ZenConverter

ZenConverter is a local file converter for Android. Pick a file on your phone, convert it on your phone, and keep it off someone else's server.

The app is built with native Kotlin and Jetpack Compose. File access goes through Android's Storage Access Framework, and longer jobs run in a foreground service. The app deliberately stays narrow: supported routes are verified on a physical Android device, and their known limits are written down instead of hidden.

Note: very large media files still need adequate free storage, memory, and power. Keep the device available while a long foreground conversion is running.

Why Build It

Desktop users already have plenty of good open-source converters. Android feels rougher. Many converter apps are cluttered, ad-heavy, oddly priced, or built around uploading your file somewhere first.

ZenConverter is the local-first Android converter I wanted to use:

• no network transfer for conversion work,
• no ads, accounts, paywalls, or remote uploads,
• INTERNET permission is only used for manual update checks and on-demand model/font downloads,
• no extra permissions unless the app actually needs them,
• large videos are treated as real use cases, even if that path is still rough,
• the support list lives in the public support matrix.

Current Status

Stable routes have been verified on a physical Android device. Beta routes work within the stated compatibility limits.

Architecture

flowchart LR
    Pick["Add files"]
    Configure["Configure each task"]
    Queue["Ready queue"]
    Service["Foreground service"]
    Engine["FFmpeg / Native / Office / WOFF2 / NCNN"]
    Output["Save output"]

    Pick --> Configure --> Queue --> Service --> Engine --> Output

The UI does not do conversion work. Each task is routed to an engine based on the input, output, and selected mode:

• Compatibility: FFmpeg true re-encode path for connected video/audio targets, GIF output, subtitles (SRT/VTT/ASS), and advanced processing.
• Native: Android platform bitmap/PDF handling, PDFBox-Android (PDF merge/text/security), and pure-Kotlin engines (WOFF, LRC).
• Office: Local first-pass Office rendering path for DOCX, PPTX, and XLSX.
• Font / WOFF2: Native google/woff2 engine for WOFF2 compression and decompression.
• AI Super-Resolution: Tencent NCNN with Vulkan GPU acceleration for local Real-ESRGAN neural network inference.
• AI Frame Interpolation: Tencent NCNN with Vulkan GPU compute acceleration for RIFE 2× video frame interpolation.
• SafeCache: fallback for file providers that cannot provide usable descriptors.

More detail lives in docs/architecture.md and docs/technical-route.md.

Development setup notes are in docs/development-setup.md.

Translations

ZenConverter welcomes community translations! You can contribute translations without knowing Kotlin or installing Android Studio. See the English source and translation guide to get started.

License

ZenConverter's own source code is licensed under the GNU Affero General Public License v3.0 or later.

Third-party libraries, native binaries, and bundled fonts keep their own licenses. Details are tracked in docs/license-and-attribution.md and third_party/THANKS.md.

Acknowledgements

• OhMyGPT provides AI API support.
• ForZTN provides the kernel compilation server.

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