Fails to parse Hebrew text from pdf using iText 7 with .net

0
votes

I am trying to read a PDF file with several pages, using iText 7 on a .NET CORE 2.1 The following is my code:

Rectangle rect = new Rectangle(0, 0, 1100, 1100);
LocationTextExtractionStrategy strategy = new LocationTextExtractionStrategy();
inputStr = PdfTextExtractor.GetTextFromPage(pdfDocument.GetPage(i), strategy);

inputStr gets the following string:

"\u0011\v\u000e\u0012\u0011\v\f)(*).=*%'\f*).5?5.5*.\a \u0011\u0002\u001b\u0001!\u0016\u0012\u001a!\u0001\u0015\u001a \u0014\n\u0015\u0017\u0001(\u001b)\u0001)\u0016\u001c*\u0012\u0001\u001d\u001a \u0016* \u0015\u0001\u0017\u0016\u001b\u001a(\n,\u0002>&\u00...

and in the Text Visualizer, it looks like that:

1
asp.net-core-2.0itext7

1 Answers

1
votes

Analysis of the shared files

file1_copyPasteWorks.pdf

The font definitions here have an invalid ToUnicode entry:

/ToUnicode/Identity-H

The ToUnicode value is specified as

A stream containing a CMap file that maps character codes to Unicode values

(ISO 32000-2, Table 119 — Entries in a Type 0 font dictionary)

Identity-H is a name, not a stream.

Nonetheless, Adobe Reader interprets this name, and for apparently any name starting with Identity- assumes the text encoding for the font to be UCS-2 (essentially UTF-16). As this indeed is the case for the character codes used in the document, copy&paste works, even if for the wrong reasons. (Without this ToUnicode value, Adobe Reader also returns nonsense.)

iText 7, on the other hand, for mapping to Unicode first follows the Encoding value with unexpected results.

Thus, in this case Adobe Reader arrives at a better result by interpreting meaning into an invalid piece of data (and without that also returns nonsense).

file2_copyPasteFails.pdf

The font definitions here have valid but incomplete ToUnicode maps which only contain entries for the used Western European characters but not for Hebrew ones. They don't have Encoding entries.

Both Adobe Reader and iText 7 here trust the ToUnicode map and, therefore, cannot map the Hebrew glyphs.

How to parse

file1_copyPasteWorks.pdf

In case of this file the "problem" is that iText 7 applies the Encoding map. Thus, for decoding the text one can temporarily replace the Encoding map with an identity map:

for (int i = 1; i <= pdfDocument.GetNumberOfPages(); i++)
{
    PdfPage page = pdfDocument.GetPage(i);
    PdfDictionary fontResources = page.GetResources().GetResource(PdfName.Font);
    foreach (PdfObject font in fontResources.Values(true))
    {
        if (font is PdfDictionary fontDict)
            fontDict.Put(PdfName.Encoding, PdfName.IdentityH);
    }

    string output = PdfTextExtractor.GetTextFromPage(page);
    // ... process output ...
}

This code shows the Hebrew characters for your file 1.

file2_copyPasteFails.pdf

Here I don't have a quick work-around. You may want to analyze multiple PDFs of that kind. If they all encode the Hebrew characters the same way, you can create your own ToUnicode map from that and inject it into the fonts like above.