If the page is "just a little off horizontal", like a crooked picture, then you want ``--deskew``. ``--rotate-pages`` is for when the cardinal angle is wrong.
If you are starting with images, you can just use Tesseract 3.04 or later directly to convert images to PDFs:
..code-block:: bash
tesseract my-image.jpg output-prefix pdf
..code-block:: bash
# When there are multiple images
tesseract text-file-containing-list-of-image-filenames.txt output-prefix pdf
Tesseract's PDF output is quite good –OCRmyPDF uses it by internally by default. However, OCRmyPDF has many features not available in Tesseract like like image processing, metadata control, and PDF/A generation.
Use a program like `img2pdf <https://gitlab.mister-muffin.de/josch/img2pdf>`_ to convert your images to PDFs, and then pipe the results to run ocrmypdf. The `-` tells ocrmypdf to read standard input.
For convenience, OCRmyPDF can also 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).
If you have multiple images, you must use ``img2pdf`` to convert the images to PDF.
..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.
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 <http://www.leptonica.com/skew-measurement.html>`_ algorithm as implemented in `Leptonica <http://www.leptonica.com/index.html>`_.
*``--clean`` uses `unpaper <https://www.flameeyes.eu/projects/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.
``--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.
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).
If you set ``--tesseract-timeout 0`` OCRmyPDF will apply its image processing without performing OCR, if all you want to is to apply image processing or PDF/A conversion.
To redo OCR on a file OCRed with other OCR software or a previous version of OCRmyPDF and/or Tesseract, you may use the ``--force-ocr`` argument. Normally, OCRmyPDF does not modify files that already appear to contain OCR text.
..code-block:: bash
ocrmypdf --force-ocr input.pdf output.pdf
Note that the method above will force rasterization of all pages, potentially reducing quality or losing vector content.
To ensure quality is preserved, one could extract all of the images and rebuild the PDF for a lossless transformation. This recipe does not work when PDFs contain multiple images per page, as many do in practice. It will also lose any page rotation information.
..code-block:: bash
pdfimages -all old-ocr.pdf prefix # extract all images
img2pdf -o temp.pdf prefix* # construct new PDF from the images
# review the new PDF to ensure it visually matches the old one
ocrmypdf --output-type pdf temp.pdf new-ocr.pdf
``--output-type pdf`` is used here to avoid using Ghostscript which will also rasterize images.
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).
