				});
+		};
+
+		if (typeof MediaStreamTrackProcessor !== 'undefined') {
+			// We can do it here directly, perfect
+			const processor = new MediaStreamTrackProcessor({ track: this._track });
+			const consumer = new WritableStream<VideoFrame>({ write: onVideoFrame });
+
+			processor.readable.pipeTo(consumer, {
+				signal: this._abortController.signal,
+			}).catch((error) => {
+				// Handle AbortError silently
+				if (error instanceof DOMException && error.name === 'AbortError') return;
+
+				this._promiseWithResolvers.reject(error);
+			});
+		} else {
+			// It might still be supported in a worker, so let's check that
+			const supportedInWorker = await mediaStreamTrackProcessorIsSupportedInWorker();
+
+			if (supportedInWorker) {
+				this._workerTrackId = nextMediaStreamTrackProcessorWorkerId++;
+
+				sendMessageToMediaStreamTrackProcessorWorker({
+					type: 'videoTrack',
+					trackId: this._workerTrackId,
+					track: this._track,
+				}, [this._track]);
+
+				this._workerListener = (event: MessageEvent) => {
+					const message = event.data as MediaStreamTrackProcessorWorkerMessage;
+
+					if (message.type === 'videoFrame' && message.trackId === this._workerTrackId) {
+						onVideoFrame(message.videoFrame);
+					} else if (message.type === 'error' && message.trackId === this._workerTrackId) {
+						this._promiseWithResolvers.reject(message.error);
+					}
+				};
+
+				mediaStreamTrackProcessorWorker!.addEventListener('message', this._workerListener);
+			} else {
+				throw new Error('MediaStreamTrackProcessor is required but not supported by this browser.');
+			}
+		}
 	}
 
 	/** @internal */
@@ -637,6 +720,32 @@ export class MediaStreamVideoTrackSource extends VideoSource {
 			this._abortController = null;
 		}
 
+		if (this._workerTrackId !== null) {
+			assert(this._workerListener);
+
+			sendMessageToMediaStreamTrackProcessorWorker({
+				type: 'stopTrack',
+				trackId: this._workerTrackId,
+			});
+
+			// Wait for the worker to stop the track
+			await new Promise<void>((resolve) => {
+				const listener = (event: MessageEvent) => {
+					const message = event.data as MediaStreamTrackProcessorWorkerMessage;
+
+					if (message.type === 'trackStopped' && message.trackId === this._workerTrackId) {
+						assert(this._workerListener);
+						mediaStreamTrackProcessorWorker!.removeEventListener('message', this._workerListener);
+						mediaStreamTrackProcessorWorker!.removeEventListener('message', listener);
+
+						resolve();
+					}
+				};
+
+				mediaStreamTrackProcessorWorker!.addEventListener('message', listener);
+			});
+		}
+
 		await this._encoder.flushAndClose();
 	}
 }
@@ -783,78 +892,86 @@ class AudioEncoderWrapper {
 	constructor(private source: AudioSource, private encodingConfig: AudioEncodingConfig) {}
 
 	async add(audioSample: AudioSample, shouldClose: boolean) {
-		this.checkForEncoderError();
-		this.source._ensureValidAdd();
+		try {
+			this.checkForEncoderError();
+			this.source._ensureValidAdd();
 
-		// Ensure audio parameters remain constant
-		if (this.lastNumberOfChannels !== null && this.lastSampleRate !== null) {
-			if (
-				audioSample.numberOfChannels !== this.lastNumberOfChannels
-				|| audioSample.sampleRate !== this.lastSampleRate
-			) {
-				throw new Error(
-					`Audio parameters must remain constant. Expected ${this.lastNumberOfChannels} channels at`
-					+ ` ${this.lastSampleRate} Hz, got ${audioSample.numberOfChannels} channels at`
-					+ ` ${audioSample.sampleRate} Hz.`,
-				);
-			}
-		} else {
-			this.lastNumberOfChannels = audioSample.numberOfChannels;
-			this.lastSampleRate = audioSample.sampleRate;
-		}
-
-		if (!this.encoderInitialized) {
-			if (!this.ensureEncoderPromise) {
-				void this.ensureEncoder(audioSample);
+			// Ensure audio parameters remain constant
+			if (this.lastNumberOfChannels !== null && this.lastSampleRate !== null) {
+				if (
+					audioSample.numberOfChannels !== this.lastNumberOfChannels
+					|| audioSample.sampleRate !== this.lastSampleRate
+				) {
+					throw new Error(
+						`Audio parameters must remain constant. Expected ${this.lastNumberOfChannels} channels at`
+						+ ` ${this.lastSampleRate} Hz, got ${audioSample.numberOfChannels} channels at`
+						+ ` ${audioSample.sampleRate} Hz.`,
+					);
+				}
+			} else {
+				this.lastNumberOfChannels = audioSample.numberOfChannels;
+				this.lastSampleRate = audioSample.sampleRate;
 			}
 
-			// No, this "if" statement is not useless. Sometimes, the above call to `ensureEncoder` might have
-			// synchronously completed and the encoder is already initialized. In this case, we don't need to await the
-			// promise anymore. This also fixes nasty async race condition bugs when multiple code paths are calling
-			// this method: It's important that the call that initialized the encoder go through this code first.
 			if (!this.encoderInitialized) {
-				await this.ensureEncoderPromise;
+				if (!this.ensureEncoderPromise) {
+					void this.ensureEncoder(audioSample);
+				}
+
+				// No, this "if" statement is not useless. Sometimes, the above call to `ensureEncoder` might have
+				// synchronously completed and the encoder is already initialized. In this case, we don't need to await
+				// the promise anymore. This also fixes nasty async race condition bugs when multiple code paths are
+				// calling this method: It's important that the call that initialized the encoder go through this
+				// code first.
+				if (!this.encoderInitialized) {
+					await this.ensureEncoderPromise;
+				}
 			}
-		}
-		assert(this.encoderInitialized);
+			assert(this.encoderInitialized);
 
-		if (this.customEncoder) {
-			this.customEncoderQueueSize++;
-			const promise = this.customEncoderCallSerializer
-				.call(() => this.customEncoder!.encode(audioSample))
-				.then(() => {
-					this.customEncoderQueueSize--;
+			if (this.customEncoder) {
+				this.customEncoderQueueSize++;
+				const promise = this.customEncoderCallSerializer
+					.call(() => this.customEncoder!.encode(audioSample))
+					.then(() => {
+						this.customEncoderQueueSize--;
 
-					if (shouldClose) {
-						audioSample.close();
-					}
-				})
-				.catch((error: Error) => {
-					this.encoderError ??= error;
-				});
+						if (shouldClose) {
+							audioSample.close();
+						}
+					})
+					.catch((error: Error) => {
+						this.encoderError ??= error;
+					});
 
-			if (this.customEncoderQueueSize >= 4) {
-				await promise;
+				if (this.customEncoderQueueSize >= 4) {
+					await promise;
+				}
+
+				await this.muxer!.mutex.currentPromise; // Allow the writer to apply backpressure
+			} else if (this.isPcmEncoder) {
+				await this.doPcmEncoding(audioSample, shouldClose);
+			} else {
+				assert(this.encoder);
+				const audioData = audioSample.toAudioData();
+				this.encoder.encode(audioData);
+				audioData.close();
+
+				if (shouldClose) {
+					audioSample.close();
+				}
+
+				if (this.encoder.encodeQueueSize >= 4) {
+					await new Promise(resolve => this.encoder!.addEventListener('dequeue', resolve, { once: true }));
+				}
+
+				await this.muxer!.mutex.currentPromise; // Allow the writer to apply backpressure
 			}
-
-			await this.muxer!.mutex.currentPromise; // Allow the writer to apply backpressure
-		} else if (this.isPcmEncoder) {
-			await this.doPcmEncoding(audioSample, shouldClose);
-		} else {
-			assert(this.encoder);
-			const audioData = audioSample.toAudioData();
-			this.encoder.encode(audioData);
-			audioData.close();
-
+		} finally {
 			if (shouldClose) {
+				// Make sure it's always closed, even if there was an error
 				audioSample.close();
 			}
-
-			if (this.encoder.encodeQueueSize >= 4) {
-				await new Promise(resolve => this.encoder!.addEventListener('dequeue', resolve, { once: true }));
-			}
-
-			await this.muxer!.mutex.currentPromise; // Allow the writer to apply backpressure
 		}
 	}
 
@@ -1186,7 +1303,7 @@ export class AudioBufferSource extends AudioSource {
 	/** @internal */
 	private _encoder: AudioEncoderWrapper;
 	/** @internal */
-	private _accumulatedFrameCount = 0;
+	private _accumulatedTime = 0;
 
 	constructor(encodingConfig: AudioEncodingConfig) {
 		validateAudioEncodingConfig(encodingConfig);
@@ -1208,47 +1325,10 @@ export class AudioBufferSource extends AudioSource {
 			throw new TypeError('audioBuffer must be an AudioBuffer.');
 		}
 
-		const MAX_FLOAT_COUNT = 64 * 1024 * 1024;
+		const audioSamples = AudioSample.fromAudioBuffer(audioBuffer, this._accumulatedTime);
+		const promises = audioSamples.map(sample => this._encoder.add(sample, true));
 
-		const numberOfChannels = audioBuffer.numberOfChannels;
-		const sampleRate = audioBuffer.sampleRate;
-		const totalFrames = audioBuffer.length;
-		const maxFramesPerChunk = Math.floor(MAX_FLOAT_COUNT / numberOfChannels);
-
-		let currentRelativeFrame = 0;
-		let remainingFrames = totalFrames;
-
-		const promises: Promise<void>[] = [];
-
-		// Create AudioData in a chunked fashion so we don't create huge Float32Arrays
-		while (remainingFrames > 0) {
-			const framesToCopy = Math.min(maxFramesPerChunk, remainingFrames);
-			const chunkData = new Float32Array(numberOfChannels * framesToCopy);
-
-			for (let channel = 0; channel < numberOfChannels; channel++) {
-				audioBuffer.copyFromChannel(
-					chunkData.subarray(channel * framesToCopy, channel * framesToCopy + framesToCopy),
-					channel,
-					currentRelativeFrame,
-				);
-			}
-
-			const audioSample = new AudioSample({
-				format: 'f32-planar',
-				sampleRate,
-				numberOfFrames: framesToCopy,
-				numberOfChannels,
-				timestamp: (this._accumulatedFrameCount + currentRelativeFrame) / sampleRate,
-				data: chunkData,
-			});
-
-			promises.push(this._encoder.add(audioSample, true));
-
-			currentRelativeFrame += framesToCopy;
-			remainingFrames -= framesToCopy;
-		}
-
-		this._accumulatedFrameCount += totalFrames;
+		this._accumulatedTime += audioBuffer.duration;
 		return Promise.all(promises);
 	}
 
@@ -1272,6 +1352,20 @@ export class MediaStreamAudioTrackSource extends AudioSource {
 	private _abortController: AbortController | null = null;
 	/** @internal */
 	private _track: MediaStreamAudioTrack;
+	/** @internal */
+	private _audioContext: AudioContext | null = null;
+	/** @internal */
+	private _scriptProcessorNode: ScriptProcessorNode | null = null; // Deprecated but goated
+	/** @internal */
+	private _promiseWithResolvers = promiseWithResolvers();
+	/** @internal */
+	private _errorPromiseAccessed = false;
+
+	/** A promise that rejects upon any error within this source. This promise never resolves. */
+	get errorPromise() {
+		this._errorPromiseAccessed = true;
+		return this._promiseWithResolvers.promise;
+	}
 
 	constructor(track: MediaStreamAudioTrack, encodingConfig: AudioEncodingConfig) {
 		if (!(track instanceof MediaStreamTrack) || track.kind !== 'audio') {
@@ -1285,41 +1379,104 @@ export class MediaStreamAudioTrackSource extends AudioSource {
 	}
 
 	/** @internal */
-	override _start() {
+	override async _start() {
+		if (!this._errorPromiseAccessed) {
+			console.warn(
+				'Make sure not to ignore the `errorPromise` field on MediaStreamVideoTrackSource, so that any internal'
+				+ ' errors get bubbled up properly.',
+			);
+		}
+
 		this._abortController = new AbortController();
 
-		let dataReceived = false;
+		if (typeof MediaStreamTrackProcessor !== 'undefined') {
+			// Great, MediaStreamTrackProcessor is supported, this is the preferred way of doing things
+			let firstAudioDataTimestamp: number | null = null;
 
-		const processor = new MediaStreamTrackProcessor({ track: this._track });
-		const consumer = new WritableStream<AudioData>({
-			write: (audioData) => {
-				if (!dataReceived) {
-					setMediaStreamTimestampOffset(this, audioData);
-					dataReceived = true;
+			const processor = new MediaStreamTrackProcessor({ track: this._track });
+			const consumer = new WritableStream<AudioData>({
+				write: (audioData) => {
+					if (firstAudioDataTimestamp === null) {
+						firstAudioDataTimestamp = audioData.timestamp / 1e6;
+
+						const muxer = this._connectedTrack!.output._muxer;
+						if (muxer.firstMediaStreamTimestamp === null) {
+							muxer.firstMediaStreamTimestamp = performance.now() / 1000;
+							this._timestampOffset = -firstAudioDataTimestamp;
+						} else {
+							this._timestampOffset = (performance.now() / 1000 - muxer.firstMediaStreamTimestamp)
+								- firstAudioDataTimestamp;
+						}
+					}
+
+					if (this._encoder.getQueueSize() >= 4) {
+						// Drop data if the encoder is overloaded
+						audioData.close();
+						return;
+					}
+
+					void this._encoder.add(new AudioSample(audioData), true)
+						.catch((error) => {
+							this._abortController?.abort();
+							this._promiseWithResolvers.reject(error);
+						});
+				},
+			});
+
+			processor.readable.pipeTo(consumer, {
+				signal: this._abortController.signal,
+			}).catch((error) => {
+				// Handle AbortError silently
+				if (error instanceof DOMException && error.name === 'AbortError') return;
+
+				this._promiseWithResolvers.reject(error);
+			});
+		} else {
+			// Let's fall back to an AudioContext approach
+			this._audioContext = new AudioContext({ sampleRate: this._track.getSettings().sampleRate });
+			const sourceNode = this._audioContext.createMediaStreamSource(new MediaStream([this._track]));
+			this._scriptProcessorNode = this._audioContext.createScriptProcessor(4096);
+
+			if (this._audioContext.state === 'suspended') {
+				await this._audioContext.resume();
+			}
+
+			sourceNode.connect(this._scriptProcessorNode);
+			this._scriptProcessorNode.connect(this._audioContext.destination);
+
+			let audioReceived = false;
+			let totalDuration = 0;
+
+			this._scriptProcessorNode.onaudioprocess = (event) => {
+				const audioSamples = AudioSample.fromAudioBuffer(event.inputBuffer, totalDuration);
+				totalDuration += event.inputBuffer.duration;
+
+				for (const audioSample of audioSamples) {
+					if (!audioReceived) {
+						audioReceived = true;
+
+						const muxer = this._connectedTrack!.output._muxer;
+						if (muxer.firstMediaStreamTimestamp === null) {
+							muxer.firstMediaStreamTimestamp = performance.now() / 1000;
+						} else {
+							this._timestampOffset = performance.now() / 1000 - muxer.firstMediaStreamTimestamp;
+						}
+					}
+
+					if (this._encoder.getQueueSize() >= 4) {
+						// Drop data if the encoder is overloaded
+						audioSample.close();
+						continue;
+					}
+
+					void this._encoder.add(audioSample, true)
+						.catch((error) => {
+							void this._audioContext!.suspend();
+							this._promiseWithResolvers.reject(error);
+						});
 				}
-
-				if (this._encoder.getQueueSize() >= 4) {
-					// Drop data if the encoder is overloaded
-					audioData.close();
-					return;
-				}
-
-				void this._encoder.add(new AudioSample(audioData), true)
-					.catch((error) => {
-						this._abortController?.abort();
-						throw error;
-					});
-			},
-		});
-
-		processor.readable.pipeTo(consumer, {
-			signal: this._abortController.signal,
-		}).catch((err) => {
-			// Handle abort error silently
-			if (err instanceof DOMException && err.name === 'AbortError') return;
-			// Handle other errors
-			console.error('Pipe error:', err);
-		});
+			};
+		}
 	}
 
 	/** @internal */
@@ -1329,22 +1486,175 @@ export class MediaStreamAudioTrackSource extends AudioSource {
 			this._abortController = null;
 		}
 
+		if (this._audioContext) {
+			assert(this._scriptProcessorNode);
+
+			this._scriptProcessorNode.disconnect();
+			await this._audioContext.suspend();
+		}
+
 		await this._encoder.flushAndClose();
 	}
 }
 
-const setMediaStreamTimestampOffset = (source: MediaSource, sample: VideoFrame | AudioData) => {
-	const timestampInSeconds = sample.timestamp / 1e6;
+// === MEDIA STREAM TRACK PROCESSOR WORKER ===
 
-	assert(source._connectedTrack);
-	const muxer = source._connectedTrack.output._muxer;
-	if (muxer.firstMediaStreamTimestamp === null) {
-		// We're the first MediaStreamTrack of this output to receive data
-		muxer.firstMediaStreamTimestamp = timestampInSeconds;
+type MediaStreamTrackProcessorWorkerMessage = {
+	type: 'support';
+	supported: boolean;
+} | {
+	type: 'videoFrame';
+	trackId: number;
+	videoFrame: VideoFrame;
+} | {
+	type: 'trackStopped';
+	trackId: number;
+} | {
+	type: 'error';
+	trackId: number;
+	error: Error;
+};
+
+type MediaStreamTrackProcessorControllerMessage = {
+	type: 'videoTrack';
+	trackId: number;
+	track: MediaStreamVideoTrack;
+} | {
+	type: 'stopTrack';
+	trackId: number;
+};
+
+const mediaStreamTrackProcessorWorkerCode = () => {
+	const sendMessage = (message: MediaStreamTrackProcessorWorkerMessage, transfer?: Transferable[]) => {
+		if (transfer) {
+			// The error is bullshit, it's using the wrong postMessage
+			// eslint-disable-next-line @typescript-eslint/no-explicit-any, @typescript-eslint/no-unsafe-argument
+			self.postMessage(message, transfer as any);
+		} else {
+			self.postMessage(message);
+		}
+	};
+
+	// Immediately send a message to the main thread, letting them know of the support
+	sendMessage({
+		type: 'support',
+		supported: typeof MediaStreamTrackProcessor !== 'undefined',
+	});
+
+	const abortControllers = new Map<number, AbortController>();
+	const stoppedTracks = new Set<number>();
+
+	self.addEventListener('message', (event) => {
+		const message = event.data as MediaStreamTrackProcessorControllerMessage;
+
+		switch (message.type) {
+			case 'videoTrack': {
+				const processor = new MediaStreamTrackProcessor({ track: message.track });
+				const consumer = new WritableStream<VideoFrame>({
+					write: (videoFrame) => {
+						if (stoppedTracks.has(message.trackId)) {
+							videoFrame.close();
+							return;
+						}
+
+						// Send it to the main thread
+						sendMessage({
+							type: 'videoFrame',
+							trackId: message.trackId,
+							videoFrame,
+						}, [videoFrame]);
+					},
+				});
+
+				const abortController = new AbortController();
+				abortControllers.set(message.trackId, abortController);
+
+				processor.readable.pipeTo(consumer, {
+					signal: abortController.signal,
+				}).catch((error: Error) => {
+					// Handle AbortError silently
+					if (error instanceof DOMException && error.name === 'AbortError') return;
+
+					sendMessage({
+						type: 'error',
+						trackId: message.trackId,
+						error,
+					});
+				});
+			}; break;
+
+			case 'stopTrack': {
+				const abortController = abortControllers.get(message.trackId);
+				if (abortController) {
+					abortController.abort();
+					abortControllers.delete(message.trackId);
+				}
+
+				stoppedTracks.add(message.trackId);
+
+				sendMessage({
+					type: 'trackStopped',
+					trackId: message.trackId,
+				});
+			}; break;
+
+			default: assertNever(message);
+		}
+	});
+};
+
+let nextMediaStreamTrackProcessorWorkerId = 0;
+let mediaStreamTrackProcessorWorker: Worker | null = null;
+
+const initMediaStreamTrackProcessorWorker = () => {
+	const blob = new Blob(
+		[`(${mediaStreamTrackProcessorWorkerCode.toString()})()`],
+		{ type: 'application/javascript' },
+	);
+	const url = URL.createObjectURL(blob);
+
+	mediaStreamTrackProcessorWorker = new Worker(url);
+};
+
+let mediaStreamTrackProcessorIsSupportedInWorkerCache: boolean | null = null;
+const mediaStreamTrackProcessorIsSupportedInWorker = async () => {
+	if (mediaStreamTrackProcessorIsSupportedInWorkerCache !== null) {
+		return mediaStreamTrackProcessorIsSupportedInWorkerCache;
 	}
 
-	// Math.min to ensure the timestamps can't get negative
-	source._timestampOffset = -Math.min(muxer.firstMediaStreamTimestamp, timestampInSeconds);
+	if (!mediaStreamTrackProcessorWorker) {
+		initMediaStreamTrackProcessorWorker();
+	}
+
+	return new Promise<boolean>((resolve) => {
+		assert(mediaStreamTrackProcessorWorker);
+
+		const listener = (event: MessageEvent) => {
+			const message = event.data as MediaStreamTrackProcessorWorkerMessage;
+
+			if (message.type === 'support') {
+				mediaStreamTrackProcessorIsSupportedInWorkerCache = message.supported;
+				mediaStreamTrackProcessorWorker!.removeEventListener('message', listener);
+
+				resolve(message.supported);
+			}
+		};
+
+		mediaStreamTrackProcessorWorker.addEventListener('message', listener);
+	});
+};
+
+const sendMessageToMediaStreamTrackProcessorWorker = (
+	message: MediaStreamTrackProcessorControllerMessage,
+	transfer?: Transferable[],
+) => {
+	assert(mediaStreamTrackProcessorWorker);
+
+	if (transfer) {
+		mediaStreamTrackProcessorWorker.postMessage(message, transfer);
+	} else {
+		mediaStreamTrackProcessorWorker.postMessage(message);
+	}
 };
 
 /**

src/output.ts

@@ -344,9 +344,8 @@ export class Output<
 
 			await this._muxer.start();
 
-			for (const track of this._tracks) {
-				track.source._start();
-			}
+			const promises = this._tracks.map(track => track.source._start());
+			await Promise.all(promises);
 
 			release();
 		})();

src/sample.ts

@@ -236,6 +236,7 @@ export class VideoSample {
 			return new VideoSample(this._data.clone(), {
 				timestamp: this.timestamp,
 				duration: this.duration,
+				rotation: this.rotation,
 			});
 		} else if (this._data instanceof Uint8Array) {
 			return new VideoSample(this._data.slice(), {
@@ -245,6 +246,7 @@ export class VideoSample {
 				timestamp: this.timestamp,
 				duration: this.duration,
 				colorSpace: this.colorSpace,
+				rotation: this.rotation,
 			});
 		} else {
 			return new VideoSample(this._data, {
@@ -254,6 +256,7 @@ export class VideoSample {
 				timestamp: this.timestamp,
 				duration: this.duration,
 				colorSpace: this.colorSpace,
+				rotation: this.rotation,
 			});
 		}
 	}
@@ -1063,6 +1066,58 @@ export class AudioSample {
 		// eslint-disable-next-line @typescript-eslint/no-unnecessary-type-assertion
 		(this.timestamp as number) = newTimestamp;
 	}
+
+	/**
+	 * Creates AudioSamples from an AudioBuffer, starting at the given timestamp in seconds. Typically creates exactly
+	 * one sample, but may create multiple if the AudioBuffer is exceedingly large.
+	 */
+	static fromAudioBuffer(audioBuffer: AudioBuffer, timestamp: number) {
+		if (!(audioBuffer instanceof AudioBuffer)) {
+			throw new TypeError('audioBuffer must be an AudioBuffer.');
+		}
+
+		const MAX_FLOAT_COUNT = 64 * 1024 * 1024;
+
+		const numberOfChannels = audioBuffer.numberOfChannels;
+		const sampleRate = audioBuffer.sampleRate;
+		const totalFrames = audioBuffer.length;
+		const maxFramesPerChunk = Math.floor(MAX_FLOAT_COUNT / numberOfChannels);
+
+		let currentRelativeFrame = 0;
+		let remainingFrames = totalFrames;
+
+		const result: AudioSample[] = [];
+
+		// Create AudioData in a chunked fashion so we don't create huge Float32Arrays
+		while (remainingFrames > 0) {
+			const framesToCopy = Math.min(maxFramesPerChunk, remainingFrames);
+			const chunkData = new Float32Array(numberOfChannels * framesToCopy);
+
+			for (let channel = 0; channel < numberOfChannels; channel++) {
+				audioBuffer.copyFromChannel(
+					chunkData.subarray(channel * framesToCopy, channel * framesToCopy + framesToCopy),
+					channel,
+					currentRelativeFrame,
+				);
+			}
+
+			const audioSample = new AudioSample({
+				format: 'f32-planar',
+				sampleRate,
+				numberOfFrames: framesToCopy,
+				numberOfChannels,
+				timestamp: timestamp + currentRelativeFrame / sampleRate,
+				data: chunkData,
+			});
+
+			result.push(audioSample);
+
+			currentRelativeFrame += framesToCopy;
+			remainingFrames -= framesToCopy;
+		}
+
+		return result;
+	}
 }
 
 const getBytesPerSample = (format: AudioSampleFormat): number => {