OpenCV .imshow（）未显示正确大小的图像

Question

I am trying to read an image using opencv, do some transformations (resize and offsets), then as a last step, do a crop on the image. In my final line crop_img = offset_image[0:1080, 0:1920].copy() , I expect a cropped 1920x1080 image to be created. My crop_img.size print out shows that that is correct. But, when I do an .imshow(), it is displaying the full sized, original image.

import numpy as np
import cv2 as cv
import copy

original = cv.imread("IMG_0015_guides.jpg", cv.IMREAD_UNCHANGED)
img_resize = cv.resize(original, (0,0), fx=.9, fy=.9) 


rows,cols,_ = img_resize.shape
M = np.float32([[1,0,100],[0,1,50]])
offset_image = cv.warpAffine(img_resize,M,(cols,rows))

crop_img = offset_image[0:1080, 0:1920].copy()

print('img_resize {}'.format(img_resize.shape))
print('offset_image {}'.format(offset_image.shape))
print('cropped {}'.format(crop_img.shape))

cv.imshow('image',crop_img)
cv.waitKey(0)
cv.destroyAllWindows()

>>> img_resize (3110, 4666, 3)
>>> offset_image (3110, 4666, 3)
>>> cropped (1080, 1920, 3)

I'm totally baffled. Why is it not showing me the cropped 1920x1080 image?

Answer 1

Working with massive images can get confusing when visualizing with OpenCV's imshow. I ran your code and it seems like its doing what you expect it to do. I suggest resizing your image again for visualization purposes only. The following code ran successfully on this image .

import numpy as np
import cv2 as cv
import copy

original = cv.imread("4k-image-tiger-jumping.jpg", cv.IMREAD_UNCHANGED)

# resize original for visualization purposes only
print('original {}'.format(original.shape))
original_resized = cv.resize(original, (0,0), fx=.1, fy=.1) 
cv.imshow('original_resize',original_resized)


img_resize = cv.resize(original, (0,0), fx=.9, fy=.9) 

rows,cols,_ = img_resize.shape
M = np.float32([[1,0,100],[0,1,50]])
offset_image = cv.warpAffine(img_resize,M,(cols,rows))

crop_img = offset_image[0:1080, 0:1920].copy()

print('img_resize {}'.format(img_resize.shape))
print('offset_image {}'.format(offset_image.shape))
print('cropped {}'.format(crop_img.shape))


# resize cropped for visualization purposes only
vis_r,vis_c,_ = original_resized.shape
cropped_resized = cv.resize(crop_img, (vis_c, vis_r))
cv.imshow('cropped_resized',cropped_resized)


# cv.imshow('image',crop_img)

cv.waitKey(0)
cv.destroyAllWindows()