NodeJS util to swap opaque lightness for black transparency in a grayscale image. Useful for making stickman images on a white background transparent.

white-to-alpha.js

const { Jimp, intToRGBA  } = require("jimp");

function rgba_to_hex(r, g, b ,a) {
    return ((r*256+g)*256+b)*256+a;
}

/**
 * grayscale image with transparent background.
 * @param inputPath Path to the input image.
 * @param outputPath Path to write the output image to.
 * @param maxOpacity Number from 1-255. Pure black on the input image will be converted to RGBA of [blackLightness,blackLightness,blackLightness,maxOpacity]
 * @param blackLightness Number from 0 to 255. Pure black on the input image will be converted to RGBA of [blackLightness,blackLightness,blackLightness,maxOpacity]
 * @returns {Promise<void>}
 */
async function convert(inputPath, outputPath, maxOpacity = 255, blackLightness = 0){
    const image = await Jimp.read(inputPath);
    image.greyscale();
    const output = new Jimp({ width: image.width, height: image.height, color: 0xff00ff88 });
    for (let x = 0; x < image.width; x++) {
        for (let y = 0; y < image.height; y++) {
            let pixelColor = image.getPixelColor(x, y);
            let c = intToRGBA(pixelColor);
            let alpha = Math.floor((255 - c.r)/(256-maxOpacity));
            let newColor = rgba_to_hex(blackLightness, blackLightness, blackLightness, alpha);
            output.setPixelColor(newColor, x, y);
        }
    }
    await output.write(outputPath);
}

const args = process.argv.slice(2);
convert(args[0], args[1], parseInt(args[2] || '255'), parseInt(args[3] || '0')).then(r => console.log('Done!'));