Hi.
The process is as follows:
1) For images: use the image
For PDFs: render each page to 300 dpi (since optimized PDFs don't
necessarily have a single big image), maybe even with text if text extraction
returned gibberish (missing unicode mapping).
2) Use tesseract to OCR image/page with PDF and HOCR output. (for pages: create
an imageless PDF). The HOCR is used for additional page layout information and
word confidence values.
3) For images, use the HOCR to filter the PDF text stream and add layout
information
For PDFs, insert the tesseract PDF text stream into the orignal PDF's page
(+add that glyphless font), use the HOCR to filter and add layout information.
For step 3, I would like to use a normal PDPageContentStream to add the content
instead of working with a raw stream. But that step fails since I cannot use
the showText() method with a Font that has an empty cmap.
I attached an empty tesseract PDF with the glyphless font. Appending text using
the font to the single page in there will fail immediately with the exception
due to the empty cmap. Adding the font to any other PDF and trying to show text
using it will fail as well.
I can probably get away with just creating/transfering the Tj commands raw, but
I was wondering if the empty cmap behaviour is ok or would it be better to
ignore empty cmaps (i.e. look for a non empty one first and return null if none
can be found in TrueTypeFont.getUnicodeCmapImpl).
Gunnar
-----Ursprüngliche Nachricht-----
Von: Tilman Hausherr <thaush...@t-online.de>
Gesendet: Donnerstag, 25. März 2021 04:37
An: users@pdfbox.apache.org
Betreff: Re: Empty cmap in TTF Files.
Am 24.03.2021 um 14:40 schrieb Gunnar Brand:
> Hi.
>
> I am working on merging original PDFs and the PDF/HOCR output of Tesseract,
> as to create a searchable PDF. Transplanting the glyphless font used by
> tesseract was no problem, it doesn’t matter if I simply use the font in the
> original PDF or use cloneutil, when saving the file the font is embedded
> properly.
>
> The problem is when I show text using a content stream, I get a “No Glyph for
> …” exception. I traced this down to the glyphless font containing empty cmap
> tables. There is a CIDToGIDMap. Coincidentally PDFBOX-5103 just addressed
> this issue with a reverse mapping if the cmap is null. But the cmap is just
> empty and will return 0 for any character code, so this new feature will
> never work in this case.
>
> For testing I modified TrueTypeFont.getUnicodeCmapImpl(isStrict) so that it
> ignores empty cmap subtables (even the fallback at the end of the method now
> being a loop). With this PDFBox will happily use the tesseract glyphless
> font. Now I lack the knowledge if empty cmaps make any sense at all and if
> they do I will simply write raw show text commands, but maybe it is something
> to consider?
>
> Gunnar
I tried tesseract some time ago and it generates searchable PDFs out of the
box, why not use that?
Can you upload one of your files to a sharehoster so that I understand what
this is about?
Tilman
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