Cropping an image after detecting the contours using opencv python

Question

I'm trying to crop an image after detecting the contours and then extract information from it using python, opencv, and numpy.

Detecting the contours was successful but then I couldn't find a way to crop. I tried some code but I didn't get the result that I wanted:

import numpy as np
import cv2
image = cv2.imread('2.jpg') # Read in your image
blurred=cv2.blur(image,(23,51))#blur the image
gray=cv2.cvtColor(blurred,cv2.COLOR_BGR2GRAY)#gray scale
ret,threshold=cv2.threshold(gray , 2 , 255 , cv2.THRESH_BINARY+cv2.THRESH_OTSU)
contours,hierachy=cv2.findContours(threshold,cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE)#detect contours
mask = np.zeros_like(image) # Create mask where white is what we want, black otherwise
cv2.drawContours(mask,  contours,  0,  (0 ,0,255),1)
out = np.zeros_like(image) # Extract out the object and place into output image
out[mask == 255] = image[mask == 255]

# Now crop
mask=mask.transpose(2,0,1).reshape(-1,mask.shape[1])
(x, y) = np.where(mask == 255)
(topx, topy) = (np.min(x), np.min(y))
(bottomx, bottomy) = (np.max(x), np.max(y))
topx=topx
topy=topy
bottomx=bottomx
bottomy=bottomy
out = out[topx:bottomx,topy:bottomy]
print(out)

enter image here

The array out is empty which is weird because when I print topx, topy, bottomx, and bottomy, I get integers, so logically cropping would give me a result.

@bechirjamoussi you have a tool in the text editor of SO to put your text as code. Text with a tabulation before each line will be formatted as code. — Clément, Jul 29 '19 at 11:27
Possible duplicate of [How to crop an image in OpenCV using Python](https://stackoverflow.com/questions/15589517/how-to-crop-an-image-in-opencv-using-python) — T A, Jul 29 '19 at 11:42

singrium · Accepted Answer · 2019-07-29T13:29:58.517

3

Normally, if you have the contours detected, you can crop the contours by adding the following lines to your code, just after contours, hierarchy = cv2.findContours(threshold, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE):

i = 0
for c in contours:
    # get the bounding rect
    x, y, w, h = cv2.boundingRect(c)
    # to save the images
    cv2.imwrite('img_{}.jpg'.format(i), image[y:y+h,x:x+w])
    i += 1

edited Jul 29 '19 at 13:29

answered Jul 29 '19 at 11:31

singrium

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Welcome Bech :p – singrium Jul 30 '19 at 08:04

score 0 · Answer 2 · answered Sep 01 '19 at 17:15

You may try this:

x, y = [], []

for contour_line in contours:
    for contour in contour_line:
        x.append(contour[0][0])
        y.append(contour[0][1])

x1, x2, y1, y2 = min(x), max(x), min(y), max(y)

cropped = img[y1:y2, x1:x2]

Source: https://stackoverflow.com/questions/41069831/opencv-python-crop-image-using-numpy-array?rq=1#=

Cropping an image after detecting the contours using opencv python

2 Answers2