How to crop an image in OpenCV using Python

Question

How can I crop images, like I've done before in PIL, using OpenCV.

Working example on PIL

im = Image.open('0.png').convert('L') im = im.crop((1, 1, 98, 33)) im.save('_0.png') 

But how I can do it on OpenCV?

This is what I tried:

im = cv.imread('0.png', cv.CV_LOAD_IMAGE_GRAYSCALE) (thresh, im_bw) = cv.threshold(im, 128, 255, cv.THRESH_OTSU) im = cv.getRectSubPix(im_bw, (98, 33), (1, 1)) cv.imshow('Img', im) cv.waitKey(0) 

But it doesn't work.

I think I incorrectly used getRectSubPix. If this is the case, please explain how I can correctly use this function.

Answer 1

It's very simple. Use numpy slicing.

import cv2 img = cv2.imread("lenna.png") crop_img = img[y:y+h, x:x+w] cv2.imshow("cropped", crop_img) cv2.waitKey(0) 

Answer 2

i had this question and found another answer here: copy region of interest

If we consider (0,0) as top left corner of image called im with left-to-right as x direction and top-to-bottom as y direction. and we have (x1,y1) as the top-left vertex and (x2,y2) as the bottom-right vertex of a rectangle region within that image, then:

roi = im[y1:y2, x1:x2] 

here is a comprehensive resource on numpy array indexing and slicing which can tell you more about things like cropping a part of an image. images would be stored as a numpy array in opencv2.

:)