Fix ghost filter for borderless boxes + remove oversized graphic artifacts

1. Skip ghost filtering for boxes with border_thickness=0 (images/graphics
   have no border lines to produce OCR artifacts like |, I)
2. Remove individual word_boxes with height > 3x zone median (OCR from
   graphics like a huge "N" from a map image below text)

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

klausur-service/backend/grid_editor_api.py

@@ -290,15 +290,18 @@ def _filter_border_ghosts(
     x_bands: List[tuple] = []
     y_bands: List[tuple] = []
     for b in boxes:
-        bx = b.x if hasattr(b, "x") else b.get("x", 0)
-        by = b.y if hasattr(b, "y") else b.get("y", 0)
-        bw = b.width if hasattr(b, "width") else b.get("w", b.get("width", 0))
-        bh = b.height if hasattr(b, "height") else b.get("h", b.get("height", 0))
         bt = (
             b.border_thickness
             if hasattr(b, "border_thickness")
             else b.get("border_thickness", 3)
         )
+        # Skip borderless boxes (images/graphics) — no border line to produce ghosts
+        if bt == 0:
+            continue
+        bx = b.x if hasattr(b, "x") else b.get("x", 0)
+        by = b.y if hasattr(b, "y") else b.get("y", 0)
+        bw = b.width if hasattr(b, "width") else b.get("w", b.get("width", 0))
+        bh = b.height if hasattr(b, "height") else b.get("h", b.get("height", 0))
         margin = max(bt * 2, 10) + 6
         x_bands.append((bx - margin, bx + margin))
         x_bands.append((bx + bw - margin, bx + bw + margin))
@@ -1518,6 +1521,44 @@ async def _build_grid_core(session_id: str, session: dict) -> dict:
                 sorted(junk_row_indices),
             )
 
+    # 4c. Remove oversized word_boxes from individual cells.
+    # OCR artifacts from graphics/images (e.g. a huge "N" from a map image)
+    # have word heights 3-5x the median.  Remove them per-word so they don't
+    # pollute cells that also contain valid text in other columns.
+    for z in zones_data:
+        cells = z.get("cells", [])
+        if not cells:
+            continue
+        all_wh = [
+            wb["height"]
+            for cell in cells
+            for wb in cell.get("word_boxes") or []
+            if wb.get("height", 0) > 0
+        ]
+        if not all_wh:
+            continue
+        med_h = sorted(all_wh)[len(all_wh) // 2]
+        oversized_threshold = med_h * 3
+        removed_oversized = 0
+        for cell in cells:
+            wbs = cell.get("word_boxes") or []
+            filtered = [wb for wb in wbs if wb.get("height", 0) <= oversized_threshold]
+            if len(filtered) < len(wbs):
+                removed_oversized += len(wbs) - len(filtered)
+                cell["word_boxes"] = filtered
+                cell["text"] = " ".join(
+                    wb.get("text", "").strip()
+                    for wb in sorted(filtered, key=lambda w: (w.get("top", 0), w.get("left", 0)))
+                    if wb.get("text", "").strip()
+                )
+        if removed_oversized:
+            # Remove cells that became empty after oversized removal
+            z["cells"] = [c for c in cells if c.get("word_boxes")]
+            logger.info(
+                "build-grid: removed %d oversized word_boxes (>%dpx) from zone %d",
+                removed_oversized, oversized_threshold, z.get("zone_index", 0),
+            )
+
     # 5. Color annotation on final word_boxes in cells
     if img_bgr is not None:
         all_wb: List[Dict] = []

klausur-service/backend/tests/test_grid_editor_api.py

@@ -384,8 +384,8 @@ class TestFilterBorderGhosts:
         assert filtered[0]["text"] == "hello"
 
     def test_multi_char_ghost_kept(self):
-        """Multi-char '(=' on a box border → NOT filtered (real content)."""
-        box = DetectedBox(x=648, y=129, width=338, height=125, confidence=0.7, border_thickness=0)
+        """Multi-char '(=' on a bordered box → NOT filtered (real content)."""
+        box = DetectedBox(x=648, y=129, width=338, height=125, confidence=0.7, border_thickness=3)
         words = [
             {"text": "(=", "left": 644, "top": 294, "width": 16, "height": 17},
             {"text": "I", "left": 665, "top": 294, "width": 9, "height": 18},
@@ -394,6 +394,17 @@ class TestFilterBorderGhosts:
         assert count == 0
         assert len(filtered) == 2
 
+    def test_borderless_box_no_ghost_filter(self):
+        """Borderless box (border_thickness=0) → no ghost filtering at all."""
+        box = DetectedBox(x=648, y=129, width=338, height=125, confidence=0.7, border_thickness=0)
+        words = [
+            {"text": "I", "left": 643, "top": 272, "width": 6, "height": 19},  # near box edge
+            {"text": "|", "left": 647, "top": 200, "width": 3, "height": 10},  # even pipe on edge
+        ]
+        filtered, count = _filter_border_ghosts(words, [box])
+        assert count == 0  # nothing filtered — borderless box
+        assert len(filtered) == 2
+
     def test_single_paren_on_border_removed(self):
         """Single ')' on border → filtered."""
         box = DetectedBox(x=100, y=100, width=200, height=150, confidence=0.9, border_thickness=2)