Here’s an implementation of the Projection Profile Method algorithm for skew angle estimation. Various angle points are projected into an accumulator array where the skew angle can be defined as the angle of projection within a search interval that maximizes alignment. The idea is to rotate the image at various angles and generate a histogram of pixels for each iteration. To determine the skew angle, we compare the maximum difference between peaks and using this skew angle, rotate the image to correct the skew.
Original ->
Corrected
Skew angle: -2
import cv2
import numpy as np
from scipy.ndimage import interpolation as inter
def correct_skew(image, delta=1, limit=5):
def determine_score(arr, angle):
data = inter.rotate(arr, angle, reshape=False, order=0)
histogram = np.sum(data, axis=1, dtype=float)
score = np.sum((histogram[1:] - histogram[:-1]) ** 2, dtype=float)
return histogram, score
gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
thresh = cv2.threshold(gray, 0, 255, cv2.THRESH_BINARY_INV + cv2.THRESH_OTSU)[1]
scores = []
angles = np.arange(-limit, limit + delta, delta)
for angle in angles:
histogram, score = determine_score(thresh, angle)
scores.append(score)
best_angle = angles[scores.index(max(scores))]
(h, w) = image.shape[:2]
center = (w // 2, h // 2)
M = cv2.getRotationMatrix2D(center, best_angle, 1.0)
corrected = cv2.warpAffine(image, M, (w, h), flags=cv2.INTER_CUBIC, \
borderMode=cv2.BORDER_REPLICATE)
return best_angle, corrected
if __name__ == '__main__':
image = cv2.imread('1.png')
angle, corrected = correct_skew(image)
print('Skew angle:', angle)
cv2.imshow('corrected', corrected)
cv2.waitKey()
Note: You may have to adjust the delta
or limit
values depending on the image. The delta
value controls iteration step, it will iterate up until the limit
which controls the maximum angle. This method is straightforward by iteratively checking each angle + delta
and currently only works to correct skew in the range of +/- 5 degrees. If you need to correct at a larger angle, adjust the limit
value. For another approach to handle skew, take a look at this alternative method.