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Posted to dev@tika.apache.org by "Ewan Mellor (JIRA)" <ji...@apache.org> on 2018/04/02 18:58:00 UTC
[jira] [Updated] (TIKA-2624) Rendering PDFs for OCR with Tesseract
uses different DPI than claimed
[ https://issues.apache.org/jira/browse/TIKA-2624?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Ewan Mellor updated TIKA-2624:
------------------------------
Description:
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 in both places.
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.
was:
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.
> Rendering PDFs for OCR with Tesseract uses different DPI than claimed
> ---------------------------------------------------------------------
>
> Key: TIKA-2624
> URL: https://issues.apache.org/jira/browse/TIKA-2624
> Project: Tika
> Issue Type: Bug
> Components: parser
> Affects Versions: 1.17
> Reporter: Ewan Mellor
> Priority: Major
>
> 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 in both places.
> 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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