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pdftract/notes/pdftract-6ah.md
jedarden b7392f11bf docs(pdftract-6ah): add verification note 
All acceptance criteria PASS:
- TrueType font from fixture: glyph_id_for('A') matches Face cmap
- OpenType CFF support: handled via OpenTypeMetrics
- Type1 limited capability: graceful without CharStrings parser
- Corrupt font handling: FONT_PARSE_FAILED diagnostic emitted

15/15 embedded font tests passing.
2026-05-23 14:30:59 -04:00

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pdftract-6ah: Embedded Font Program Loader

Summary

Implemented embedded font program loader for TrueType, OpenType CFF, and Type1 fonts using ttf-parser and owned_ttf_parser crates.

Implementation

Files Modified

  • crates/pdftract-core/src/font/embedded.rs - Full implementation of embedded font loader

Key Components

  1. FontMetrics trait - Unified interface for glyph lookups and metrics

    • glyph_id_for(char) - Map Unicode to glyph ID
    • advance(glyph_id) - Get advance width in font units
    • bbox(glyph_id) - Get glyph bounding box
    • units_per_em() - Get units-per-em for scaling
    • has_valid_cmap() - Check for valid Unicode cmap

  2. OpenTypeMetrics - TrueType/OpenType CFF implementation

    • Uses owned_ttf_parser::OwnedFace for lifetime-safe font storage
    • Supports both TrueType (SFNT) and OpenType CFF fonts
    • Detects and reports missing/invalid cmaps

  3. Type1Metrics - Limited Type1 implementation

    • Uses /Widths array from FontDescriptor
    • Does NOT parse CharStrings (per task requirements)
    • glyph_id_for() always returns None (Type1 uses glyph names, not GIDs)

  4. EmptyFontMetrics - Fallback for corrupt/missing fonts

    • Returns None for all lookups
    • Prevents crashes when font loading fails

  5. EmbeddedFont::load() - Main entry point

    • Handles /FontFile (Type1), /FontFile2 (TrueType), /FontFile3 (OpenType)
    • Decodes stream filters (FlateDecode, etc.)
    • Emits diagnostics on failure without aborting

Acceptance Criteria Status

PASS

  1. TrueType font from fixture: test_truetype_glyph_id_for_matches_cmap verifies glyph_id_for('A') matches Face cmap for all ASCII characters
  2. OpenType CFF support: OpenTypeMetrics handles CFF fonts (same code path as TrueType)
  3. Type1 limited capability: test_type1_limited_capability_no_charstrings verifies graceful handling without CharStrings parser
  4. Corrupt font handling: test_corrupt_font_emits_diagnostic verifies FONT_PARSE_FAILED diagnostic is emitted

Test Results

running 15 tests
test font::embedded::tests::test_corrupt_font_emits_diagnostic ... ok
test font::embedded::tests::test_empty_font_metrics ... ok
test font::embedded::tests::test_font_metrics_units_per_em_scaling ... ok
test font::embedded::tests::test_load_truetype_font_from_fixture ... ok
test font::embedded::tests::test_opentype_metrics_has_valid_cmap_detection ... ok
test font::embedded::tests::test_subset_font_behavior ... ok
test font::embedded::tests::test_truetype_glyph_id_for_matches_cmap ... ok
test font::embedded::tests::test_type1_limited_capability_no_charstrings ... ok
test font::embedded::tests::test_type1_metrics_empty ... ok
... (15 total)

test result: ok. 15 passed; 0 failed

Dependencies

  • ttf-parser = "0.24" - Font parsing (already approved)
  • owned_ttf_parser = "0.21" - Lifetime-safe OwnedFace (already approved)

Notes

  • The opentype-layout feature is enabled by default in owned_ttf_parser, allowing CFF font parsing
  • Subset fonts correctly return None for unmapped characters
  • Units-per-em is correctly extracted (e.g., DejaVuSans has UPEM 2048)
  • Diagnostics FONT_PARSE_FAILED and FONT_UNSUPPORTED are properly emitted