egg/OCR

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egg c12ea0b9f6 proposal: add-layout-preprocessing for improved table detection

Problem: PP-Structure misses tables with faint lines/borders
Solution: Preprocess images (contrast, sharpen) for layout detection
- Preprocessed image only used for layout detection
- Original image preserved for element extraction (quality)

Includes: proposal.md, design.md, tasks.md, spec delta

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

2025-11-27 14:24:23 +08:00

2.6 KiB

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Change: Add Image Preprocessing for Layout Detection

Why

PP-StructureV3's layout detection (PP-DocLayout_plus-L) sometimes fails to detect tables with faint lines, low contrast borders, or poor scan quality. This results in missing table elements in the output, even when the table structure recognition models (SLANeXt) are correctly configured.

The root cause is that layout detection happens before table structure recognition - if a region isn't identified as a "table" in the layout detection stage, the table recognition models never get invoked.

What Changes

Add image preprocessing module for layout detection input
- Contrast enhancement (histogram equalization, CLAHE)
- Optional binarization (adaptive thresholding)
- Sharpening for faint lines
Preserve original images for extraction
- Preprocessing ONLY affects layout detection input
- Image element extraction continues to use original (preserves quality)
- Raw OCR continues to use original image
Configurable preprocessing options
- Enable/disable preprocessing per track
- Adjustable preprocessing intensity

Impact

Affected Specs

ocr-processing - New preprocessing configuration requirements

Affected Code

backend/app/services/ocr_service.py - Add preprocessing before PP-Structure
backend/app/core/config.py - New preprocessing configuration options
backend/app/services/preprocessing_service.py - New service (to be created)

Track Impact Analysis

Track	Impact	Reason
OCR	Improved layout detection	Preprocessing enhances PP-Structure input
Hybrid	Potentially improved	Uses PP-Structure for layout
Direct	No impact	Does not use PP-Structure
Raw OCR	No impact	Continues using original image

Quality Impact

Component	Impact	Reason
Table detection	Improved	Enhanced contrast reveals faint borders
Image extraction	No change	Uses original image for quality
Text recognition	No change	Raw OCR uses original image
Reading order	Improved	Better element detection → better ordering

Risks

Performance overhead: Preprocessing adds compute time per page
- Mitigation: Make preprocessing optional, cache preprocessed images
Over-processing: Strong enhancement may introduce artifacts
- Mitigation: Configurable intensity levels, default to moderate enhancement
Memory usage: Keeping both original and preprocessed images
- Mitigation: Preprocessed image is temporary, discarded after layout detection

2.6 KiB Raw Blame History