---
title: Manipulate image pixels in Python
layout: post
excerpt: I had a black and white image but I wanted a black image with the white bits transparent.

thumbnail: /assets/blog/manipulate_pixels.png
social_image: /assets/blog/manipulate_pixels.png
assets: 
alt: A screenshot of some python code showing the line "rgb_array = np.array(..." and then multiple that say "rgb_array = ...". To be honest this is kinda lazy but I wanted to have some kind of image for each blog post. You, oh reader of the alt text, are getting more out of me than someone who just looked at the picture. 
image_class: invertable

---

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.


<figure class="two-wide">
<img src="/assets/blog/micropython/simulated_display.png" class="invertable">
<img src="/assets/blog/micropython/simulated_display_transparent.png" class="invertable">
<figcaption>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.</figcaption>
</figure>

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:

```python
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}")
```