从路径列表中读取文件

Question

I have downloaded a dataset for a deep learning project that contains images (.png) and the corresponding label for each image (.txt). I have all the images' paths in a list x . I want to iterate through these paths, preprocess the images using cv2 , and append the new images to a new list images_data . However, every time I try to loop through it, I keep getting this same error: cv2.error: OpenCV(4.1.1) /io/opencv/modules/imgproc/src/color.cpp:182: error: (-215:Assertion failed)._src.empty() in function 'cvtColor'

Even when I comment out that line of code that throws the error, I get another error when trying to resize the image.

This is the for loop I'm using to iterate through the list:

images_data = []
    for file in x:
        img = cv2.imread(file)
        img = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
        img = cv2.resize(img, (80, 80))
        images_data.append(img)

My x list looks pretty much like this: x = [car1.png, car2.png, car3.png, car4.png, car5.png]

How can I solve this error?

Answer 1

That error indicates your img variable is empty so cv2.imread(file) did not read an image. You can check this after reading the image and before converting the color or resizing, with a simple if case:

if img is None: 
    print('Error reading image')
else:
    img = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

or check if the file exists using the os module:

img = cv2.imread(file)
if os.path.isfile(file): 
    img = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

os.path.isfile(file) will return True if the file exists.

I am assuming your files, car1.png, car2.png etc are in a different folder and not in the same path with this script in which case, you need to append the directory of the images to the file name before you can read it. So for example, if your images are in, let's say, '/home/Deep_Learning/Car_Images/' then when reading the images, the variable file must contain the string: /home/Deep_Learning/Car_Images/car1.png , for the first image and not just car1.png . You can do this using python's os module like this:

import cv2
import os

# Directory of the images. Change it according your path
data_dir = '/home/Deep_Learning/Car_Images/' 

images_data = []
for file in x:
    file_name = os.path.join(data_dir, file) # Append the image folder with the image name

    if os.path.isfile(file_name) is False: #Check if image file exists
        print("Image file ", file_name, "does not exist")
    else:
        img = cv2.imread(file_name)
        img = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
        img = cv2.resize(img, (80, 80))
        images_data.append(img)