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DualSub for Stremio

DualSub is a Stremio addon for learning English from movies. It shows the English subtitle and a translation together, in Burmese, Thai or Chinese, so you follow the story and pick up the English as you watch. It's public, and I designed and built it on my own.

Role
Solo designer and developer
Product
Stremio addon for learning English
Platform
Stremio addon
Status
Live, public

The player and install page are rebuilt in code for this page. The film and the dialogue are sample content.

The idea

Follow the story. Keep the English.

Subtitles in your own language help you follow a film, but you stop listening to the English. English-only subtitles keep the English, but you lose the story. DualSub shows both, line by line, so every scene is a small lesson.

The challenge

Three things had to be true for people to keep it on.

  • Two lines, one glance

    Both lines have to read at a glance without covering the film. English stays on top as the line you learn from; the translation sits under it in a softer style.

  • Ready when you press play

    Translating a whole film line by line is slow. The subtitles have to be ready in about the time it takes a movie to start.

  • Pay once per line

    AI translation costs money on every call. The same film should never be translated twice, however many people watch it.

Install

Pick a language, press Install.

Stremio addons are added from a web page. I kept it to two choices: which language to translate to, and whether to show English with it. A live preview shows exactly what the subtitle will look like, and one button opens Stremio and adds the addon.

How it works

Translate once, reuse forever.

Every translated line is saved. The first person to watch a film waits a moment while it's translated; everyone after that gets the subtitles straight away, at no extra cost.

  1. StremioAsks

    The player asks for subtitles for the film or episode being played.

  2. CacheChecks

    Lines already translated come back at once from the cache.

  3. AITranslates

    The rest go out in batches with their neighbouring lines, several batches at a time, so tone and names carry across.

  4. PlayerShows

    English and the translation are merged on the original timing, and the new lines are saved for the next viewer.

Design details

  • English on top, translation below in a softer colour and style, so the eye reads the English first
  • Both lines share the original timing, so they appear and leave together
  • Two modes: English with the translation for learning, or the translation alone for just watching
  • Lines are translated with their neighbours, so a reply still sounds like a reply

My role

  • The idea, the design and the code, on my own
  • Subtitle source, AI translation and the translation cache
  • The install and configure page
  • Shipped as a public Stremio addon

Tools

Node.jsStremio Addon SDKSQLiteDeepSeekRailway
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