@hevcjs/dashjs-plugin

HEVC/H.265 playback plugin for dash.js. Transparently transcodes HEVC segments to H.264 via WebAssembly when native HEVC is unavailable. When native HEVC is available, the plugin detects it and does nothing.

Install

npm install @hevcjs/dashjs-plugin dashjs

Setup

Copy the static assets from @hevcjs/core to your public directory:

cp node_modules/@hevcjs/core/dist/transcode-worker.js public/
cp node_modules/@hevcjs/core/dist/wasm/hevc-decode.js public/
cp node_modules/@hevcjs/core/dist/wasm/hevc-decode.wasm public/

Adjust public/ to match your project structure.

Usage

import dashjs from 'dashjs';
import { attachHevcSupport } from '@hevcjs/dashjs-plugin';

const video = document.querySelector('video');
const player = dashjs.MediaPlayer().create();

await attachHevcSupport(player, {
  workerUrl: '/transcode-worker.js',
  wasmUrl: '/hevc-decode.js',
});

player.initialize(video, 'https://example.com/stream/manifest.mpd', true);

How It Works

When attachHevcSupport(player) is called:

1. Probes native HEVC support — creates a real SourceBuffer (not just isTypeSupported, which can lie on Firefox)
2. If native HEVC works — does nothing, zero overhead
3. If not — patches MediaSource.addSourceBuffer() to intercept HEVC and return an H.264 proxy
4. The proxy SourceBuffer intercepts appendBuffer():
   • Init segments: extracts VPS/SPS/PPS from hvcC
   • Media segments: demux (mp4box.js) → decode HEVC (WASM) → encode H.264 (WebCodecs) → mux fMP4 → append to real H.264 SourceBuffer
5. Proper updating state management — the proxy reports updating = true during transcoding, so dash.js waits between segments

Audio and subtitle tracks pass through untouched.

API

attachHevcSupport(player, config?)

const cleanup = await attachHevcSupport(player, {
  workerUrl: '/transcode-worker.js',
  wasmUrl: '/hevc-decode.js',
  fps: 25,              // Target framerate (optional, default: 25)
  bitrate: 4_000_000,   // H.264 encode bitrate (optional)
  forceTranscode: false, // Bypass native HEVC detection (optional)
});

// Remove patches when done
cleanup();

Requirements

• Chrome 94+, Edge 94+, or Firefox with WebCodecs H.264 encoding support
• Secure Context (HTTPS or localhost)
• dash.js >= 4.0.0

License

MIT