docs(pdftract-25k4x): verify figure and caption detection implementation
Add verification note confirming all acceptance criteria PASS. - Figure classifier: 16/16 tests pass - Caption classifier: 8/8 tests pass - All acceptance criteria verified against code Closes pdftract-25k4x
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# pdftract-25k4x: Figure Detection + Caption Detection
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# Figure and Caption Detection Verification - pdftract-25k4x
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## Status: COMPLETE
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## Acceptance Criteria Verification
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## Overview
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Figure detection and caption detection were already implemented in the codebase in:
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- `crates/pdftract-core/src/layout/figure.rs` (517 lines, 16 tests)
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- `crates/pdftract-core/src/layout/caption.rs` (342 lines, 8 tests)
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### 1. Image XObject, no text overlap: 1 Figure block
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**Location:** `crates/pdftract-core/src/layout/figure.rs:130`
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- Checks `text_overlap_area / image_area < 0.5`
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- Creates Block with kind="figure"
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- **PASS** - Tests: `test_classify_figure_pure_visual_image`, `test_classify_figure_no_glyphs`
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## Verification Summary
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### 2. Image + small-font caption 1 line below: Figure + Caption
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**Location:** `crates/pdftract-core/src/layout/caption.rs:126-140`
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- Checks `block.median_font_size < ctx.page_body_median` (small font)
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- Checks `vertical_distance < 2.0 * ctx.line_height` (within 2 lines)
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- Sets kind to "caption"
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- **PASS** - Test: `test_caption_immediately_below_figure`
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### Figure Detection (`classify_figure`)
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**Algorithm:**
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1. Walks image XObjects from Phase 3.3 Do + Phase 3.5 inline images
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2. For each image, computes union area of all text glyph bboxes intersecting the image
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3. Uses sweep line algorithm for precise union area computation
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4. If `text_overlap_area / image_area < 0.5`, creates a Figure block
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5. Sorts figures by bbox top Y (descending)
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### 3. Image overlapping text (background): NOT Figure
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**Location:** `crates/pdftract-core/src/layout/figure.rs:130`
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- Images with >= 50% text overlap are NOT classified as figures
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- **PASS** - Tests: `test_classify_figure_text_on_image`, `test_classify_figure_partial_text_above_threshold`
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**Acceptance Criteria Verification:**
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| Criteria | Test | Status |
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|----------|------|--------|
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| Image XObject, no text overlap → 1 Figure block | `test_five_figures_no_text` | ✅ PASS |
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| Image + small-font caption 1 line below → Figure + Caption | `test_caption_immediately_below_figure` | ✅ PASS |
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| Image overlapping text (background) → NOT Figure | `test_text_covered_image_not_figure` | ✅ PASS |
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| Text overlap < 50% → Figure | `test_classify_figure_partial_text_below_threshold` | ✅ PASS |
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| Text overlap ≥ 50% → NOT Figure | `test_classify_figure_partial_text_above_threshold` | ✅ PASS |
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### 4. Caption 5 lines below: NOT Caption
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**Location:** `crates/pdftract-core/src/layout/caption.rs:145-148`
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- Checks `vertical_distance >= 2.0 * ctx.line_height`
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- Returns false if too far below
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- **PASS** - Test: `test_caption_too_far_below_figure`
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### Caption Detection (`classify_caption`)
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**Algorithm:**
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1. Checks font size < page_body_median
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2. Requires previous block is a Figure
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3. Vertical distance < 2 * line_height
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4. Same column (when num_columns > 1)
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**Acceptance Criteria Verification:**
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| Criteria | Test | Status |
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|----------|------|--------|
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| Small font + follows Figure + within 2 lines + same column → Caption | `test_caption_immediately_below_figure` | ✅ PASS |
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| Caption 5 lines below → NOT Caption | `test_caption_too_far_below_figure` | ✅ PASS |
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| Caption different column → NOT Caption | `test_caption_different_column` | ✅ PASS |
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| Font not smaller than body → NOT Caption | `test_caption_font_not_smaller` | ✅ PASS |
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| No previous Figure → NOT Caption | `test_no_previous_figure` | ✅ PASS |
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### 5. Caption different column: NOT Caption
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**Location:** `crates/pdftract-core/src/layout/caption.rs:152-154`
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- Checks `block.column != figure.column` in multi-column layouts
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- Returns false if different column
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- **PASS** - Test: `test_caption_different_column`
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## Test Results
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```
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Figure tests: 16 passed; 0 failed
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Caption tests: 8 passed; 0 failed
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```
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## Key Implementation Details
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### Figure Classifier Tests (16/16 PASS)
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- test_bboxes_intersect
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- test_classify_figure_no_images
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- test_classify_figure_partial_text_below_threshold
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- test_classify_figure_partial_text_above_threshold
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- test_classify_figure_exactly_at_threshold
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- test_classify_figure_no_glyphs
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- test_classify_figure_pure_visual_image
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- test_bbox_area
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- test_classify_figure_sort_order
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- test_classify_figure_empty_context
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- test_classify_figure_text_on_image
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- test_compute_text_overlap_area_multiple_glyphs
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- test_compute_text_overlap_area_union
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- test_figure_block_properties
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- test_five_figures_no_text
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- test_text_covered_image_not_figure
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### INV (Invariants)
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- ✅ Figure block has empty `lines` Vec (lines=[], but Block uses `text: String` instead)
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- ✅ Figure blocks have `median_font_size: 0.0`
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- ✅ Caption blocks have `kind: "caption"` set via `set_caption()`
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### Caption Classifier Tests (8/8 PASS)
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- test_caption_above_figure
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- test_caption_font_not_smaller
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- test_caption_too_far_below_figure
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- test_no_previous_figure
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- test_caption_different_column
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- test_caption_immediately_below_figure
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- test_block_accessors
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- test_page_classification
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### Critical Considerations Addressed
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- **Text overlap union algorithm**: Uses sweep line for accurate union area (not naive sum)
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- **Sorting**: Figures sorted by top Y descending for consistent page order
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- **Column assignment**: TODO comment present for column assignment based on image center
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- **Above-figure captions**: NOT detected in v0.1.0 (as specified in bead)
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## INV Verification
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- **INV: Figure block has empty lines Vec** - SATISFIED: Block created with text=String::empty(), median_font_size=0.0
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- **Caption above figure NOT detected in v0.1.0** - SATISFIED: caption.rs test_caption_above_figure returns false
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## Files Modified
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None - implementation was already complete
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## Files Verified
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- crates/pdftract-core/src/layout/figure.rs (517 lines)
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- crates/pdftract-core/src/layout/caption.rs (342 lines)
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- crates/pdftract-core/src/layout/mod.rs (exports classifiers)
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## Retrospective
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### What worked
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- The existing implementation is clean, well-tested, and follows the bead specification exactly
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- Sweep line algorithm for text overlap union is mathematically correct
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- Test coverage is comprehensive with edge cases (thresholds, empty contexts, multiple figures)
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### What didn't
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- N/A - implementation was already complete and passing
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### Surprise
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- The bead was already fully implemented despite being in the ready queue
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- Both modules share a common `Block` type via `pub use` from caption.rs
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### Reusable pattern
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- The sweep line algorithm in `compute_text_overlap_area` is a reusable pattern for union rectangle area computation
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- The `classify_caption` pattern of checking: (1) font metric, (2) spatial relationship, (3) column membership is a template for other block classifiers
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## Verification Status
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**ALL ACCEPTANCE CRITERIA PASS** - Implementation complete and tested.
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