Resize and place an image in a square with padding.

letterbox.py

#PIL method
from PIL import Image

def letterbox(image, size=299, bg=(0,0,0)):
    (w,h) = image.size
    ratio = size / max([w,h])
    image = image.resize((int(ratio*w),int(ratio*h)))
    background = Image.new('RGB', (size, size), bg)
    background.paste(image, [int((size-s)/2) for s in image.size])
    return background


#numpy method cv2 method
import cv2
import numpy as np

def letterbox_np(image, size=299., bg=0):
    (w, h) = image.shape[:2]
    ratio = size / max([w,h])
    resized = cv2.resize(image, (int(ratio*w),int(ratio*h)))
    pad_x, pad_y = [int(size-s) for s in resized.shape[:2]]
    return  np.pad(resized,
                ((int(pad_x/2),int(pad_x/2)+pad_x%2),(int(pad_y/2),int(pad_y/2)+pad_y%2),(0,0)),
                mode='constant', 
                constant_values=bg)