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CHAPTER
5
Cookbook
Basic examples
Help!
ocrmypdf has built-in help.
ocrmypdf --help
Add an OCR layer and convert to PDF/A
ocrmypdf input.pdf output.pdf
Add an OCR layer and output a standard PDF
ocrmypdf --output-type pdf input.pdf output.pdf
Create a PDF/A with all color and grayscale images converted to JPEG
ocrmypdf --output-type pdfa --pdfa-image-compression jpeg input.pdf output.pdf
Modify a file in place
The file will only be overwritten if OCRmyPDF is successful.
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ocrmypdf myfile.pdf myfile.pdf
Correct page rotation
OCR will attempt to automatic correct the rotation of each page. This can help fix a scanning job that contains a mix of landscape and portrait pages.
ocrmypdf --rotate-pages myfile.pdf myfile.pdf
You can increase (decrease) the parameter --rotate-pages-threshold to make page rotation more (less) aggressive.
OCR languages other than English
By default OCRmyPDF assumes the document is English.
ocrmypdf -l fre LeParisien.pdf LeParisien.pdf
ocrmypdf -l eng+fre Bilingual-English-French.pdf Bilingual-English-French.pdf
Language packs must be installed for all languages specified. See
Installing additional language packs
Produce PDF and text file containing OCR text
This produces a file named “output.pdf” and a companion text file named “output.txt”. The pdftotext program from Poppler is used to extract text from the finished PDF.
ocrmypdf input.pdf - | tee output.pdf | pdftotext - output.txt
Note: To get pdftotext, Debian/Ubuntu users may apt-get install poppler-utils and macOS users may brew install poppler respectively.
OCR images, not PDFs
Use a program like img2pdf to convert your images to PDFs, and then pipe the results to run ocrmypdf: img2pdf my-images*.jpg | ocrmypdf - myfile.pdf
img2pdf also has features to control the position of images on a page, if desired.
For convenience, OCRmyPDF can convert single images to PDFs on its own. If the resolution (dots per inch, DPI) of an image is not set or is incorrect, it can be overridden with --image-dpi. (As 1 inch is 2.54 cm, 1 dpi = 0.39
dpcm).
ocrmypdf --image-dpi 300 image.png myfile.pdf
If you have multiple images, you must use img2pdf to convert the images to PDF.
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Note: ImageMagick convert can also convert a group of images to PDF, but in the author’s experience it takes a long time, transcodes unnecessarily and gives poor results.
You can also use Tesseract 3.04+ directly to convert single page images or multi-page TIFFs to PDF: tesseract my-image.jpg output-prefix pdf
Image processing
OCRmyPDF perform some image processing on each page of a PDF, if desired. The same processing is applied to each page. It is suggested that the user review files after image processing as these commands might remove desirable content, especially from poor quality scans.
• --rotate-pages attempts to determine the correct orientation for each page and rotates the page if necessary.
• --remove-background attempts to detect and remove a noisy background from grayscale or color images.
Monochrome images are ignored. This should not be used on documents that contain color photos as it may remove them.
• --deskew will correct pages were scanned at a skewed angle by rotating them back into place. Skew determination and correction is performed using Postl’s variance of line sums algorithm as implemented in Leptonica .
• --clean uses unpaper to clean up pages before OCR, but does not alter the final output. This makes it less likely that OCR will try to find text in background noise.
• --clean-final uses unpaper to clean up pages before OCR and inserts the page into the final output. You will want to review each page to ensure that unpaper did not remove something important.
Note: In many cases image processing will rasterize PDF pages as images, potentially losing quality.
Warning:
--clean-final and -remove-background may leave undesirable visual artifacts in some images where their algorithms have shortcomings. Files should be visually reviewed after using these options.
OCR and correct document skew (crooked scan)
Deskew: ocrmypdf --deskew input.pdf output.pdf
Image processing commands can be combined. The order in which options are given does not matter. OCRmyPDF always applies the steps of the image processing pipeline in the same order (rotate, remove background, deskew, clean).
ocrmypdf --deskew --clean --rotate-pages input.pdf output.pdf
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Improving OCR quality
The
Image processing
features can improve OCR quality.
Rotating pages and deskewing helps to ensure that the page orientation is correct before OCR begins. Removing the background and/or cleaning the page can also improve results. The --oversample DPI argument can be specified to resample images to higher resolution before attempting OCR; this can improve results as well.
OCR quality will suffer if the resolution of input images is not correct (since the range of pixel sizes that will be checked for possible fonts will also be incorrect).
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