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fix: keep the plateau rule and the full-resolution fine sweep in SkewCorrector #21

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Merged
NastyBoget merged 1 commit into from
Sep 8, 2026
Merged

fix: keep the plateau rule and the full-resolution fine sweep in SkewCorrector #21

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17 changes: 10 additions & 7 deletions dedocutils/preprocessing/skew_corrector.py
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Original file line number Diff line number Diff line change
Expand Up @@ -16,8 +16,7 @@ class SkewCorrector(AbstractPreprocessor):
def __init__(self) -> None:
self._step = 1 # step
self._max_angle = 45 # max angle
self._min_side = 1000 # the fine sweep runs on an image downscaled to this long side (never upscales a small page)
self._coarse_side = 512 # the coarse guess runs on this smaller thumbnail
self._coarse_side = 512 # the coarse guess runs on a thumbnail with this long side

def preprocess(self, image: np.ndarray, parameters: Optional[dict] = None) -> Tuple[np.ndarray, dict]:
parameters = {} if parameters is None else parameters
Expand All @@ -28,18 +27,22 @@ def preprocess(self, image: np.ndarray, parameters: Optional[dict] = None) -> Tu

gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
thresh = cv2.threshold(gray, 0, 255, cv2.THRESH_BINARY_INV + cv2.THRESH_OTSU)[1]
scale = min(1.0, self._min_side / max(thresh.shape[:2]))
if scale < 1.0:
thresh = cv2.resize(thresh, None, fx=scale, fy=scale, interpolation=cv2.INTER_AREA)

coarse_scale = min(1.0, self._coarse_side / max(thresh.shape[:2]))
thumb = cv2.resize(thresh, None, fx=coarse_scale, fy=coarse_scale, interpolation=cv2.INTER_AREA) if coarse_scale < 1.0 else thresh

coarse_angles = np.arange(-self._max_angle, self._max_angle + 1, 3)
coarse = float(coarse_angles[int(np.argmax([self._score(thumb, angle) for angle in coarse_angles]))])
lo, hi = max(coarse - 4, -self._max_angle), min(coarse + 4, self._max_angle)
fine_angles = np.arange(lo, hi + 0.001, self._step)
best_angle = float(fine_angles[int(np.argmax([self._score(thresh, angle) for angle in fine_angles]))])
scores = [self._score(thresh, angle) for angle in fine_angles]
max_idx = int(np.argmax(scores))
# a document with short lines gives a plateau instead of a peak: take the angle between two equally good ones
if max_idx >= 2 and scores[max_idx - 2] > scores[max_idx] * 0.98:
best_angle = float(fine_angles[max_idx - 1])
elif max_idx < len(scores) - 2 and scores[max_idx + 2] > scores[max_idx] * 0.98:
best_angle = float(fine_angles[max_idx + 1])
else:
best_angle = float(fine_angles[max_idx])

rotated = image if best_angle == 0 else rotate_image(image, best_angle)
return rotated, {"rotated_angle": float(orientation_angle + best_angle)}
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