How can I extract and map PDF chunk bounding boxes in a Python RAG pipeline for highlighting?

Below is an approach that might help you connect PDF chunk text to their bounding boxes and deal with potential cross-page overlap issues:

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1. Track Text Positions During Preprocessing

When you convert a PDF to text using libraries like pdf.js (on the client) or a Python PDF converter, you typically extract a text layer. This layer has metadata about each text item (including page number and coordinates). To later map your text chunks back to their positions, you must:

• Extend your extraction process:
  Modify or wrap your existing PDF-to-text conversion (for example, using pdf.js’s getTextContent or using a more advanced Python library like PyMuPDF) to also output coordinates (bounding boxes) along with the text content and page number.

• Save positional metadata with text:
  For each text item, store information such as:

  • The page number.
  • Bounding box coordinates (x, y, width, height).
    This way, when your preprocessor splits the PDF into chunks, you can include a reference back to these positional data.

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2. Mapping Chunks to Bounding Boxes

Once you have both the raw text with positions and the text chunks produced by your Haystack pipeline:

• Maintain positional references:
  Instead of splitting the raw string alone, keep track of the character index offset for each text item. For example, if you know that “chunk A” spans characters 1500–1700 in the full document, you can scan your positional data to find which text items (and hence bounding boxes) fall into that range.

• Aggregate boxes per chunk:
  Combine the bounding boxes from the text items that share the same chunk. This may involve:

  • Creating an array of bounding boxes for a single chunk.
  • Optionally computing a “union” bounding box if you want one highlight rectangle. Note that sometimes combining all individual boxes is more accurate for highlighting.

Example (pseudo-code for mapping text positions):

PYTHONCopied
# Pseudo-code: Map document offsets to bounding boxes
def map_chunk_to_bboxes(chunk_start, chunk_end, text_items):
    bboxes = []
    for item in text_items:
        item_start = item['char_offset']
        item_end = item_start + len(item['str'])
        if item_end >= chunk_start and item_start <= chunk_end:
            bboxes.append({
                "page": item['page'],
                "bbox": item['bbox']  # e.g. (x, y, width, height)
            })
    return bboxes

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3. Handling Overlapping Bounding Boxes Across Pages

Since PDF pages are rendered separately in pdf.js, overlapping bounding boxes from different pages should naturally spread across separate div containers corresponding to each page’s canvas:

• Store page numbers with each position:
  When aggregating bounding boxes, ensure you record which page each box belongs to. This allows your front end to highlight only on the relevant page.

• Multiple highlights:
  If a chunk spans text that appears on two different pages, your highlighting routine should iterate over each page’s bounding box list and add highlights separately.

• Avoid merging boxes from different pages:
  Since the coordinate systems differ page to page, always handle each page’s boxes individually. In your pdf.js frontend, loop through the retrieved bounding boxes by page and create overlays on each page’s text layer.

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4. Frontend Highlights with pdf.js

On the client side, pdf.js’s text layer typically gives you a DOM structure with individual <span> or <div> elements for each text item. You can run a script that:

• Maps the bounding boxes (which include page number and coordinates) to the corresponding page’s text layer.
• Creates absolutely positioned highlight <div> elements over the relevant text. Be sure to adjust for scaling if the PDF is zoomed.

Example (JavaScript snippet):

JAVASCRIPTCopied
// Assume you have a bounding box object: { page: 1, bbox: { x, y, width, height } }
function addHighlight(pageNumber, bbox) {
  const pageDiv = document.getElementById(`pageContainer${pageNumber}`);
  if (!pageDiv) return;
  
  const highlight = document.createElement("div");
  highlight.className = "pdfHighlight";
  highlight.style.position = "absolute";
  highlight.style.left = `${bbox.x}px`;
  highlight.style.top = `${bbox.y}px`;
  highlight.style.width = `${bbox.width}px`;
  highlight.style.height = `${bbox.height}px`;
  highlight.style.backgroundColor = "rgba(255,255,0,0.3)";
  
  pageDiv.appendChild(highlight);
}

And for each retrieved chunk, loop through its bounding boxes to call addHighlight.

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Summary

1. Extract and store positional metadata: Update your conversion from PDF to text to include bounding box (and page number) information.
2. Map text chunks to bounding boxes: Use the raw text offsets from the extraction to associate each chunk with one or more bounding boxes.
3. Deal with cross-page chunks by keeping page info: Render highlights separately per page in pdf.js.
4. Render with pdf.js: Use absolute positioning on the text layer to highlight the relevant boxes.

By integrating these steps, your frontend can accurately highlight the retrieved text chunks even if they span multiple or overlapping elements across pages.