feat(ocr-pipeline): distinguish header/footer vs margin_top/margin_bottom

Check for actual ink content in detected top/bottom regions:
- 'header'/'footer' when text is present (e.g. title, page number)
- 'margin_top'/'margin_bottom' when the region is empty page margin

Also update all skip-type sets and color maps for the new types.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

klausur-service/backend/cv_vocab_pipeline.py

@@ -98,7 +98,7 @@ ENGLISH_FUNCTION_WORDS = {'the', 'a', 'an', 'is', 'are', 'was', 'were', 'to', 'o
 @dataclass
 class PageRegion:
     """A detected region on the page."""
-    type: str           # 'column_en', 'column_de', 'column_example', 'page_ref', 'column_marker', 'column_text', 'header', 'footer'
+    type: str           # 'column_en', 'column_de', 'column_example', 'page_ref', 'column_marker', 'column_text', 'header', 'footer', 'margin_top', 'margin_bottom'
     x: int
     y: int
     width: int
@@ -924,12 +924,10 @@ def analyze_layout(layout_img: np.ndarray, ocr_img: np.ndarray) -> List[PageRegi
     # Add header/footer info (gap-based detection with fallback)
     _add_header_footer(regions, top_y, bottom_y, w, h, inv=inv)
 
-    has_header = any(r.type == 'header' for r in regions)
+    top_region = next((r.type for r in regions if r.type in ('header', 'margin_top')), 'none')
-    has_footer = any(r.type == 'footer' for r in regions)
+    bottom_region = next((r.type for r in regions if r.type in ('footer', 'margin_bottom')), 'none')
     col_count = len([r for r in regions if r.type.startswith('column')])
-    logger.info(f"Layout: {col_count} columns, "
+    logger.info(f"Layout: {col_count} columns, top={top_region}, bottom={bottom_region}")
-                f"header={'yes' if has_header else 'no'}, "
-                f"footer={'yes' if has_footer else 'no'}")
 
     return regions
 
@@ -2042,7 +2040,8 @@ def _build_margin_regions(
 
     # Right margin: from end of last content column to image edge
     non_margin = [r for r in all_regions
-                  if r.type not in ('margin_left', 'margin_right', 'header', 'footer')]
+                  if r.type not in ('margin_left', 'margin_right', 'header', 'footer',
+                                    'margin_top', 'margin_bottom')]
     if non_margin:
         last_col_end = max(r.x + r.width for r in non_margin)
     else:
@@ -2625,13 +2624,37 @@ def _detect_header_footer_gaps(
     return header_y, footer_y
 
 
+def _region_has_content(inv: np.ndarray, y_start: int, y_end: int,
+                        min_density: float = 0.005) -> bool:
+    """Check whether a horizontal strip contains meaningful ink.
+
+    Args:
+        inv: Inverted binarized image (white-on-black).
+        y_start: Top of the region (inclusive).
+        y_end: Bottom of the region (exclusive).
+        min_density: Fraction of white pixels required to count as content.
+
+    Returns:
+        True if the region contains text/graphics, False if empty margin.
+    """
+    if y_start >= y_end:
+        return False
+    strip = inv[y_start:y_end, :]
+    density = float(np.sum(strip)) / (strip.shape[0] * strip.shape[1] * 255)
+    return density > min_density
+
+
 def _add_header_footer(regions: List[PageRegion], top_y: int, bottom_y: int,
                        img_w: int, img_h: int,
                        inv: Optional[np.ndarray] = None) -> None:
-    """Add header/footer regions in-place.
+    """Add header/footer/margin regions in-place.
 
-    When *inv* is provided, uses gap-based detection to find header/footer
+    Uses gap-based detection when *inv* is provided, otherwise falls back
-    boundaries. Falls back to simple top_y/bottom_y check otherwise.
+    to simple top_y/bottom_y bounds.
+
+    Region types depend on whether there is actual content (text/graphics):
+      - 'header' / 'footer'       — region contains text (e.g. title, page number)
+      - 'margin_top' / 'margin_bottom' — region is empty page margin
     """
     header_y: Optional[int] = None
     footer_y: Optional[int] = None
@@ -2639,17 +2662,28 @@ def _add_header_footer(regions: List[PageRegion], top_y: int, bottom_y: int,
     if inv is not None:
         header_y, footer_y = _detect_header_footer_gaps(inv, img_w, img_h)
 
-    # Gap-based header
+    # --- Top region ---
-    if header_y is not None and header_y > 10:
+    top_boundary = header_y if header_y is not None and header_y > 10 else (
-        regions.append(PageRegion(type='header', x=0, y=0, width=img_w, height=header_y))
+        top_y if top_y > 10 else None
-    elif top_y > 10:
+    )
-        regions.append(PageRegion(type='header', x=0, y=0, width=img_w, height=top_y))
+    if top_boundary is not None:
+        has_content = inv is not None and _region_has_content(inv, 0, top_boundary)
+        rtype = 'header' if has_content else 'margin_top'
+        regions.append(PageRegion(type=rtype, x=0, y=0, width=img_w, height=top_boundary))
+        logger.info(f"HeaderFooter: top region type={rtype} height={top_boundary}px "
+                    f"(has_content={has_content})")
 
-    # Gap-based footer
+    # --- Bottom region ---
-    if footer_y is not None and footer_y < img_h - 10:
+    bottom_boundary = footer_y if footer_y is not None and footer_y < img_h - 10 else (
-        regions.append(PageRegion(type='footer', x=0, y=footer_y, width=img_w, height=img_h - footer_y))
+        bottom_y if bottom_y < img_h - 10 else None
-    elif bottom_y < img_h - 10:
+    )
-        regions.append(PageRegion(type='footer', x=0, y=bottom_y, width=img_w, height=img_h - bottom_y))
+    if bottom_boundary is not None:
+        has_content = inv is not None and _region_has_content(inv, bottom_boundary, img_h)
+        rtype = 'footer' if has_content else 'margin_bottom'
+        regions.append(PageRegion(type=rtype, x=0, y=bottom_boundary, width=img_w,
+                                  height=img_h - bottom_boundary))
+        logger.info(f"HeaderFooter: bottom region type={rtype} y={bottom_boundary} "
+                    f"height={img_h - bottom_boundary}px (has_content={has_content})")
 
 
 # --- Main Entry Point ---
@@ -2690,7 +2724,7 @@ def analyze_layout_by_words(ocr_img: np.ndarray, dewarped_bgr: np.ndarray) -> Li
     col_count = len([r for r in regions if r.type.startswith('column') or r.type == 'page_ref'])
     methods = set(r.classification_method for r in regions if r.classification_method)
     logger.info(f"LayoutByWords: {col_count} columns detected (methods: {methods}): "
-                f"{[(r.type, r.x, r.width, r.classification_confidence) for r in regions if r.type not in ('header','footer')]}")
+                f"{[(r.type, r.x, r.width, r.classification_confidence) for r in regions if r.type not in ('header','footer','margin_top','margin_bottom')]}")
 
     return regions
 
@@ -3797,7 +3831,7 @@ def build_cell_grid(
         return [], []
 
     # Use columns only — skip ignore, header, footer, page_ref
-    _skip_types = {'column_ignore', 'header', 'footer', 'page_ref', 'margin_left', 'margin_right'}
+    _skip_types = {'column_ignore', 'header', 'footer', 'margin_top', 'margin_bottom', 'page_ref', 'margin_left', 'margin_right'}
     relevant_cols = [c for c in column_regions if c.type not in _skip_types]
     if not relevant_cols:
         logger.warning("build_cell_grid: no usable columns found")
@@ -3959,7 +3993,7 @@ def build_cell_grid_streaming(
     if not content_rows:
         return
 
-    _skip_types = {'column_ignore', 'header', 'footer', 'page_ref', 'margin_left', 'margin_right'}
+    _skip_types = {'column_ignore', 'header', 'footer', 'margin_top', 'margin_bottom', 'page_ref', 'margin_left', 'margin_right'}
     relevant_cols = [c for c in column_regions if c.type not in _skip_types]
     if not relevant_cols:
         return
@@ -4444,7 +4478,7 @@ def run_multi_pass_ocr(ocr_img: np.ndarray,
     """
     results: Dict[str, List[Dict]] = {}
 
-    _ocr_skip = {'header', 'footer', 'margin_left', 'margin_right'}
+    _ocr_skip = {'header', 'footer', 'margin_top', 'margin_bottom', 'margin_left', 'margin_right'}
     for region in regions:
         if region.type in _ocr_skip:
             continue  # Skip non-content regions

klausur-service/backend/ocr_pipeline_api.py

@@ -840,6 +840,8 @@ async def _get_columns_overlay(session_id: str) -> Response:
         "column_ignore": (180, 180, 180), # Light Gray
         "header": (128, 128, 128),        # Gray
         "footer": (128, 128, 128),        # Gray
+        "margin_top": (100, 100, 100),    # Dark Gray
+        "margin_bottom": (100, 100, 100), # Dark Gray
     }
 
     overlay = img.copy()
@@ -1226,7 +1228,7 @@ async def _word_stream_generator(
 
     # Compute grid shape upfront for the meta event
     n_content_rows = len([r for r in row_geoms if r.row_type == 'content'])
-    _skip_types = {'column_ignore', 'header', 'footer', 'page_ref'}
+    _skip_types = {'column_ignore', 'header', 'footer', 'margin_top', 'margin_bottom', 'page_ref'}
     n_cols = len([c for c in col_regions if c.type not in _skip_types])
 
     # Determine layout
@@ -1712,6 +1714,8 @@ async def _get_rows_overlay(session_id: str) -> Response:
         "content": (255, 180, 0),     # Blue
         "header": (128, 128, 128),    # Gray
         "footer": (128, 128, 128),    # Gray
+        "margin_top": (100, 100, 100),    # Dark Gray
+        "margin_bottom": (100, 100, 100), # Dark Gray
     }
 
     overlay = img.copy()

klausur-service/backend/tests/test_cv_vocab_pipeline.py

@@ -35,6 +35,8 @@ from cv_vocab_pipeline import (
     _filter_narrow_runs,
     _build_margin_regions,
     _detect_header_footer_gaps,
+    _region_has_content,
+    _add_header_footer,
     analyze_layout,
     _group_words_into_lines,
     match_lines_to_vocab,
@@ -340,7 +342,8 @@ class TestLayoutAnalysis:
         ocr_img = create_ocr_image(text_like_image)
         layout_img = create_layout_image(text_like_image)
         regions = analyze_layout(layout_img, ocr_img)
-        valid_types = {'column_en', 'column_de', 'column_example', 'header', 'footer'}
+        valid_types = {'column_en', 'column_de', 'column_example',
+                       'header', 'footer', 'margin_top', 'margin_bottom'}
         for r in regions:
             assert r.type in valid_types, f"Unexpected region type: {r.type}"
 
@@ -976,7 +979,7 @@ class TestMarginRegions:
 
     def test_margins_in_skip_types(self):
         """Verify margin types are in the skip set used by build_cell_grid."""
-        skip = {'column_ignore', 'header', 'footer', 'page_ref', 'margin_left', 'margin_right'}
+        skip = {'column_ignore', 'header', 'footer', 'margin_top', 'margin_bottom', 'page_ref', 'margin_left', 'margin_right'}
         assert 'margin_left' in skip
         assert 'margin_right' in skip
 
@@ -1090,6 +1093,72 @@ class TestHeaderFooterGapDetection:
         assert footer_y is None
 
 
+class TestRegionContentCheck:
+    """Tests for _region_has_content() and _add_header_footer() type selection."""
+
+    def _make_inv(self, height: int, width: int, bands: list) -> np.ndarray:
+        inv = np.zeros((height, width), dtype=np.uint8)
+        for y1, y2 in bands:
+            inv[y1:y2, :] = 255
+        return inv
+
+    def test_region_with_text_has_content(self):
+        """Strip with ink → True."""
+        inv = self._make_inv(1000, 800, [(10, 50)])
+        assert _region_has_content(inv, 0, 100) is True
+
+    def test_empty_region_no_content(self):
+        """Strip without ink → False."""
+        inv = self._make_inv(1000, 800, [(500, 600)])
+        assert _region_has_content(inv, 0, 100) is False
+
+    def test_header_with_text_is_header(self):
+        """Top region with text → type='header' (via content bounds fallback)."""
+        h, w = 1000, 800
+        # Header text at 20-60, body starts at 200
+        inv = self._make_inv(h, w, [(20, 60), (200, 900)])
+        regions: list = []
+        # Simulate content bounds detecting body start at y=200
+        _add_header_footer(regions, top_y=200, bottom_y=h, img_w=w, img_h=h, inv=inv)
+        top_regions = [r for r in regions if r.type in ('header', 'margin_top')]
+        assert len(top_regions) == 1
+        assert top_regions[0].type == 'header'  # text at 20-60 → header
+
+    def test_empty_top_is_margin_top(self):
+        """Top region without text → type='margin_top'."""
+        h, w = 1000, 800
+        # Content only in body area (200-900), nothing in top 200px
+        inv = self._make_inv(h, w, [(200, 900)])
+        regions: list = []
+        # Simulate top_y=200 from content bounds
+        _add_header_footer(regions, top_y=200, bottom_y=h, img_w=w, img_h=h, inv=inv)
+        top_regions = [r for r in regions if r.type in ('header', 'margin_top')]
+        assert len(top_regions) == 1
+        assert top_regions[0].type == 'margin_top'
+
+    def test_empty_bottom_is_margin_bottom(self):
+        """Bottom region without text → type='margin_bottom'."""
+        h, w = 1000, 800
+        # Content only in top/body (50-700), nothing below 700
+        inv = self._make_inv(h, w, [(50, 700)])
+        regions: list = []
+        _add_header_footer(regions, top_y=50, bottom_y=700, img_w=w, img_h=h, inv=inv)
+        bottom_regions = [r for r in regions if r.type in ('footer', 'margin_bottom')]
+        assert len(bottom_regions) == 1
+        assert bottom_regions[0].type == 'margin_bottom'
+
+    def test_footer_with_page_number_is_footer(self):
+        """Bottom region with page number text → type='footer'."""
+        h, w = 1000, 800
+        # Body 50-700, page number at 900-930
+        inv = self._make_inv(h, w, [(50, 700), (900, 930)])
+        regions: list = []
+        _add_header_footer(regions, top_y=50, bottom_y=700, img_w=w, img_h=h, inv=inv)
+        bottom_regions = [r for r in regions if r.type in ('footer', 'margin_bottom')]
+        assert len(bottom_regions) == 1
+        assert bottom_regions[0].type == 'footer'
+
+
 # =============================================
 # RUN TESTS
 # =============================================