CV2 Image Error: error: (-215:Assertion failed) !ssize.empty() in function 'cv::resize'

Question

I'm making an image classifier with TensorFlow and Python, but I'm having an error reading images with CV2. I'm very new to CV2, and I haven't been able to find anything that sufficiently solves my problem. Can anyone explain how to solve this?

def train_data_with_label():
  train_images = []
  for i in tqdm(os.listdir(train_data)):
    path = os.path.join(train_data, i)
    img = cv2.imread(path, 3)
    img = cv2.resize(img, (64,64))
    train_images.append([np.array(img), one_hot_label(i)])
  shuffle(train_images)
  return train_images
  
def test_data_with_label():
  test_images = []
  for i in tqdm(os.listdir(test_data)):
    path = os.path.join(test_data, i)
    img = cv2.imread(path, 3)
    img = cv2.resize(img, (64,64))
    test_images.append([np.array(img), one_hot_label(i)])
  shuffle(test_images)
  return test_images

This is the error I got:

Using TensorFlow backend.
  0%|                                                                                            | 0/2 [00:00<?, ?it/s]
Traceback (most recent call last):
  File "retrain.py", line 47, in <module>
    training_images = train_data_with_label()
  File "retrain.py", line 32, in train_data_with_label
    img = cv2.resize(img, (64,64))
cv2.error: OpenCV(4.1.1) C:\projects\opencv-python\opencv\modules\imgproc\src\resize.cpp:3720: error: (-215:Assertion failed) !ssize.empty() in function 'cv::resize'

Answer 1

Your imread() call failed. It returned None.

When you try to imread() a file that doesn't exist, or it's not a proper picture file, imread signals that by returning None (in Python), instead of a proper numpy array.

Now, when you pass this None to resize(), resize() notices and throws this error. It tries to check if the argument is a proper array, and that it's not an empty array. In this case, your argument isn't even an array, but the error is the same.

How to fix this: check if the result of imread() is None. If it is None, reading failed. Just continue the loop on to the next picture file name.

img = cv2.imread(path)
if img is None:
    continue

by the way: the 3 in the second argument to imread means IMREAD_COLOR | IMREAD_ANYDEPTH

Answer 2

check if there _DS_Store file in the folder you're trying to iterate in the for loop, as it is not a image file cv2.resize() is not able to resize it.