From 0532b2a79764564f7a69e5a3cdb5325dc205a0c6 Mon Sep 17 00:00:00 2001
From: Benjamin Admin <benjaminadmin@MacBookPro.fritz.box>
Date: Mon, 2 Mar 2026 17:54:49 +0100
Subject: [PATCH] fix(ocr-pipeline): skip edge-touching gaps in header/footer
 detection
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Gaps that extend to the image boundary (top/bottom edge) are not valid
content separators — they typically represent dewarp padding. Only gaps
with content on both sides qualify as header/footer boundaries.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
---
 klausur-service/backend/cv_vocab_pipeline.py          |  7 +++++++
 .../backend/tests/test_cv_vocab_pipeline.py           | 11 +++++++++++
 2 files changed, 18 insertions(+)

diff --git a/klausur-service/backend/cv_vocab_pipeline.py b/klausur-service/backend/cv_vocab_pipeline.py
index 3a9dde9..2bd8bd1 100644
--- a/klausur-service/backend/cv_vocab_pipeline.py
+++ b/klausur-service/backend/cv_vocab_pipeline.py
@@ -2593,6 +2593,9 @@ def _detect_header_footer_gaps(
     large_gap_threshold = median_gap * GAP_MULTIPLIER
 
     # Step 6: Find largest qualifying gap in header / footer zones
+    # A separator gap must have content on BOTH sides — edge-touching gaps
+    # (e.g. dewarp padding at bottom) are not valid separators.
+    EDGE_MARGIN = max(5, actual_h // 400)
     header_zone_limit = int(actual_h * HEADER_FOOTER_ZONE)
     footer_zone_start = int(actual_h * (1.0 - HEADER_FOOTER_ZONE))
 
@@ -2601,6 +2604,8 @@ def _detect_header_footer_gaps(
 
     best_header_size = 0
     for gs, ge in raw_gaps:
+        if gs <= EDGE_MARGIN:
+            continue  # skip gaps touching the top edge
         gap_mid = (gs + ge) / 2
         gap_size = ge - gs
         if gap_mid < header_zone_limit and gap_size > large_gap_threshold:
@@ -2610,6 +2615,8 @@ def _detect_header_footer_gaps(
 
     best_footer_size = 0
     for gs, ge in raw_gaps:
+        if ge >= actual_h - EDGE_MARGIN:
+            continue  # skip gaps touching the bottom edge
         gap_mid = (gs + ge) / 2
         gap_size = ge - gs
         if gap_mid > footer_zone_start and gap_size > large_gap_threshold:
diff --git a/klausur-service/backend/tests/test_cv_vocab_pipeline.py b/klausur-service/backend/tests/test_cv_vocab_pipeline.py
index 0ce25d7..4d764c7 100644
--- a/klausur-service/backend/tests/test_cv_vocab_pipeline.py
+++ b/klausur-service/backend/tests/test_cv_vocab_pipeline.py
@@ -1092,6 +1092,17 @@ class TestHeaderFooterGapDetection:
         assert header_y is None
         assert footer_y is None
 
+    def test_edge_gaps_ignored_dewarp_padding(self):
+        """Trailing gap at bottom edge (dewarp padding) should not be detected as footer."""
+        h, w = 2000, 800
+        # Body lines from 10 to 1700
+        bands = self._make_body_with_lines(h, w, 10, 1700)
+        # Gap from 1700 to 2000 = bottom edge padding (no content after)
+        inv = self._make_inv(h, w, bands)
+        header_y, footer_y = _detect_header_footer_gaps(inv, w, h)
+        # The trailing gap touches the image edge → not a valid separator
+        assert footer_y is None
+
 
 class TestRegionContentCheck:
     """Tests for _region_has_content() and _add_header_footer() type selection."""