OCRmyPDF — Image Processing Guide
Overview
OCRmyPDF includes powerful image processing capabilities to improve scan quality before OCR. These tools help fix skewed pages, remove noise, clean borders, and enhance readability.
For core OCR functionality, see the ocrmypdf skill. For optimization and PDF/A options, see ocrmypdf-optimize. For batch/Docker/scripting, see ocrmypdf-batch.
Deskew
Deskew corrects pages that are slightly rotated (e.g., from feed scanner skew).
# Auto deskew (recommended)
ocrmypdf --deskew input.pdf output.pdf
# Force deskew even if rotation is minimal
ocrmypdf --deskew --force-ocr input.pdf output.pdf
Rotation
Rotate pages to correct upside-down or sideways scans:
# Auto-rotate based on text orientation
ocrmypdf --rotate-pages input.pdf output.pdf
# Force rotate all pages
ocrmypdf --rotate-pages --force-ocr input.pdf output.pdf
Remove Borders / Cleaning
Remove unwanted borders, artifacts, and noise from scanned pages:
# Remove borders (dots, solid borders)
ocrmypdf --remove-bordering input.pdf output.pdf
# Combine with cleanup
ocrmypdf --remove-bordering --clean input.pdf output.pdf
Despeckle
Remove speckles and isolated noise pixels:
# Remove speckles
ocrmypdf --despeckle input.pdf output.pdf
# Aggressive despeckle for very noisy scans
ocrmypdf --despeckle --clean input.pdf output.pdf
Unpaper
unpaper provides advanced post-processing:
# Apply unpaper with default settings
ocrmypdf --unpaper input.pdf output.pdf
# Custom unpaper board options
ocrmypdf --unpaper-args "--board A4" input.pdf output.pdf
Oversampling
Increase image resolution before OCR for better accuracy:
# Oversample to 300 DPI before OCR
ocrmypdf --oversample 300 input.pdf output.pdf
# Common for low-resolution scans
ocrmypdf --oversample 400 input.pdf output.pdf
Combined Recipes
Fix a skewed scan
ocrmypdf --deskew --remove-bordering --despeckle scanned.pdf fixed.pdf
Clean up a very noisy scan
ocrmypdf --deskew --rotate-pages --despeckle --clean --oversample 300 noisy.pdf clean.pdf
Remove all artifacts
ocrmypdf --remove-bordering --unpaper --despeckle dirty.pdf clean.pdf
Quick Reference
| Task | Command |
|------|---------|
| Auto deskew | --deskew |
| Auto rotate | --rotate-pages |
| Remove borders | --remove-bordering |
| Remove speckles | --despeckle |
| Unpaper | --unpaper |
| Oversample DPI | --oversample N |
Troubleshooting
- Poor OCR after cleaning: Try
--oversample 300to increase input quality. - Artifacts remain: Use
--unpaperfor aggressive cleanup. - Over-cleaned image: Reduce cleaning options for preserve original quality.