Tom Hodson - Manipulate image pixels in Python

The way I’ve implemented dark mode on this site is to mark some images with a class invertable that means they still look good inverted. And then in CSS I go ahead and invert them if you’re in dark mode. For other images I just dim them a bit.

Try switching back and forth between dark mode and light mode, the one on the right works, the one on the left gets and ugly black background.

However for some images like this black and white png it looks a bit weird when inverted because the background becomes hard black but my site’s background is a dark grey. So I wanted to make the white pixels transparent instead.

Anyway the point of this post is that I knew in terms of pixel values what I wanted to do but wasn’t sure how to do this in an image editors. So here’s the code for you and my future reference:

import sys

import numpy as np
from PIL import Image

if len(sys.argv) < 3:
    print("Usage: python convert_to_transparent.py <input_image_path> <output_image_path>")
    sys.exit(1)

input_path, output_path = sys.argv[1], sys.argv[2]

grey = np.array(Image.open(input_path).convert("L"))
alpha_channel = 255 - grey

rgba_array = np.zeros((grey.shape[0], grey.shape[1], 4), dtype=np.uint8)
rgba_array[..., 0] = 0  # Red channel
rgba_array[..., 1] = 0  # Green channel
rgba_array[..., 2] = 0  # Blue channel
rgba_array[..., 3] = alpha_channel  # Alpha channel

# Create a new image for the output
rgba_img = Image.fromarray(rgba_array, mode="RGBA")

# Save the result
rgba_img.save(output_path, "PNG")
print(f"Image saved to {output_path}")