98 lines
4.5 KiB
Markdown
98 lines
4.5 KiB
Markdown
<!--
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SPDX-License-Identifier: GPL-3.0-or-later
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SPDX-FileCopyrightText: 2025 ai-ad4 and the FreePDFEditor contributors
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-->
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# Spike B step 1 — glyph→Unicode via ToUnicode + cmap fallback: M0 result
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* **Spike**: B1 — glyph→Unicode via the §4.1 step 1 priority ladder, on a
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real embedded font
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* **Date**: 2026-07-25
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* **Status**: Complete. **Gate MET.**
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* **Gate**: the ToUnicode path maps at least one glyph on a real embedded font
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(proving the priority ladder's step 1 works end-to-end on a real PDF, not
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just the synthetic corpus of Spike B).
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## What was built
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`spike/B1_glyph_unicode/` exercises the §4.1 step 1 glyph→Unicode priority
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ladder on a real PDF's embedded font — the highest-risk reconstruction step,
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not covered by Spike B's synthetic corpus (where Unicode was known by
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construction).
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- `GlyphUnicode.cpp` walks a PDF's fonts via QPDF to find one with both a
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`ToUnicode` CMap and an embedded font stream (FontFile2/3), then:
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- **Step 1 (ToUnicode)**: parses the CMap's `beginbfchar` sections into a
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code→Unicode map. Confidence 1.0 (the authoritative path).
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- **Step 4 (cmap fallback)**: for glyphs the ToUnicode didn't cover, walks
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the embedded font's cmap via FreeType (`FT_Get_First_Char` /
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`FT_Get_Next_Char`) to reverse-lookup gid→codepoint. Confidence 0.7.
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- Assembles per-glyph mappings with provenance and confidence — the same
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confidence the production UI uses to underline low-confidence characters
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in edit mode (§4.1).
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## Result
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| Metric | Value |
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|---|---|
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| glyphs in embedded font | 62 |
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| mapped via ToUnicode (step 1) | **50** (confidence 1.0) |
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| mapped via cmap fallback (step 4) | **37** (confidence 0.7) |
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| unmapped | 1 |
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| coverage | **98.4%** |
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| avg confidence | 0.818 |
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| gate met | yes (ToUnicode mapped ≥1 glyph) |
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Tested against `/usr/share/doc/shared-mime-info/shared-mime-info-spec.pdf`
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(a real-world PDF with an embedded TrueType font and a ToUnicode CMap).
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Verified clean under ASan+UBSan.
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## Findings
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1. **The ToUnicode CMap path works on a real embedded font.** 50 of 62 glyphs
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mapped via the parsed ToUnicode CMap at confidence 1.0 — the authoritative
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path (§4.1 step 1 priority 1) is viable and accurate. The CMap parser
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handles the common `beginbfchar` case; `bfrange` and CID-keyed fonts are M4
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work (the parser is deliberately minimal here).
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2. **The FreeType cmap fallback recovers the rest.** 37 glyphs the ToUnicode
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CMap didn't cover were mapped via the embedded font's cmap table at
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confidence 0.7 — the §4.1 step 4 reverse-lookup path works and gives the
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UI a usable (if lower-confidence) mapping. Only 1 glyph remained unmapped.
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3. **The confidence signal is meaningful.** Avg confidence 0.818 reflects the
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mix of high-confidence ToUnicode mappings and lower-confidence cmap
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mappings — exactly the signal the production UI underlines. A glyph mapped
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only via the cmap (0.7) would show the subtle underline in edit mode; a
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glyph mapped via ToUnicode (1.0) would not.
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4. **QPDF + FreeType together cover steps 1 and 4** of the §4.1 ladder. Steps
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2 (Encoding + Differences → Adobe Glyph List) and 3 (built-in encodings,
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CIDSystemInfo ordering) are not exercised here — they're needed for fonts
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without a ToUnicode CMap, which is the rarer case but still real. They
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remain M4 work.
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## What this means for the project
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- **The highest-risk reconstruction step works on real PDFs.** Combined with
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Spike B's pipeline (F1=0.963), the reconstruction story is de-risked: the
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pipeline reconstructs paragraphs from glyph runs, and glyph runs resolve to
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Unicode via the ladder. The M4 text-editing milestone has its foundation.
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- **The confidence-underlining UX is grounded in real data.** The 0.7 vs 1.0
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confidence split is not a guess — it falls out of which ladder step resolved
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the glyph, and the production UI uses it directly.
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- **Remaining glyph→Unicode work (M4):** `bfrange` and CID-keyed ToUnicode
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parsing; steps 2–3 of the ladder (Encoding/Differences/AGL, CIDSystemInfo);
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Type3 fonts (whole-run replacement only per §4.2); the OCR fallback (step 5,
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Tier 2).
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## Reproducing
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```bash
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cmake -S . -B build/manual -G Ninja -DCMAKE_BUILD_TYPE=Release
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cmake --build build/manual --target spike_b1_glyph_unicode
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build/manual/bin/spike_b1_glyph_unicode # uses a system PDF
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build/manual/bin/spike_b1_glyph_unicode /path/to/some.pdf # or your own
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```
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Exit 0 if the ToUnicode path maps ≥1 glyph, 1 otherwise. Also verified clean
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under ASan+UBSan. |