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Expand documentation of filter_page_image

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James R. Barlow 2020-09-14 14:36:12 -07:00
parent 624df9bb23
commit 8b5b02e0d8
1 changed files with 20 additions and 2 deletions
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22
src/ocrmypdf/pluginspec.py
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@ -154,11 +154,29 @@ def filter_page_image(page: 'PageContext', image_filename: Path) -> Path:
produced for a given page, this function will not be called. This is not
the image that will be shown to OCR.
ocrmypdf will create the PDF page based on the image format used. If you
If the function does not want to modify the image, it should return
``image_filename``. The hook may overwrite ``image_filename`` with a new file.
The output image should preserve the same physical unit dimensions, that is
(width * dpi_x, height * dpi_y). That is, if the image is resized, the DPI
must be adjusted by the reciprocal. If this is not preserved, the PDF page
will be resized and the OCR layer misaligned. OCRmyPDF does not nothing
to enforce these constraints; it is up to the plugin to do sensible things.
OCRmyPDF will create the PDF page based on the image format used. If you
convert the image to a JPEG, the output page will be created as a JPEG, etc.
Note that the ocrmypdf image optimization stage may ultimately chose a
If you change the colorspace, that change will be kept. Note that the
OCRmyPDF image optimization stage, if enabled, may ultimately chose a
different format.
If the return value is a file that does not exist, ``FileNotFoundError``
will occur. The return value should be a path to a file in the same folder
as ``image_filename``.
Implementation detail: If the value returned is falsy, OCRmyPDF will ignore
the return value and assume the input file was unmodified. This is deprecated.
To leave the image unmodified, ``image_filename`` should be returned.
Note:
This hook will be called from child processes. Modifying global state
will not affect the main process or other child processes.