Rotating QR code to the correct position using Python OpenCV

Issue

I’m a beginner in python and currently studying QR code detection and decoding. I’m having a hard time rotating the detected QR code to the right position. I already used minAreaRect() to rotate my QR code but it doesn’t work. Is there any workaround or a right way to do this? thanks!

ROI2 = cv2.imread('ROI.png')
gray2 = cv2.cvtColor(ROI2, cv2.COLOR_BGR2GRAY)
blur2 = cv2.GaussianBlur(gray2, (9, 9), 0)
thresh2 = cv2.threshold(blur2, 0, 255, cv2.THRESH_BINARY_INV + cv2.THRESH_OTSU)[1]

# Morph close
# kernel2 = cv2.getStructuringElement(cv2.MORPH_RECT, (5, 5))
# close2 = cv2.morphologyEx(thresh2, cv2.MORPH_CLOSE, kernel2, iterations=10)

# Find contours and filter for QR code
cnts2 = cv2.findContours(thresh2, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
cnts2 = cnts2[0] if len(cnts2) == 2 else cnts2[1]
c = sorted(cnts2, key=cv2.contourArea, reverse=True)[0]

draw = cv2.cvtColor(thresh2, cv2.COLOR_GRAY2BGR)
cv2.drawContours(draw, [c], 0, (0, 255, 0), 2)

rotrect = cv2.minAreaRect(c)
box = cv2.boxPoints(rotrect)
box = numpy.int0(box)
cv2.drawContours(draw, [box], 0, (0, 0, 255), 2)

cv2.imshow('thresh', thresh2)
cv2.imshow('ROI', ROI2)
cv2.imshow('minarearect', draw)

thresh
ROI
minAreaRect output

Solution

From my understanding, you’re trying to deskew an image. To do this, we need to first compute the rotated bounding box angle then perform a linear transformation. The idea is to use
cv2.minAreaRect + cv2.warpAffine. According to the documentation, cv2.minAreaRect returns

(center(x, y), (width, height), angle of rotation) = cv2.minAreaRect(...)

The third parameter gives us the angle we need to deskew the image.


Input image -> Output result


Skew angle: -39.99416732788086

Code

import cv2
import numpy as np

# Load image, grayscale, Otsu's threshold
image = cv2.imread('2.png')
gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
gray = 255 - gray
thresh = cv2.threshold(gray, 0, 255, cv2.THRESH_BINARY + cv2.THRESH_OTSU)[1]

# Compute rotated bounding box
coords = np.column_stack(np.where(thresh > 0))
angle = cv2.minAreaRect(coords)[-1]

if angle < -45:
    angle = -(90 + angle)
else:
    angle = -angle
print("Skew angle: ", angle)

# Rotate image to deskew
(h, w) = image.shape[:2]
center = (w // 2, h // 2)
M = cv2.getRotationMatrix2D(center, angle, 1.0)
rotated = cv2.warpAffine(image, M, (w, h), flags=cv2.INTER_CUBIC, borderMode=cv2.BORDER_REPLICATE)

cv2.imshow('rotated', rotated)
cv2.waitKey()

Note: See Python OpenCV skew correction for another approach using the Projection Profile Method to correct skew.

Answered By – nathancy

Answer Checked By – Timothy Miller (AngularFixing Admin)

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