Ipython cv2.imwrite() not saving image

As a general and absolute rule, you have to protect your windows path strings (containing backslashes) with r prefix or some characters are interpreted (ex: \n,\b,\v,\x aaaaand \t, full list here):

so when doing this:

cv2.imwrite('C:\Users\Niladri\Desktop\tropical_image_sig5.bmp', img2)

you're trying to save to C:\Users\Niladri\Desktop<TAB>ropical_image_sig5.bmp

And the annoying thing with imread and imwrite is that those functions don't throw exceptions on errors, but fail silently. imwrite returns False

>>> cv2.imread("D:/nonexisting.jpg")  # this returns None, no error
>>> s = cv2.imread("D:/sloth_book.jpg")  # this works
>>> s
array([[[250, 250, 250],
        [246, 246, 246],
        [255, 255, 255],
        ...,
>>> cv2.imwrite("inexistent_dir/file.jpg",s)  # dir doesn't exist, write fails
False

So you have to check return value of those functions.

Do this:

if not cv2.imwrite(r'C:\Users\Niladri\Desktop\tropical_image_sig5.bmp', img2):
     raise Exception("Could not write image")

Note: the read works fine because "escaped" uppercase letters have no particular meaning in python 2 (\U and \N have a meaning in python 3 so it wouldn't have worked)

And if there's an error, the program now complains loudly.

As Jean suggested, the error is due to the \ being interpreted as an escape sequence. It is hence always safer to use os.path.join() as it is more cross platform and you need not worry about the escape sequence problem. For instance, in your case, you further need not worry about the first few arguments, as that is your home directory

import os
cv2.imwrite(os.path.join(os.path.expanduser('~'),'Desktop','tropical_image_sig5.bmp'), img2)

os.path.expanduser('~') will directly return your home directory.