Himotoki Sub – learn Japanese from subtitles

Browser extension that turns the subtitles of YouTube, Netflix and other streaming sites into a Japanese study tool. Every subtitle line is split into words with a local model, and hovering or clicking a word opens a Himotoki dictionary popup with readings, meanings, pitch accent, JLPT/frequency and grammar. Words can be saved to your Himotoki account or to Anki. Everything on the hover path runs on your device once the offline dictionary is installed. Japanese-only.

Demo

Furigana over the subtitles	Click a word for the dictionary

How it works

1. Word splitting runs locally. A character-level BiLSTM + CRF model (public/models/default.onnx, trained in himotoki-split) runs inside the extension through onnxruntime-web in an offscreen document. No text leaves the browser for segmentation. Until the model has run, cues are painted with Intl.Segmenter so subtitles appear instantly.
2. Dictionary lookup is local. On first use the extension popup offers to download a trimmed Jitendex database (about 38 MB compressed, 185 MB unpacked) into the browser's origin-private file system. Lookups then run in an extension worker with SQLite compiled to WebAssembly, including deinflection, and answer in a few milliseconds. Word lookup has no online fallback — until the dictionary is installed the popup prompts you to install it. (The dictionary file itself is a self-hosted GitHub release; see "Offline dictionary file".)
3. Whole-line translation is optional: clicking a subtitle line outside a word shows a machine translation of the full line via Google Translate, or DeepL with your own API key (DeepL's free API tier works).
4. Saving words. Sign in with Google from the extension popup to save words (with the sentence, video URL and timestamp) to Himotoki favorites, or choose Anki in settings to send them to a local AnkiConnect (http://localhost:8765).

Supported sites

YouTube, Netflix, KinoPub, Coursera, Plex, Udemy, Kinopoisk, Amazon Prime Video, inoriginal.online. On YouTube a Japanese caption track is preferred automatically when the video has one.

Features

• Local word splitting with an instant Intl.Segmenter first paint.
• Offline dictionary (Jitendex, SQLite WebAssembly) with deinflection; a one-time ~38 MB download, then every lookup stays on your device.
• Configurable hover and click — each can be furigana, meaning, furigana + meaning, pop-up dictionary, or nothing.
• Dictionary pop-up with senses, pitch accent, JLPT and frequency, an example, the deconjugation chain and an on-demand conjugation table, an entry switcher, word audio (speech synthesis + replay the line from the video), and save / mark-known. Pitch accent can show every recorded contour, compact numbers, or be hidden.
• Furigana ruby over the kanji only; always/on-hover/never, and can be limited to words above a chosen JLPT level.
• New words only (beta): pick your JLPT level (N3, N2 or N1) and each subtitle line becomes a short glossary of the words above it (harder JLPT words and rare ones) with their meanings. Move the mouse to get a Show line chip beside the glossary (it fades out while you watch), click the glossary, press H, or assign a mouse button to reveal the full line; mark a missed word Learning and it is always listed.
• Colour words by JLPT difficulty, and dim known words with per-video coverage and i+1 stats.
• Dual subtitles (a second track or a translation) and whole-line translation on click.
• Reading & shadowing: searchable transcript, sentence breakdown, replay/loop the current line, and arrow-key subtitle navigation.
• Adjustable overlay (size, background, delay, scale) and support for your own .srt / .vtt.

Keyboard shortcuts

Key	Action
← / →	Previous / next subtitle (alt + arrow forces the jump)
D	Cycle the second subtitle line
R	Replay the current line
L	Loop the current line (shadowing)
B	Toggle the sentence breakdown
T	Toggle the searchable transcript
H	Peek: show the full line in listening mode or new-words-only mode
, / .	Playback speed down / up
\	Replay the current line slowly (0.75×)
Esc	Close a pinned pop-up

Mouse controls

The middle button and the two side buttons (back / forward) can each be set to previous line, next line, replay, loop, slow replay, play/pause or show full line, from the in-player panel or the settings page. They are off by default and only act while the pointer is over the video; elsewhere they keep their normal browser behaviour.

Settings live in the in-player panel, the full settings page (toolbar popup → Open settings), and a welcome page on first install.

Build

1. Install Node 22.13+ and pnpm 11 (corepack enable sets it up).
2. pnpm i
3. pnpm build (Chrome) or pnpm build:firefox
4. Load the dist/ folder as an unpacked extension (chrome://extensions, developer mode, "Load unpacked").

pnpm dev starts a watch build with hot reload.

Offline dictionary file

The extension downloads jitendex-lite.sqlite.gz from the URL in src/shared/himotokiConfig.ts (HIMOTOKI_DICT_URL, default the repo's own GitHub release: https://github.com/msr2903/himotoki-sub/releases/latest/download/jitendex-lite.sqlite.gz). It is self-hosted so the offline dictionary does not depend on himotoki.my.id. Build the file from a Jitendex SQLite and publish it as a release asset:

python3 scripts/build-dict.py /path/to/jitendex.sqlite dist-dict \
  --pitch pitch-kanjium.sqlite --freq freq-jpdb-v2.sqlite --jlpt jlpt.sqlite
# → dist-dict/jitendex-lite.sqlite.gz (+ .json manifest with revision and sha256)
gh release create dict-<revision> dist-dict/jitendex-lite.sqlite.gz dist-dict/jitendex-lite.json --latest

latest/download always resolves to the newest release, and a new release with a higher manifest revision triggers the in-app "update available" prompt.

All deployment-tied endpoints are runtime-configurable so a domain/backend move needs no rebuild: the Advanced section of the options page overrides the dictionary URL and the Google OAuth client ID used by Save to Himotoki. Each falls back to the compiled default when left blank, and setting a custom dictionary URL requests host permission for that origin. The same values can be set directly as plain chrome.storage.local keys (himotokiDictUrl, himotokiGoogleClientId) — the end-to-end scripts set himotokiDictUrl this way.

Save to Himotoki signs in with Google once (from the popup or options page), then every save goes straight into the same Himotoki account as himotoki.web.app and the Himotoki app — no confirm page. It writes only the signed-in user's own saved-words document in Himotoki's Firebase project, with the same merge rules as the website, and retries instead of overwriting when another device saves at the same moment. For sign-in to work, the himotoki project's Google Web client must list this extension's redirect URL (https://<extension-id>.chromiumapp.org/, shown under Advanced) as an Authorized redirect URI.

Testing

pnpm test:unit                                  # Vitest unit tests (pure logic)
pnpm test                                        # deterministic Chromium regression checks
npx playwright install chromium
pnpm test:e2e                                    # YouTube: split, popup, settings, hotkeys
node scripts/e2e/dict.mjs /tmp/himotoki-dict     # offline dictionary: install, lookups, repair, persistence
HIMOTOKI_DICT_DIR=/tmp/himotoki-dict pnpm test:e2e   # YouTube with the offline dictionary installed

tsc --noEmit, pnpm test:unit and pnpm build also run in CI on every push and pull request.

Firefox note (untested target): Firefox does not implement chrome.offscreen. The word splitter falls back to Intl.Segmenter, and the offline dictionary worker is started directly in the background page instead of an offscreen document; this path has not been verified in a real Firefox yet. If a browser can run neither, the dictionary panel reports that the dictionary is unavailable in this browser and word lookups are off (whole-line translation still works).

Permissions

Permission	Why
Content scripts on the supported sites	Read the site's caption track and draw the subtitle overlay.
storage, unlimitedStorage	Settings, known words, and the offline dictionary (~185 MB unpacked) in the browser's private file system.
offscreen	Run the word-splitting model and the dictionary worker outside the page.
scripting, activeTab	Inject the overlay on a site after you grant it access from the popup.
identity, accounts.google.com	Optional Google sign-in for Save to Himotoki.
github.com, *.githubusercontent.com	Download the offline dictionary release.
*.convex.cloud	Optional Save to Himotoki backend.
translate.google.com, api.deepl.com, api-free.deepl.com	Whole-line translation, only when you click a line.
localhost:8765	AnkiConnect, only when you choose Anki for saving words.
Optional: any site	Only requested when you set a custom dictionary or backend URL in Advanced settings.

Nothing is sent anywhere while you only watch and hover; see PRIVACY.md.

Using alongside Yomitan

Yomitan scans any text on the page, including this extension's subtitle overlay, so with both enabled you may see two pop-ups. Either add youtube.com (and the other video sites) to Yomitan's excluded sites, or set this extension's hover action to "No action" and keep click for the Himotoki pop-up.

Contributing

Issues and pull requests are welcome. See CONTRIBUTING.md for setup, checks and conventions, and ROADMAP.md for planned work. Please open an issue to discuss larger features before implementing them.

Licence

Himotoki Sub is licensed under GNU Affero General Public License v3.0 or later (AGPL-3.0-or-later), matching asbplayer's licence for the planned playback/mining work. Copies and forks are allowed, including commercial use, subject to the licence's source-sharing and notice requirements. This change does not revoke permissions granted for earlier MIT releases.

Third-party code and dictionary data keep their own licences. The inherited MIT notice is preserved in LICENSES/MIT-upstream.txt; see THIRD_PARTY_NOTICES.md for data, models, fonts and other dependencies. Source and build information is included with the extension build. No asbplayer code is included by this licence change.

Credits

Built on EasySubs by Nitrino — this project is a Japanese-only fork that removes its multi-language features (Google word translation, phrasal verbs, English dictionaries, LinguaLeo, Puzzle English) and builds Japanese learning around the Himotoki dictionary and a local word splitter. Dictionary data from Jitendex (CC BY-SA 4.0), built from JMdict (EDRDG) with Tatoeba examples; pitch accent from Kanjium (CC BY-SA 4.0); JLPT levels from jlpt-word-list (based on the tanos.co.uk lists); frequency ranks from JPDB. Fonts: Newsreader and Zen Maru Gothic (SIL OFL 1.1). See THIRD_PARTY_NOTICES.md for every component and licence.