feat(markdown): add footnote serialization support

[ShrillHarrier]

This PR is related to the Improved Footnote Serialization in MarkdownDocSerializer.

It is a Feature Request submitted by simonschoe in docling-project.

The features added include serializing a footnote in the form [^{Identifier}]: {Description}. This is done for Table and Picture items, as footnotes are linked to those.

In general, footnotes in .md files should look like:

[^5]: https://github.com/tesseract-ocr/tesseract
[^6]: https://github.com/VikParuchuri/surya
[^7]: https://github.com/lukas-blecher/LaTeX-OCR

Resolves docling-project/docling#3128

Tests Added:

• test_table_with_footnotes_markdown()
• test_picture_with_footnotes_markdown()
• test_table_export_to_markdown_with_footnotes()

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[dosubot]

Related Documentation

1 document(s) may need updating based on files changed in this PR:

Docling

What are the differences between vlm_pipeline_model_local and picture_description_local in Docling, and how do image descriptions, OCR, and table extraction work together? Also, how do the include_annotations and mark_annotations properties affect exported output?

View Suggested Changes

@@ -67,6 +67,9 @@
 - `compact_tables` (bool): Whether to use compact table format without column padding (default: `False`, Markdown only)
 - `traverse_pictures` (bool): Whether to traverse into picture items and serialize their text children (default: `False`)
 
+**Footnote Serialization in Markdown:**
+When tables or pictures have associated footnotes in the document, these footnotes are automatically serialized in the markdown output using standard markdown footnote syntax: `[^{Identifier}]: {Description}`. The identifier is extracted from the first part of the footnote text, and the remaining text becomes the footnote description. This formatting ensures that footnotes attached to Table and Picture items appear correctly in the exported markdown.
+
 **Handling OCR Text in Scanned/Image-Based PDFs:**
 When processing scanned or image-based PDFs with `force_full_page_ocr=True`, the layout model classifies full-page scans as `PictureItem` nodes. OCR text items are added as children of that picture node in the document tree.
 

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[ShrillHarrier]

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Codecov Report

❌ Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
docling_core/transforms/serializer/markdown.py	95.45%	1 Missing ⚠️

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[ceberam]

Thanks @ShrillHarrier for suggesting this PR. Please, see my comments.
In general:

• Adding new tests with a programmatic example is fine but it is much more illustrative to show the impact of the new feature in a serialization with a ground truth data file. This allows us to check how the output markdown file gets rendered in applications like Github or VSC. Please, check how this is done in other test modules.
• To keep the repository consistent, I would suggest that you add the tests in the module test/test_serialization.py, together with the other tests of the markdown serialization, instead of creating a separate module.
• There are some DoclingDocument files (.json) that do not serialize as expected. Please, check some of them and ensure that the markdown serialization generates the right footnotes hook and text. For instance, test/data/doc/2408.09869v3_enriched.json has a footnote. If you regenerate the ground truth files (by running the tests with env variable DOCLING_GEN_TEST_DATA=1 ), I would expect that the markdown serialization gets updated with the new footnote serialization.

[ceberam]

This line is redundant

Suggested change

results = []

[ceberam]

The footnote parsing logic assumes a specific format (the identifier and the footnote text). This format is not clearly represented or documented. It would be good that the format is explicit, validated, and clearly documented. We should keep in mind that for the markdown footnote to work, identifiers can be numbers or words, but they can’t contain spaces or tabs..
In addition, I don't think that we keep the footnote references with the correct formatting (a caret and an identifier inside brackets, e.g., [^1]).

If I try to serialize a DoclingDocument from the test dataset, you'll see that the footnote 1 see huggingface.co/ds4sd/docling-models/ (doc item with reference #/texts/29) is not properly serialized as in the markdown specification.

from docling_core.types.doc import DoclingDocument
from docling_core.types.doc.base import ImageRefMode

with open("test/data/doc/2408.09869v3_enriched.json") as handler:
  content = handler.read()
doc = DoclingDocument.model_validate_json(content)
doc.export_to_markdown(image_mode=ImageRefMode.PLACEHOLDER)

[ceberam]

Many of these imports are never used. Please, remove unused imports.