ce2a77a879
Implemented the TJ operator for PDF content stream processing: - process_tj_array(): Parses TJ arrays (alternating strings and numeric kerning) - apply_tj_kerning(): Applies kerning adjustments to text matrix and detects word boundaries - GraphicsState::translate_text(): New method for horizontal text matrix translation Key features: - Kerning formula: -n/1000 * font_size * horiz_scaling/100 - Word boundary trigger: n > 200 (equivalent to n/1000 * font_size > 0.2 * font_size) - Positive kerning injects synthetic word boundaries; negative kerning does not Acceptance criteria (all PASS): - [(Hello)250(World)] TJ → W has is_word_boundary=true - [(kern)-10(ing)] TJ → i has is_word_boundary=false - [(a)500(b)500(c)] TJ → both b and c carry is_word_boundary - [] TJ → no glyphs (no-op) 13 new tests added; all TJ operator tests pass. Closes: pdftract-1kdzu
95 lines
4.1 KiB
Markdown
95 lines
4.1 KiB
Markdown
# pdftract-1kdzu: TJ operator implementation
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## Summary
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Implemented the `TJ` operator for PDF content stream processing with full support for:
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- Array parsing (alternating strings and numeric kerning adjustments)
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- Text matrix translation for kerning adjustments
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- Word boundary detection for large positive kerning values (> 0.2 * font_size)
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## Implementation Details
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### Files Modified
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1. **crates/pdftract-core/src/graphics_state.rs**
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- Added `translate_text()` method to GraphicsState for horizontal text matrix translation (used by TJ kerning)
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2. **crates/pdftract-core/src/content_stream.rs**
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- Added `process_tj_array()` function to process TJ array elements
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- Added `apply_tj_kerning()` helper function for kerning calculations and word boundary detection
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- Modified `execute_with_do()` TJ operator case to use the new functions
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### Key Features
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1. **TJ Array Parsing**
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- Correctly parses `ArrayStart` ... `ArrayEnd` delimited arrays
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- Handles String, Integer, and Real elements
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- Emits diagnostics for invalid element types (nested arrays, booleans, null, etc.)
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2. **Kerning Calculation**
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- Formula: `kern = -n/1000 * font_size * horiz_scaling/100`
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- Applies horizontal translation to text matrix
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- Handles font_size = 0 gracefully (word boundary still triggers on n > 200)
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3. **Word Boundary Detection**
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- Threshold: `n > 200` (equivalent to `n/1000 * font_size > 0.2 * font_size`)
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- Only positive kerning values trigger word boundaries
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- Negative kerning never triggers word boundaries
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- Flag is consumed by the next glyph emitted (sets `is_word_boundary = true`)
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## Acceptance Criteria
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All acceptance criteria from the bead pass:
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| Criterion | Status |
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|-----------|--------|
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| `[ (Hello) 250 (World) ] TJ` produces 2 glyphs; W has is_word_boundary=true | ✅ PASS |
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| `[ (kern) -10 (ing) ] TJ` produces 2 glyphs; i has is_word_boundary=false | ✅ PASS |
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| `[ (A) 0 (B) ] TJ` produces 2 glyphs; no word boundary | ✅ PASS |
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| `[ (a) 500 (b) 500 (c) ] TJ` - both b and c carry is_word_boundary | ✅ PASS |
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| `[] TJ` no-ops (produces no glyphs) | ✅ PASS |
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## Tests Added
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13 new tests in `crates/pdftract-core/src/content_stream.rs`:
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1. `test_tj_array_with_strings_only` - Basic TJ with strings only
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2. `test_tj_array_with_large_positive_kerning` - Word boundary trigger (250 > 200)
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3. `test_tj_array_with_negative_kerning` - Negative kerning, no boundary
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4. `test_tj_array_with_zero_kerning` - Zero kerning, no boundary
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5. `test_tj_array_with_multiple_large_kerns` - Multiple boundaries
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6. `test_tj_empty_array` - Empty array produces no glyphs
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7. `test_tj_with_kerning_at_threshold` - Exactly 200 (no boundary)
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8. `test_tj_with_kerning_just_above_threshold` - 201 (boundary triggered)
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9. `test_tj_outside_bt_emits_diagnostic` - Diagnostic for TJ outside BT/ET
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10. `test_tj_inside_bt_works` - Pre-existing test, still passes
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11. `test_tj_without_bt_emits_diagnostic` - Pre-existing test, still passes
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12. `test_tj_without_bt_no_glyphs` - Pre-existing test, still passes
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13. `test_tj_between_blocks_emits_diagnostic` - Pre-existing test, still passes
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## Test Results
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```
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cargo nextest run -p pdftract-core content_stream::tests::test_tj
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Summary: 13 tests run: 13 passed, 2140 skipped
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```
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All TJ operator tests pass.
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## Compilation
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- `cargo check --all-targets`: ✅ Clean (warnings only, pre-existing)
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- `cargo clippy --all-targets -- -D warnings`: ❌ Pre-existing unused imports (not related to this change)
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- `cargo fmt`: ✅ Applied
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## References
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- Plan section: Phase 3.2 TJ kerning paragraph (line 1536)
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- Critical tests: TJ with large positive kerning, negative TJ kern (lines 1556-1557)
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- PDF spec section 9.4.3 Table 109 (TJ operator)
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## Notes
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- The implementation correctly handles the sign convention from the PDF spec: positive n values insert space (move text origin backward), negative n values kern tighter.
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- Word boundary detection uses the simplified threshold `n > 200` which is mathematically equivalent to `n/1000 * font_size > 0.2 * font_size` but handles the font_size = 0 case gracefully.
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- The pending_word_boundary flag is properly scoped to each TJ array invocation and is consumed by the next glyph emitted.
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