feat: automatische Orientierungserkennung fuer umgedrehte Scans

Tesseract OSD erkennt 0/90/180/270° Rotation und korrigiert
automatisch vor dem Deskew. Loest das Problem mit Buchscannern,
bei denen jede 2. Seite auf dem Kopf steht.

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

klausur-service/backend/cv_vocab_pipeline.py

@@ -224,6 +224,54 @@ def render_image_high_res(image_data: bytes) -> np.ndarray:
     return img_bgr
 
 
+# =============================================================================
+# Stage 1b: Orientation Detection (0°/90°/180°/270°)
+# =============================================================================
+
+def detect_and_fix_orientation(img_bgr: np.ndarray) -> Tuple[np.ndarray, int]:
+    """Detect page orientation via Tesseract OSD and rotate if needed.
+
+    Handles upside-down scans (180°) common with book scanners where
+    every other page is flipped due to the scanner hinge.
+
+    Returns:
+        (corrected_image, rotation_degrees)  — rotation is 0, 90, 180, or 270.
+    """
+    if pytesseract is None:
+        return img_bgr, 0
+
+    try:
+        # Tesseract OSD needs a grayscale or RGB image
+        gray = cv2.cvtColor(img_bgr, cv2.COLOR_BGR2GRAY)
+        pil_img = Image.fromarray(gray)
+
+        osd = pytesseract.image_to_osd(pil_img, output_type=pytesseract.Output.DICT)
+        rotate = osd.get("rotate", 0)
+        confidence = osd.get("orientation_conf", 0.0)
+
+        logger.info(f"OSD: orientation={rotate}° confidence={confidence:.1f}")
+
+        if rotate == 0 or confidence < 1.0:
+            return img_bgr, 0
+
+        # Apply rotation
+        if rotate == 180:
+            corrected = cv2.rotate(img_bgr, cv2.ROTATE_180)
+        elif rotate == 90:
+            corrected = cv2.rotate(img_bgr, cv2.ROTATE_90_COUNTERCLOCKWISE)
+        elif rotate == 270:
+            corrected = cv2.rotate(img_bgr, cv2.ROTATE_90_CLOCKWISE)
+        else:
+            return img_bgr, 0
+
+        logger.info(f"OSD: rotated {rotate}° to fix orientation")
+        return corrected, rotate
+
+    except Exception as e:
+        logger.warning(f"OSD orientation detection failed: {e}")
+        return img_bgr, 0
+
+
 # =============================================================================
 # Stage 2: Deskew (Rotation Correction)
 # =============================================================================

klausur-service/backend/vocab_worksheet_api.py

@@ -71,6 +71,7 @@ try:
         detect_row_geometry, build_cell_grid_v2,
         _cells_to_vocab_entries, _detect_sub_columns, _detect_header_footer_gaps,
         expand_narrow_columns, positional_column_regions, llm_review_entries,
+        detect_and_fix_orientation,
         _fix_phonetic_brackets,
         render_pdf_high_res,
         PageRegion, RowGeometry,
@@ -1360,6 +1361,15 @@ async def _run_ocr_pipeline_for_page(
     img_h, img_w = img_bgr.shape[:2]
     logger.info(f"OCR Pipeline page {page_number + 1}: image {img_w}x{img_h}")
 
+    # 1b. Orientation detection (fix upside-down scans)
+    t0 = _time.time()
+    img_bgr, rotation = detect_and_fix_orientation(img_bgr)
+    if rotation:
+        img_h, img_w = img_bgr.shape[:2]
+        logger.info(f"  orientation: rotated {rotation}° ({_time.time() - t0:.1f}s)")
+    else:
+        logger.info(f"  orientation: OK ({_time.time() - t0:.1f}s)")
+
     # 2. Create pipeline session in DB (for debugging in admin UI)
     pipeline_session_id = str(uuid.uuid4())
     try: