WebCodecs Encoder

Function-First API to encode video and audio using WebCodecs API.

A TypeScript library to encode video (H.264/AVC, HEVC, VP9, VP8, AV1) and audio (AAC, MP3, Opus, Vorbis, FLAC) using the WebCodecs API and mux them into MP4 or WebM containers with a simple, function-first API.

Features

• 🚀 Function-First API: Simple encode(), encodeStream(), and canEncode() functions
• 🎯 Zero Configuration: Automatic resolution, frame rate, and codec detection
• 📊 Quality Presets: Simple low, medium, high, lossless presets
• 🔄 Multiple Input Types: Frame arrays, AsyncIterable, MediaStream, VideoFile
• ⚡ Real-time Streaming: Progressive encoding with encodeStream()
• 🎨 Progressive Enhancement: Start simple, add complexity as needed
• 📦 Optimized Bundle Size: Tree-shakable with ES Modules and sideEffects: false for efficient bundling.
• 🛡️ Type Safety: Full TypeScript support with comprehensive types
• 🎵 Audio Support: Automatic AAC↔MP3 fallback for MP4 and Opus/Vorbis/FLAC support for WebM
• 🎤 Audio-Only Encoding: Support for video: false option (v0.2.2)
• 📹 VideoFile Audio: Extract and encode audio from video files (v0.2.2)
• ⚡ Performance Optimized: Transferable objects for faster data transfer (v0.2.2)

Installation

npm install webcodecs-encoder
# or
yarn add webcodecs-encoder

Worker Setup

The encoder runs inside a dedicated Web Worker (/webcodecs-worker.js). Ship that file with your app and ensure it is publicly reachable at the site root.

By default the library:

• Prefers the external worker in browsers and production builds.
• Falls back to an inline mock only when running under known test runners (Vitest, Jest, NODE_ENV=test) or when you explicitly opt in.
• Never uses the inline mock in production unless you override the safety check.

Inline worker controls:

| Flag | Effect | | --- | --- | | WEBCODECS_USE_INLINE_WORKER=true or window.__WEBCODECS_USE_INLINE_WORKER__ = true | Force the inline mock (useful for Storybook, unit tests, etc.). | | WEBCODECS_DISABLE_INLINE_WORKER=true or window.__WEBCODECS_DISABLE_INLINE_WORKER__ = true | Always require the external worker. | | WEBCODECS_ALLOW_INLINE_IN_PROD=true or window.__WEBCODECS_ALLOW_INLINE_IN_PROD__ = true | Explicitly permit the inline mock on production builds (not recommended). |

⚠️ The inline worker is a test stub that returns placeholder bytes. Use it only for wiring/UI development. Real MP4/WebM output requires the external worker bundle.

Copy the worker file from node_modules into your public assets directory during build/deploy:

# Example for a Next.js/Vite project with a 'public' directory
cp node_modules/webcodecs-encoder/dist/webcodecs-worker.js public/

Example: Vite + TypeScript

// vite.config.ts
import { defineConfig } from 'vite';
import copy from 'rollup-plugin-copy';

export default defineConfig({
  plugins: [
    copy({
      targets: [
        {
          src: 'node_modules/webcodecs-encoder/dist/webcodecs-worker.js',
          dest: 'public'
        }
      ]
    })
  ]
});

// main.ts (development helper)
if (import.meta.env.DEV) {
  window.__WEBCODECS_USE_INLINE_WORKER__ = true;
}

Quick Start

Basic Encoding

import { encode } from 'webcodecs-encoder';

// Encode frames with automatic configuration
const frames = [/* VideoFrame, Canvas, ImageData objects */];
const mp4Data = await encode(frames, { quality: 'medium' });

// Save or use the encoded MP4
const blob = new Blob([mp4Data], { type: 'video/mp4' });
const url = URL.createObjectURL(blob);

Audio-Only Encoding

import { encode } from 'webcodecs-encoder';

// Encode a microphone stream to an Opus audio file in a WebM container
const micStream = await navigator.mediaDevices.getUserMedia({ audio: true, video: false });
const webmAudio = await encode(micStream, {
  video: false, // Required for audio-only
  audio: {
    codec: 'opus',
    bitrate: 128_000
  },
  container: 'webm'
});

const blob = new Blob([webmAudio], { type: 'audio/webm' });

Streaming Encoding

import { encodeStream } from 'webcodecs-encoder';

// Real-time encoding for live streaming
const stream = await navigator.mediaDevices.getUserMedia({ video: true });

for await (const chunk of encodeStream(stream, { quality: 'high' })) {
  // Send chunk to MediaSource, server, or save incrementally
  mediaSource.appendBuffer(chunk);
}

Check Browser Support

import { canEncode } from 'webcodecs-encoder';

// Check if encoding is supported
const isSupported = await canEncode();

// Check specific configuration
const canEncodeHEVC = await canEncode({
  video: { codec: 'hevc' },
  quality: 'high'
});

API Reference

Core Functions

encode(source, options?)

Encode video to a complete MP4/WebM file.

async function encode(
  source: VideoSource,
  options?: EncodeOptions
): Promise<Uint8Array>

encodeStream(source, options?)

Stream encoding with real-time chunks.

async function* encodeStream(
  source: VideoSource,
  options?: EncodeOptions
): AsyncGenerator<Uint8Array>

canEncode(options?)

Check if encoding is supported with given options.

async function canEncode(options?: EncodeOptions): Promise<boolean>

Video Sources

The API supports multiple input types:

type VideoSource =
  | Frame[]                    // Static frame array
  | AsyncIterable<Frame>       // Dynamic frame generation
  | MediaStream               // Camera/screen capture
  | VideoFile;                // Existing video file

type Frame = VideoFrame | HTMLCanvasElement | OffscreenCanvas | ImageBitmap | ImageData;

Encode Options

interface EncodeOptions {
  // Basic settings (auto-detected if not specified)
  width?: number;
  height?: number;
  frameRate?: number;

  // Quality preset (recommended)
  quality?: 'low' | 'medium' | 'high' | 'lossless';

  // Advanced settings
  /** Set to `false` for audio-only encoding. */
  video?: {
    codec?: 'avc' | 'hevc' | 'vp9' | 'vp8' | 'av1';
    bitrate?: number;
    hardwareAcceleration?: 'no-preference' | 'prefer-hardware' | 'prefer-software';
    keyFrameInterval?: number;
  } | false;

  /** Set to `false` to disable audio. */
  audio?: {
    codec?: 'aac' | 'mp3' | 'opus' | 'vorbis' | 'flac';
    bitrate?: number;
    sampleRate?: number;
    channels?: number;
    bitrateMode?: 'constant' | 'variable';
  } | false;

  container?: 'mp4' | 'webm';

  // --- Advanced Control ---

  /**
   * Latency mode for encoder and muxer.
   * `encodeStream()` automatically uses 'realtime'.
   */
  latencyMode?: 'quality' | 'realtime';

  /**
   * How to handle the first timestamp. 'offset' is recommended for streams.
   * - 'offset': Shifts all timestamps so the first one is zero.
   * - 'strict': Uses the original timestamps.
   */
  firstTimestampBehavior?: 'offset' | 'strict';

  /**
   * Maximum video encode queue size before applying backpressure (default: 30).
   */
  maxVideoQueueSize?: number;

  /**
   * Maximum audio encode queue size before applying backpressure (default: 30).
   */
  maxAudioQueueSize?: number;

  /**
   * Strategy for handling encode queue overflow (default: 'drop').
   * - 'drop': Discard new frames when the queue is full.
   * - 'wait': Block the processing loop until there is space in the queue.
   */
  backpressureStrategy?: 'drop' | 'wait';

  // Callbacks
  onProgress?: (progress: ProgressInfo) => void;
  onError?: (error: EncodeError) => void;
}

interface ProgressInfo {
  /** Percentage of completion (0-100) */
  percent: number;
  /** Total number of frames processed */
  processedFrames: number;
  /** Total number of frames to encode (if known) */
  totalFrames?: number;
  /** Current encoding speed in frames per second */
  fps: number;
  /** Current stage label ("streaming", "finalizing", etc.) */
  stage: string;
  /** Estimated remaining time in milliseconds */
  estimatedRemainingMs?: number;
}

Audio codec compatibility

• container: 'mp4' supports aac (default) and automatically falls back to mp3 if AAC isn’t available.
• container: 'webm' supports opus (default) with vorbis and flac as fallbacks.
• Other codec hints are treated as best-effort; if they can’t be muxed into the requested container the encoder switches to the first compatible alternative.

Real-time MediaStream Recording

Use encodeStream() for real-time recording from camera, microphone, or screen sharing. This provides progressive encoding with streaming output:

import { encodeStream } from 'webcodecs-encoder';

const stream = await navigator.mediaDevices.getUserMedia({ video: true, audio: true });

// Collect encoded chunks as they're generated
const chunks: Uint8Array[] = [];

for await (const chunk of encodeStream(stream, {
  quality: 'medium',
  container: 'mp4',
  onProgress: (progress) => {
    console.log(`Recording: ${progress.percent.toFixed(1)}%`);
  }
})) {
  chunks.push(chunk);
  console.log(`Received chunk: ${chunk.byteLength} bytes`);
  
  // Optional: Send chunks to server for real-time streaming
  // await sendChunkToServer(chunk);
}

// Stop recording by stopping the media tracks
setTimeout(() => {
  stream.getTracks().forEach(track => track.stop());
}, 5000); // Record for 5 seconds

// Combine chunks into final video file
const totalSize = chunks.reduce((sum, chunk) => sum + chunk.byteLength, 0);
const finalVideo = new Uint8Array(totalSize);
let offset = 0;
for (const chunk of chunks) {
  finalVideo.set(chunk, offset);
  offset += chunk.byteLength;
}

const blob = new Blob([finalVideo], { type: 'video/mp4' });

Real-time streaming benefits:

• Progressive encoding as data flows
• Immediate chunk availability for streaming
• Built-in cancellation support
• Memory efficient for long recordings

Usage Examples

1. Canvas Animation to MP4

import { encode } from 'webcodecs-encoder';

// Create animation frames
const frames = [];
const canvas = new OffscreenCanvas(800, 600);
const ctx = canvas.getContext('2d');

for (let i = 0; i < 120; i++) { // 4 seconds at 30fps
  ctx.clearRect(0, 0, 800, 600);
  ctx.fillStyle = `hsl(${i * 3}, 70%, 50%)`;
  ctx.fillRect(i * 6, 200, 100, 200);
  frames.push(canvas.transferToImageBitmap());
}

// Encode with automatic settings
const mp4 = await encode(frames, {
      quality: 'high',
  frameRate: 30
});

// Save the file
const blob = new Blob([mp4], { type: 'video/mp4' });
    const url = URL.createObjectURL(blob);
const a = document.createElement('a');
    a.href = url;
a.download = 'animation.mp4';
    a.click();

2. Camera Recording with Progress

import { encode } from 'webcodecs-encoder';

const stream = await navigator.mediaDevices.getUserMedia({
  video: { width: 1280, height: 720 },
  audio: true
});

const mp4 = await encode(stream, {
  quality: 'medium',
  container: 'mp4',
  onProgress: (progress) => {
    console.log(`Progress: ${progress.percent.toFixed(1)}%`);
    console.log(`Speed: ${progress.fps.toFixed(1)} fps`);
    if (progress.estimatedRemainingMs) {
      console.log(`ETA: ${(progress.estimatedRemainingMs / 1000).toFixed(1)}s`);
    }
  }
});

3. Real-time Streaming

import { encodeStream } from 'webcodecs-encoder';

const stream = await navigator.mediaDevices.getDisplayMedia({ video: true });
const chunks = [];

for await (const chunk of encodeStream(stream, {
  quality: 'medium',
  video: { latencyMode: 'realtime' }
})) {
  // Send to server or MediaSource immediately
  chunks.push(chunk);

  // Or stream to MediaSource Extensions
  if (mediaSource.readyState === 'open') {
    sourceBuffer.appendBuffer(chunk);
  }
}

// Combine all chunks for final file
const fullVideo = new Uint8Array(chunks.reduce((acc, chunk) => acc + chunk.length, 0));
let offset = 0;
for (const chunk of chunks) {
  fullVideo.set(chunk, offset);
  offset += chunk.length;
}

4. Custom Frame Generation

import { encode } from 'webcodecs-encoder';

// Generate frames dynamically
async function* generateFrames() {
  const canvas = new OffscreenCanvas(640, 480);
  const ctx = canvas.getContext('2d');

  for (let frame = 0; frame < 300; frame++) { // 10 seconds at 30fps
    // Draw your animation
    ctx.fillStyle = '#000';
    ctx.fillRect(0, 0, 640, 480);
    ctx.fillStyle = '#fff';
    ctx.font = '48px Arial';
    ctx.fillText(`Frame ${frame}`, 50, 240);

    yield canvas.transferToImageBitmap();

    // Optional: add timing control
    await new Promise(resolve => setTimeout(resolve, 33)); // ~30fps
  }
}

const mp4 = await encode(generateFrames(), {
  quality: 'high',
  frameRate: 30
});

Advanced Usage

The main package entry webcodecs-encoder exports all core functionalities. Sub-path imports like webcodecs-encoder/factory are no longer necessary.

Custom Encoder Factory

For repeated encoding with the same settings:

import { createEncoder, encoders } from 'webcodecs-encoder';

// Create custom encoder
const myEncoder = createEncoder({
  quality: 'high',
  video: { codec: 'avc' },
  audio: { codec: 'aac', bitrate: 192_000 }
});

// Use multiple times
const video1 = await myEncoder.encode(frames1);
const video2 = await myEncoder.encode(frames2);

// Or use predefined encoders
const youtubeVideo = await encoders.youtube.encode(frames);
const twitterVideo = await encoders.twitter.encode(frames);

Error Handling

import { encode, EncodeError } from 'webcodecs-encoder';

try {
  const mp4 = await encode(frames, { quality: 'high' });
} catch (error) {
  if (error instanceof EncodeError) {
    // The 'type' property provides specific details
    console.error(`Encoding failed: ${error.type}`, error.message);

    switch (error.type) {
      case 'not-supported':
        console.log('WebCodecs not supported in this browser');
        break;
      case 'invalid-input':
        console.log('Invalid input frames or configuration');
        break;
      case 'configuration-error':
        console.log('The provided configuration is not supported.');
        break;
      case 'initialization-failed':
      case 'video-encoding-error':
      case 'audio-encoding-error':
      case 'muxing-failed':
      case 'worker-error':
        console.log('A critical error occurred during the encoding process.');
        break;
      case 'cancelled':
        console.log('The encoding was cancelled.');
        break;
      // ... handle other specific error types
      default:
        console.log('Unknown encoding error:', error.message);
    }
  }
}

The EncodeError.type can be one of: 'not-supported', 'invalid-input', 'initialization-failed', 'configuration-error', 'video-encoding-error', 'audio-encoding-error', 'muxing-failed', 'cancelled', 'timeout', 'worker-error', 'filesystem-error', 'unknown'.

Browser Support

• Chrome 113+: Full support.
• Edge 113+: Full support.
• Firefox: Experimental support (enable dom.media.webcodecs.enabled).
• Safari: Not yet supported.

Note: While WebCodecs was available in earlier versions, versions 113+ are recommended for stability.

Check support at runtime:

import { canEncode } from 'webcodecs-encoder';

const supported = await canEncode();
if (!supported) {
  // Fallback to MediaRecorder or other solutions
}

Performance Tips

1. Use quality presets instead of manual bitrate calculation.
2. Enable hardware acceleration when available: { video: { hardwareAcceleration: 'prefer-hardware' } }.
3. Use streaming for large videos: encodeStream() instead of encode().
4. Optimize frame rate for your use case (30fps is usually sufficient).
5. Tune queue limits for real-time streams: Adjust queue size and backpressure strategy to balance latency and frame drops.

   encode(stream, {
     latencyMode: 'realtime',
     maxVideoQueueSize: 15, // Lower queue size for lower latency
     backpressureStrategy: 'drop' // Drop frames if the system can't keep up
   });

6. Consider container format: MP4 for compatibility, WebM for smaller files.

Migration Guide

From v0.2.x to v0.3.0

Breaking Changes: The MediaStreamRecorder class has been removed in favor of the function-first API.

Before (v0.2.x)

import { MediaStreamRecorder } from 'webcodecs-encoder';

const recorder = new MediaStreamRecorder(options);
await recorder.start(stream);
// ... recording in progress
const mp4Data = await recorder.stop();

After (v0.3.0+)

import { encodeStream } from 'webcodecs-encoder';

const chunks: Uint8Array[] = [];
for await (const chunk of encodeStream(stream, options)) {
  chunks.push(chunk);
}

// Combine chunks into final video
const totalSize = chunks.reduce((sum, c) => sum + c.byteLength, 0);
const mp4Data = new Uint8Array(totalSize);
let offset = 0;
for (const chunk of chunks) {
  mp4Data.set(chunk, offset);
  offset += chunk.byteLength;
}

Migration Benefits

• Better tree-shaking: Smaller bundle sizes with function imports
• Streaming support: Real-time chunk processing
• Memory efficiency: Progressive encoding without buffering entire video
• Error handling: Standard async/await error handling

Common Migration Patterns

Recording Control:

// Before: recorder.start() / recorder.stop()
// After: Control via MediaStream tracks
setTimeout(() => {
  stream.getTracks().forEach(track => track.stop());
}, 5000);

Progress Tracking:

// Before: constructor options
new MediaStreamRecorder({ onProgress })

// After: encodeStream options  
encodeStream(stream, { onProgress })

Cancellation:

// Before: recorder.cancel()
// After: Stop MediaStream tracks or break out of the loop
const stopRecording = () => {
  stream.getTracks().forEach(track => track.stop());
};

setTimeout(stopRecording, 5000);

The current implementation does not accept an AbortSignal. To cancel encode / encodeStream, stop the MediaStream tracks or end the async generator manually.

See examples/realtime-mediastream.ts for complete examples.

Changelog

v0.2.2 (2025-06-14)

🚀 Major Features

• Real-time streaming: Fixed encodeStream() MediaStream processing - no longer throws errors
• Audio-only encoding: Added video: false option support for pure audio encoding
• VideoFile audio extraction: Automatic audio track processing from video files using AudioContext
• Transferable objects optimization: Improved performance with optimized VideoFrame/AudioData transfer

🔧 Improvements

• Enhanced MediaStream track detection for audio-only streams
• Better error handling for AudioContext unavailability
• Optimized worker communication with transferable objects
• Extended type definitions for video: false configurations

🐛 Bug Fixes

• Fixed real-time MediaStream processing in encodeStream()
• Resolved audio processing issues in VideoFile inputs
• Improved configuration inference for audio-only scenarios

📝 Documentation

• Added comprehensive examples for new features
• Updated API documentation with v0.2.2 features
• Added performance optimization guidelines

v0.2.1 (2025-06-13)

• Added VideoFile support and removed AudioWorklet feature
• Updated MediaStreamRecorder to use MediaStreamTrackProcessor
• Improved build configuration and exports
• Enhanced test coverage and documentation

License

MIT License - see LICENSE file for details.

Contributing

Contributions are welcome! Please see our Contributing Guide for details.

Support

• 📖 Documentation
• 🐛 Issue Tracker
• 💬 Discussions