使用 PIL.resize 调整 JPG 大小会得到一个完全黑色的图像

Question

I'm using PIL to resize a JPG. I'm expecting the same image, resized as output, but instead I get a correctly sized black box. The new image file is completely devoid of any information, just an empty file. Here is an excerpt for my script:

basewidth = 300
img = Image.open(path_to_image)
wpercent = (basewidth/float(img.size[0]))
hsize = int((float(img.size[1])*float(wpercent)))
img = img.resize((basewidth,hsize))
img.save(dir + "/the_image.jpg")

I've tried resizing with Image.LANCZOS as the second argument, (defaults to Image.NEAREST with 1 argument), but it didn't make a difference. I'm running Python3 on Ubunutu 16.04. Any ideas on why the image file is empty?

Answer 1

I also encountered the same issue when trying to resize an image with transparent background. The "resize" works after I add a white background to the image.

Code to add a white background then resize the image:

from PIL import Image

im = Image.open("path/to/img")

if im.mode == 'RGBA':
    alpha = im.split()[3]
    bgmask = alpha.point(lambda x: 255-x)
    im = im.convert('RGB')
    im.paste((255,255,255), None, bgmask)

im = im.resize((new_width, new_height), Image.ANTIALIAS)

ref:

1. Other's code for making thumbnail

2. Python: Image resizing: keep proportion - add white background

Answer 2

The simplest way to get to the bottom of this is to post your image! Failing that, we can check the various aspects of your image.

So, import Numpy and PIL, open your image and convert it to a Numpy ndarray , you can then inspect its characteristics:

import numpy as np
from PIL import Image

# Open image
img = Image.open('unhappy.jpg')

# Convert to Numpy Array
n = np.array(img) 

Now you can print and inspect the following things:

n.shape       # we are expecting something like (1580, 1725, 3)

n.dtype       # we expect dtype('uint8')

n.max()       # if there's white in the image, we expect 255

n.min()       # if there's black in the image, we expect 0

n.mean()      # we expect some value between 50-200 for most images