cv2 minAreaRect 中心旋转 90 度？

Question

I'm not sure if I encountered a bug or if I'm missing something (opencv 4.4.0.46 and 4.5.3.56, maybe others).

I'm trying to find the rotated bounding box for this image:

This is the result:

here is the code:

import cv2
import numpy as np

base_image = cv2.imread("so_sample.png", 0)

thresh = cv2.threshold(base_image, 0, 255, cv2.THRESH_BINARY | cv2.THRESH_OTSU)[1]

coords = np.column_stack(np.where(thresh > 0))
rect = cv2.minAreaRect(coords)
print("RECT", rect)

box = np.int0(cv2.boxPoints(rect))
drawImg = cv2.cvtColor(thresh, cv2.COLOR_GRAY2BGR)
drawImg = cv2.copyMakeBorder(drawImg, 0, 100, 0, 100, cv2.BORDER_REPLICATE) # to see the whole box
cv2.drawContours(drawImg, [box], 0,(0,0,255), 2)

cv2.imshow("base_image", base_image)
cv2.imshow("thresh", thresh)
cv2.imshow("drawImg", drawImg)
cv2.waitKey(0)

This code works fine for the "thunder" sample image and it looks like all the sample code I could find around. Am I missing something? Thanks

Answer 1

You are using np.column_stack(np.where(thresh > 0)) to find the contours. OpenCV uses (x, y) notation whereas NumPy uses (row, col) . You can check it by printing coords .

I suggest using OpenCV functions where possible. The following code works.

coords, _ = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
rect = cv2.minAreaRect(np.vstack(coords))

See Contour Features .