Batch cropping images (black bar removal)

autocrop.py

from PIL import Image
import glob

for filepath in glob.iglob(r'*.png'):   
    im = Image.open(filepath)
    #print("Processing: ", filepath, " ", im.format, im.size, im.mode)
    box = im.convert('RGB').getbbox()

    #size of black bars in each axis
    x1 = box[0]
    x2 = im.size[0] - box[2]
    y1 = box[1]
    y2 = im.size[1] - box[3]

     #first, assert that the bars are symmetric!
    if x1 != x2 or y1 != y2:
        print("asymmetric borders, skipping...")
        continue

    if x1 == 0 and y1 == 0:
        print("no borders detected, skipping...")
        continue

    box = list(box) #cannot override elements otherwise

    #then make sure we only trim one axis, e.g. strong vignette
    #can cause detection of black bars on both sides
    
    if y1 > x1:        
        box[0] = 0               
        box[2] = im.size[0]     
    else:
        box[1] = 0              
        box[3] = im.size[1]

    cropped = im.crop(tuple(box))
    print(filepath, im.format, im.mode)
    print("cropping: ", im.size, "->", cropped.size)
    cropped.save(filepath)