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PDF conformance

How close the Rust PDF pipeline gets to docling's default Markdown, measured byte-for-byte against the committed groundtruth (tests/data/pdf/groundtruth/*.md). The groundtruth is regenerated from live published docling, so it agrees with scripts/conformance.sh pdf.

Measure locally with scripts/pdf_groundtruth.sh (diffs the checked-in reference; no docling install needed) or scripts/conformance.sh pdf (installs docling and diffs against it). Diff = changed lines vs the groundtruth (one changed line counts as 2).

Current state

4 / 14 groundtruth PDFs are byte-for-byte exact.

PDF diff dominant remaining blocker
picture_classification exact —
code_and_formula exact —
multi_page exact —
2305.03393v1-pg9 exact — (TableFormer table, cell-for-cell)
right_to_left_01 2 RTL justified double-space
right_to_left_02 8 RTL bidi spacing
amt_handbook_sample 14 justified spacing + figure captions
2305.03393v1 40 title-page reading order + author-ID run spacing
right_to_left_03 66 RTL bidi
normal_4pages 74 reading order
table_mislabeled_as_picture 110 layout over-detects tables (survey rendered as tables)
2206.01062 234 TableFormer multi-row headers + title-page reading order
2203.01017v2 247 TableFormer structure + reading order
redp5110_sampled 271 TOC mis-classified as a picture; cover-page ordering

The close ones (right_to_left_01/02, amt_handbook) are 1–2 fixes from exact — the realistic path to 50% (7/14).

How the pipeline works

pdfium extracts the glyph layer and renders each page to a bitmap; an ONNX stack (layout detection, TableFormer, PaddleOCR) interprets it; regions are assembled in reading order into a DoclingDocument. Tables use TableFormer (image encoder

  • autoregressive OTSL structure decoder + cell-bbox decoder, ported and exported to ONNX in tableformer.rs) on a cv2-exact preprocessed crop (resample.rs); the structure + matched cell text reproduce docling's padded GitHub tables (2305-pg9 is cell-for-cell exact).

Text reconstruction: docling-parse line sanitizer (ported)

The byte-exact ceiling used to be the text extractor — pdfium differs from docling's own docling-parse C++ parser at text-run boundaries. We closed it by porting docling-parse's line sanitizer (src/parse/page_item_sanitators/cells.h → dp_lines.rs): a 3-pass corner-distance contraction (LTR → RTL → LTR-reverse) with merge_with space insertion (one space when the gap exceeds 0.33×avg-char-width, plus literal space glyphs), enforce_same_font, ligature recomposition (same-loose-box glyphs become one cell), and loose-box geometry (uniform font ascent/descent), fed by pdfium glyph cells. It is the default (set DOCLING_LEGACY_LINES to fall back to the old gap heuristic). This fixed justified double-spacing, the space before :, lam-alef ordering, and fi/ffi ligatures — and got multi_page to byte-exact.

Other text/serializer fixes matching docling: markdown escaping (_→\_, then HTML-escape &/</>), U+2212→-, @-glue (mAP @0.5), wrap dehyphenation, paragraph-continuation merging across column/page breaks, and band-aware two-column reading order (full-width regions break the columns into bands).

Remaining blockers (model-level)

These yield smaller or uncertain gains than the text-layer work already shipped.

  1. TableFormer structure on complex tables. Multi-row headers / spans on the big papers (2206, 2203) differ from docling's OTSL prediction; one cell- structure diff cascades through the padded columns into many row diffs (2206's ~92 table-row diffs trace to ~4 structure diffs).
  2. Layout classification. The layout ONNX classifies redp5110's table-of-contents as a picture (docling renders it as a table) and table_mislabeled's survey as tables (docling renders lists/text) — opposite classifications, not a text problem.
  3. Complex title-page reading order. Author-block / abstract interleaving on the academic papers (band reading-order handles the full-width title; the in-column author/abstract order is still off).
  4. RTL justified double-spaces. pdfium emits zero-width space boxes, losing the literal space that, with the inserted one, forms a justified double (right_to_left_01). Needs space-box reconstruction that estimates width.