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fix: remove morph close that merged balloons into giant blob
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The 5x5 MORPH_CLOSE was connecting scattered color pixels into one
page-spanning contour that swallowed individual balloons. Fix:
- Remove MORPH_CLOSE, keep only MORPH_OPEN for speckle removal
- Lower sat threshold 50→40 to catch more colored elements
- Filter contours spanning >50% of width OR height (was AND)
- Filter contours >10% of image area

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
This commit is contained in:
Benjamin Admin
2026-03-16 14:42:51 +01:00
parent 1653e7cff4
commit eeee61108a
1 changed files with 7 additions and 8 deletions
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15
klausur-service/backend/cv_graphic_detect.py
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@@ -137,15 +137,14 @@ def detect_graphic_elements(
# PASS 1 — COLOR CHANNEL (no word exclusion needed) # PASS 1 — COLOR CHANNEL (no word exclusion needed)
# ===================================================================== # =====================================================================
# Saturated pixels = colored ink. Black text has sat ≈ 0 → invisible. # Saturated pixels = colored ink. Black text has sat ≈ 0 → invisible.
sat_mask = (hsv[:, :, 1] > 50).astype(np.uint8) * 255 sat_mask = (hsv[:, :, 1] > 40).astype(np.uint8) * 255
# Exclude very bright backgrounds (white/near-white with color cast) # Exclude very bright backgrounds (white/near-white with color cast)
val_mask = (hsv[:, :, 2] < 235).astype(np.uint8) * 255 val_mask = (hsv[:, :, 2] < 240).astype(np.uint8) * 255
color_mask = cv2.bitwise_and(sat_mask, val_mask) color_mask = cv2.bitwise_and(sat_mask, val_mask)
# Morphological cleanup: close small gaps, remove speckle # Only remove tiny speckle — NO closing, which would merge nearby
kernel = cv2.getStructuringElement(cv2.MORPH_ELLIPSE, (5, 5)) # colored elements into one giant blob spanning half the page.
color_mask = cv2.morphologyEx(color_mask, cv2.MORPH_CLOSE, kernel) kernel_open = cv2.getStructuringElement(cv2.MORPH_RECT, (2, 2))
kernel_open = cv2.getStructuringElement(cv2.MORPH_RECT, (3, 3))
color_mask = cv2.morphologyEx(color_mask, cv2.MORPH_OPEN, kernel_open) color_mask = cv2.morphologyEx(color_mask, cv2.MORPH_OPEN, kernel_open)
contours_color, _ = cv2.findContours( contours_color, _ = cv2.findContours(
@@ -162,8 +161,8 @@ def detect_graphic_elements(
if bw < 8 or bh < 8: if bw < 8 or bh < 8:
continue continue
# Skip page-spanning contours (background color cast) # Skip page-spanning contours (background color cast / merged blobs)
if bw > w * 0.8 and bh > h * 0.8: if bw > w * 0.5 or bh > h * 0.5 or area > img_area * 0.10:
continue continue
perimeter = cv2.arcLength(cnt, True) perimeter = cv2.arcLength(cnt, True)