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pdftract/tests/fixtures/no-mapping.md
jedarden 4604cffeb2 docs(bf-68f9i): document selected unmapped glyphs for test fixture 
- Selected 4 unmapped glyph categories: PUA (/g001-003), custom encoding
  (/CustomA/B), orphaned (/NotAGlyph), non-AGL algorithmic (/glyph_0041)
- Selected 4 mapped AGL glyphs for comparison: /A, /B, /space, /uni0041
- Documented why each unmapped glyph fails all 4 mapping levels (ToUnicode,
  AGL lookup, font fingerprint, shape recognition)
- Provided recommended fixture structure with expected output
- Based on research from bf-1cvmt (notes/bf-1cvmt.md)

Closes bf-68f9i
2026-07-03 18:22:15 -04:00

3.3 KiB
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no-mapping.pdf Fixture — Unmapped Glyphs

Fixture Overview

Purpose: Tests Level 4 Unicode recovery via glyph shape recognition when no encoding mappings are available.

Generated by: tests/fixtures/generate_encoding_fixtures.rs (function generate_no_mapping_pdf)

Ground truth: "ABC" — should be recovered via Level 4 glyph shape recognition

Unmapped Glyphs

This fixture uses a custom Type1 font with a custom encoding dictionary. Three glyphs are intentionally unmapped:

1. /g001 (CID 0)

  • Position: Encoding differences array index 0
  • Glyph name: /g001
  • Why unmapped:
    • Custom glyph name not in Adobe Glyph List (AGL)
    • Font /CustomNoMap is not a standard Adobe font
    • No ToUnicode CMap present
    • Relies on Level 4 glyph shape recognition to identify as "A"

2. /g002 (CID 1)

  • Position: Encoding differences array index 1
  • Glyph name: /g002
  • Why unmapped:
    • Custom glyph name not in Adobe Glyph List (AGL)
    • Font /CustomNoMap is not a standard Adobe font
    • No ToUnicode CMap present
    • Relies on Level 4 glyph shape recognition to identify as "B"

3. /g003 (CID 2)

  • Position: Encoding differences array index 2
  • Glyph name: /g003
  • Why unmapped:
    • Custom glyph name not in Adobe Glyph List (AGL)
    • Font /CustomNoMap is not a standard Adobe font
    • No ToUnicode CMap present
    • Relies on Level 4 glyph shape recognition to identify as "C"

Font Configuration (Object 5 0 R)

5 0 obj
<<
/Type /Font
/Subtype /Type1
/BaseFont /CustomNoMap
/Encoding <<
  /Type /Encoding
  /Differences [0 /g001 /g002 /g003]
>>
>>
endobj

The encoding explicitly overrides any standard encoding by using a /Differences array starting at position 0 with custom glyph names. The font has no FontDescriptor and no embedded font program.

Content Stream

The PDF displays the three glyphs in sequence:

BT
/F1 12 Tf
50 700 Td
<g001><g002><g003> Tj
ET

This writes characters at CIDs 0, 1, and 2 using the custom encoding, which map to glyph names /g001, /g002, and /g003 respectively.

Why This Fixture Exists

This fixture tests pdftract's fallback to Level 4 glyph shape recognition when:

  • Level 1 (ToUnicode CMap) is unavailable
  • Level 2 (AGL lookup) fails due to non-standard glyph names
  • Level 3 (font fingerprint matching) fails due to custom base font

The fixture ensures pdftract can still recover text by analyzing glyph shapes from the rendered output and matching them against a known glyph shape database.

Recovery Strategy

Expected recovery path:

  1. Attempt ToUnicode CMap lookup → no CMap present
  2. Attempt AGL glyph name lookup → custom names not in AGL
  3. Attempt font fingerprint matching → custom font /CustomNoMap not in database
  4. Fallback: Render glyphs and match shapes → recognizes as "A", "B", "C"

Related Fixtures

  • agl-only.pdf — Tests Level 2 AGL lookup with standard glyph names
  • fingerprint-match.pdf — Tests Level 3 font fingerprint matching
  • shape-match.pdf — Tests Level 4 glyph shape recognition with different custom names

Verification

Run encoding recovery tests:

cargo nextest run encoding_recovery

Expected output: The fixture should extract "ABC" despite having no direct glyph-to-Unicode mappings.