This script will resize to an intermediate size and pad an image to desired square size while keeping its aspect ratio unchanged.

resize_and_pad_image_to_square.py

import cv2
import numpy as np


def intermediate_scaling(img, intermediate_size):
    (rows, cols) = img.shape
    image_size = max(rows, cols)
    # Rescale to intermediate_size pixels so that image size resembles average size of dataset item.
    # For example a sample character is resized to average size of dataset
    scaling_factor = intermediate_size / image_size
    rows = int(scaling_factor * rows)
    cols = int(scaling_factor * cols)
    img = cv2.resize(img, (cols, rows), interpolation=cv2.INTER_CUBIC)
    return img


# Set your desired size and intermediate sizes
desired_size = 512
intermediate_size = 400

# Change the image path to a valid one before running
im_pth = "image.jpg"
im = cv2.imread(im_pth)
im = intermediate_scaling(im, intermediate_size)
(rows, cols) = im.shape[:2] # shape of im is in (height, width) format

# Padding
top = bottom = int((desired_size - rows)/2)
left = right = int((desired_size - cols)/2)

# color = [0, 0, 0] for black and color = [255, 255, 255] for white
color = [0, 0, 0]
new_im = cv2.copyMakeBorder(im, top, bottom, left, right, cv2.BORDER_CONSTANT,
    value=color)

cv2.imwrite('resized.jpg', new_im)
cv2.imshow("resized", new_im)
cv2.waitKey(0)
cv2.destroyAllWindows()