Python OpenCV How to draw ractangle center of image and crop image inside rectangle?

Question

I use Python3 OpenCV3. I want to draw ractangle center of image and crop image inside rectangle. I try to run this code but rectangle not show at the center of image.

width, height, channels = img.shape
cv2.rectangle(img,(0,0),(int(width/2), int(height/2)), (0,255,0) , 2)

How to draw ractangle center of image and crop image inside rectangle ?

Solution 1

create an image

import cv2
import numpy as np

img = np.random.randint(0, 256, size=(100, 150, 3), dtype=np.uint8)
cv2.imshow('img', img)

draw a rectangle

height, width, channels = img.shape
upper_left = (width // 4, height // 4)
bottom_right = (width * 3 // 4, height * 3 // 4)
# draw in the image
cv2.rectangle(img, upper_left, bottom_right, (0, 255, 0), thickness=1)
cv2.imshow('draw_img', img)

fill values by indexing

# indexing array
rect_img = img[upper_left[1]: bottom_right[1] + 1, upper_left[0]: bottom_right[0] + 1]
rect_img[:] = (255, 0, 0)  # modify value
cv2.imshow('index_img', img)
cv2.waitKey()

Solution 2

You need to use numpy slicing in order to crop the image.

The way OpenCV stores an image is as a numpy array. This means that you can 'crop' them as you can a numpy array.

The way to do this is with the following syntax:

cropped = img[top_edge : bottom_edge, left_edge : right_edge]

where top_edge, bottom_edge etc. are pixels vals.

The reason this works is because numpy slicing allows you to slice along any axis - each one separated by a comma.

So here, we are slicing the rows of the image to between top_edge and bottom_edge and then the comma says that what comes next is going to effect the columns in each row. So for a given row, we slice it between left_edge and right_edge. And that's it!

Hope this is useful! :)