如何用Java/Scala提取PDF字形？天城文PDF映射错误处理需求

Got it, let's break this down step by step. Devanagari PDFs often have wonky glyph-to-Unicode mappings because creators sometimes use custom encodings or broken ToUnicode CMaps. Here's how to extract the glyphs, fix their mappings, and code this in Java/Scala.

First: Diagnose the Fonts in Your PDF #

Before writing any code, use the pdffonts tool (part of Poppler) to check what's going on with your PDF's fonts. Run this in your terminal:

pdffonts your-devanagari-document.pdf

Look for entries where the ToUnicode column says no or unknown—that's almost certainly where your mapping errors are coming from. If the font is embedded in the PDF, you can extract it later to build a correct mapping.

Step 2: Glyph Extraction & Mapping with Java/Scala #

We'll use Apache PDFBox—it's the go-to open-source library for PDF processing, and it has solid support for font and glyph handling.

Add Dependencies #

For Java (Maven):

<dependency>
    <groupId>org.apache.pdfbox</groupId>
    <artifactId>pdfbox</artifactId>
    <version>2.0.32</version>
</dependency>
<dependency>
    <groupId>org.apache.pdfbox</groupId>
    <artifactId>fontbox</artifactId>
    <version>2.0.32</version>
</dependency>

For Scala (sbt):

libraryDependencies += "org.apache.pdfbox" % "pdfbox" % "2.0.32"
libraryDependencies += "org.apache.pdfbox" % "fontbox" % "2.0.32"

Example Code: Extract Glyphs & Fix Mappings #

Here's a Scala example (easily tweakable for Java) that pulls glyphs, checks the existing mapping, and falls back to a custom correction map when things go wrong:

import org.apache.pdfbox.pdmodel.PDDocument
import org.apache.pdfbox.text.{PDFTextStripper, TextPosition}
import java.io.File

class DevanagariGlyphFixer extends PDFTextStripper {
  // Replace this with your own corrected GID-to-Unicode pairs
  private val fixedGlyphMap: Map[Int, String] = Map(
    123 -> "\u0915", // क
    124 -> "\u0916", // ख
    125 -> "\u0917"  // ग
    // Add all problematic glyph IDs and their correct Unicode here
  )

  override def writeString(text: String, textPositions: Array[TextPosition]): Unit = {
    textPositions.foreach { textPos =>
      val font = textPos.getFont
      val glyphIds = textPos.getGlyphCodes
      
      glyphIds.foreach { gid =>
        // First try the PDF's built-in ToUnicode mapping
        var unicode = font.toUnicode(gid)
        // If the mapping is missing or wrong, use our fixed map
        if (unicode == null || unicode.isEmpty) {
          unicode = fixedGlyphMap.getOrElse(gid, s"[Unmapped GID: $gid]")
        }
        print(unicode)
      }
    }
    println()
  }
}

object DevanagariPDFProcessor {
  def main(args: Array[String]): Unit = {
    val doc = PDDocument.load(new File("your-devanagari-file.pdf"))
    val fixer = new DevanagariGlyphFixer()
    fixer.getText(doc)
    doc.close()
  }
}

How to Build the Fixed Mapping: #

1. Extract the embedded font: Use PDFBox to pull the font file from the PDF (look into PDTrueTypeFont or PDType0Font methods).
2. Inspect glyphs: Open the extracted font in a tool like FontForge to see what each glyph ID (GID) looks like.
3. Map to Unicode: Match each glyph's shape to its correct Devanagari Unicode character (e.g., conjuncts like क्ष are \u0915\u094D\u0937).

Backup Plan: OCR for Rasterized/Unreadable Text #

If your PDF uses rasterized images instead of vector text, or the font is completely unparseable, use Tesseract OCR with Devanagari support. Integrate it with Java/Scala using tess4j:

Maven dependency:

<dependency>
    <groupId>net.sourceforge.tess4j</groupId>
    <artifactId>tess4j</artifactId>
    <version>5.6.0</version>
</dependency>

Just make sure you install the hin (Hindi) language pack for Tesseract—it handles Devanagari perfectly.

Quick Tips #

• Test with a small section of your PDF first to make sure your mapping works.
• Don't forget about Devanagari ligatures and conjuncts—they often use combined Unicode code points, not single characters.

内容的提问来源于stack exchange，提问作者Kabir Manandhar