Using multiprocessing to process images

Question

I created a for loop that would loop through a directory of images and resize every image and then saves it to another directory. The code works but I'm trying to parallelize the process to make it faster.

This is the resize function

import cv2 
import os

def resize_image(img):
    # get the name of the file
    name = os.path.basename(img)
    # read the image
    img = cv2.imread(img)
    # resize and save to new directory
    resize = cv2.resize(img, (700, 700)) 
    resized_image = cv2.imwrite("Resized/"+name, resize)

And here is the for loop that would loop through the images in the directory (takes around 700 seconds to resize all the images in the directory).

SOURCE_DIRECTORY = "Source/"
directory_list = os.listdir(SOURCE_DIRECTORY)

for source_file in directory_list:
    source_path = os.path.join(SOURCE_DIRECTORY, source_file) 
    if os.path.isfile(source_path):
        resize_image(source_path)

In an effort to parallelize the process I tried using concurrent.futures and map it to the resize function.

import concurrent.futures

SOURCE_DIRECTORY = "Source/"
directory_list = os.listdir(SOURCE_DIRECTORY)

with concurrent.futures.ProcessPoolExecutor() as executor: 
    executor.map(resize_image, directory_list)

But I instantly get this error.

BrokenProcessPool: A child process terminated abruptly, the process pool is not usable anymore

How can I parallelize the process of resizing the images. Any help would be appreciated.

Answer 1

Here is sample skeleton you can use to parallelize the task (using multiprocessing.Pool):

import os
from multiprocessing import Pool

import cv2


def resize_image(file_name):
    # get the name of the file
    name = os.path.basename(file_name)

    # just to be sure the file exists (skip if not necessary):
    if not os.path.exists(name):
        return f"{name} does not exist!"

    # read the image
    img = cv2.imread(img)
    # resize and save to new directory
    resize = cv2.resize(img, (700, 700))
    resized_image = cv2.imwrite("Resized/" + name, resize)

    return f"{name} resized."


if __name__ == "__main__":

    SOURCE_DIRECTORY = "Source/"
    directory_list = os.listdir(SOURCE_DIRECTORY)

    filelist = []
    for source_file in directory_list:
        source_path = os.path.join(SOURCE_DIRECTORY, source_file)
        if os.path.isfile(source_path):
            filelist.append(source_path)

    with Pool(4) as pool:  # 4 is number of processes we want to use
        for result in pool.imap_unordered(resize_image, filelist):
            print(result)