modest improvement to cropping (safer)

copienator/ink_detection.py

@@ -65,13 +65,26 @@ def _large_blank_ink(gray: np.ndarray, clean: np.ndarray, dpi: float):
     n, labels, stats, centers = cv2.connectedComponentsWithStats(
         (residual > 35).astype(np.uint8), 8)
     keep = np.zeros(n, bool)
+    xx = stats[1:, 0]
     ww, hh, area = stats[1:, 2], stats[1:, 3], stats[1:, 4]
-    keep[1:] = ((area >= .8*px*px) & (np.minimum(ww, hh) >= .6*px)
-                & (area / (ww*hh) > .3)
-                & (np.maximum(ww, hh) < 4*np.minimum(ww, hh)))
+    density = area / (ww*hh)
+    shortest, longest = np.minimum(ww, hh), np.maximum(ww, hh)
+    compact_ink = ((area >= .8*px*px) & (shortest >= .6*px)
+                   & (density > .3) & (longest < 4*shortest))
+    # Ruling suppression can fragment faint pencil handwriting into sparse,
+    # elongated components. Admit those moderately more readily in the central
+    # 80% of the sheet. The outermost 9% deliberately uses a stricter filter:
+    # punched holes, torn binding edges, and page numbers usually occur there.
+    center_x = xx + ww/2
+    central = (center_x > width*.1) & (center_x < width*.9)
+    central_ink = (central & (area >= .55*px*px) & (shortest >= .45*px)
+                   & (density > .18) & (longest < 6*shortest))
+    outer = (center_x < width*.09) | (center_x > width*.91)
+    outer_ink = (outer & (area >= 1.2*px*px) & (shortest >= .8*px)
+                 & (density >= .4) & (longest < 3*shortest))
+    keep[1:] = np.where(outer, outer_ink, compact_ink | central_ink)
     # Use side columns as a prior, then require repeated size and alignment. An
     # isolated note in the same column remains eligible to protect the margin.
-    xx = stats[1:, 0]
     candidates = np.flatnonzero(
         ((xx < 15*px) | (xx+ww > width-15*px))
         & (ww > px) & (ww < 9*px) & (hh > px) & (hh < 12*px)) + 1