Convert images to black or white (No grey)

Using NodeJS (>18), you can achieve this easily.

Using the package named sharp.

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Setup

npm init -y

npm install sharp

Images that are already grey-ish

Create an index.js file.

const sharp = require("sharp");

// Works with .jpg and .png files (tested only these two.)
const src = "image.png";

(async () => {
  await sharp(src)
    .greyscale()
    .linear(1, 0)
    .png({ colors: 2 })
    .toFile(`bw-${src}`);
})();

Change the src variable to your image, and run the script using : node index.js

The converted image will be prefixed with bw-

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You can tweak the linear value to change the amount of white vs black pixels. (I tested between 1 to 6 and having pretty good results)

I tested that solution with images that are mostly already black and white.

Images with colors

index.js

(async () => {
  await sharp(src)
    .gamma(1)
    .greyscale()
    .linear(2.5, 0)
    .png({ colors: 2 })
    .toFile(`bw-${src}`);
})();

This code is preferable when your image have more colors (more than grey-ish colors),

You need to tweak the gamma (between 1 to 3), the linear value (between 1 to 10) to get good results.

The dithering effect is quite nice. If you do not want the dithering effect at all (yes you will loose details), you can try that snippet:

index.js

(async () => {
  await sharp(src)
    .sharpen()
    .greyscale()
    .gamma(1)
    .linear(1, 0)
    .png({ colors: 2 })
    .median(3)
    .threshold(128)
    .toFile(`bw-${src}`);
})();

Yet again, you have to tweak the gamma, linear, median and threshold values

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You can also try to reorder the manipulation, but so far it was the best output I had...

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If you have a bunch of images to process you can create a loop and go through all of your images quite easily and quickly !
You can also create arrays for each parameters and loop for values, this way you can generate alternate output automatically.

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