Auto-crop jpeg image without recompressing (requires Python 3 with scikit-image, and jpegtran)

autocropjpg.py

#!/usr/bin/env python

import subprocess
import sys
from skimage import io, filters
import numpy as np

filename = sys.argv[1]
base, ext = filename.rsplit('.', 1)
outfile = f'{base} cropped.{ext}'

if not (filename.endswith('.jpg') or filename.endswith('.jpeg')):
    print("Can only losslessly crop JPEG images")

image = io.imread(filename).sum(axis=2)
coords = np.array(np.nonzero(image != image.max())).T
top_left = coords.min(axis=0)
bottom_right = coords.max(axis=0)

height = bottom_right[0] - top_left[0]
width = bottom_right[1] - top_left[1]
y, x = top_left

subprocess.run([
    'jpegtran', '-outfile', outfile,
    '-crop', f'{width}x{height}+{x}+{y}',
    filename
])