Ewan Mellor created PDFBOX-4178:
-----------------------------------
Summary: Rendering PDFs for OCR with Tesseract uses different DPI
than claimed
Key: PDFBOX-4178
URL: https://issues.apache.org/jira/browse/PDFBOX-4178
Project: PDFBox
Issue Type: Bug
Components: Parsing
Affects Versions: 2.0.9
Reporter: Ewan Mellor
Tika has two properties in `PDFParser.properties` that control what happens in
AbstractPDF2XHTML when a PDF is rendered before being passed to Tesseract for
OCR. These are `ocrDPI` (default 300) and `ocrImageScale` (default 2.0).
`ocrDPI` is passed to `ImageIOUtil.writeImage`, which uses it as the metadata
in the image (i.e. it doesn't control scaling at all, it's just an advertised
metadata field).
`ocrImageScale` is passed to PDFBox's `PDFRenderer.renderImage`, which uses it
to specify the scale for rendering. This value is such that 1.0 == 72dpi, and
therefore Tika's default is to request 144dpi for rendering.
This means that Tika is asking PDFBox to render at 144dpi, and then advertising
300dpi in the image metadata. This makes no sense to me, and is surely going
to confuse Tesseract.
Instead of doing this, we should remove `ocrImageScale`, and use the same DPI
value for rendering as we advertise in the image metadata.
We should keep the existing default DPI value, since Tesseract is trained at
300dpi by default, so this will mean that all stages between PDFRenderer and
Tesseract are defaulting to 300dpi.
This change will have the side-effect that the temporary images between the PDF
rendering and Tesseract will be 4x larger (144dpi to 300dpi). This will have a
memory and temporary disk space impact, but I think that it's still best to
have the whole pipeline using 300dpi. People who have memory constraints will
need to reduce ocrDPI and make the corresponding changes on the Tesseract side.
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