FAQ

FAQ

recognize returns empty text

• The image may be too low-resolution or low-contrast. See Performance.
• Wrong PSM: a full-page mode on a single word (or vice-versa) can yield nothing. Try pageSegMode = 6 or 7.
• Confirm the language is right for the text (getVersion() / getAvailableLanguages()).

Low confidence / garbled output

• Upscale small text (glyphs ≥ 20 px), increase contrast, deskew.
• For numbers/codes, set whitelistChars.
• Try tessdata_best models (Pro) for hard inputs.

TesseractException.LanguageDataMissing

The requested language's *.traineddata isn't on disk. English is bundled; for others, download the file and call addLanguage(context, code, file) before initialize. See Languages.

UnsatisfiedLinkError / lib not found

Ensure your app includes the arm64-v8a ABI. The Free AAR is arm64-v8a only — it will not load on an x86_64 emulator. Use a physical arm64 device, or Pro which adds x86_64.

Does it work offline?

Yes. Everything is on-device. English works with zero network. Additional languages just need their traineddata present (bundled or added once).

Can I OCR a PDF?

Not directly — render the PDF page to a Bitmap (e.g. PdfRenderer) and pass that to recognize.

Is it thread-safe?

A single TesseractOCR instance is not thread-safe. Serialise calls, or use one instance per worker thread.

Why is my first call slow?

initialize extracts the bundled traineddata to storage and loads the LSTM model into memory. Subsequent recognize calls are much faster. Keep the instance alive and reuse it.

License / commercial use?

The library is Apache 2.0 — fine for commercial apps. Tesseract (Apache 2.0), Leptonica (BSD-2-Clause), and the bundled English model (Apache 2.0) permit redistribution.

How is Pro different?

12 bundled languages, tessdata_best models, OpenMP multi-threading, batch/advanced API, and x86_64 support. See Pro Integration.