Replaces black backgrounds with transparent ones. Essentially scales the transparency with the respective brightness of the each pixel.

BlackgroundRemover.kt

import java.awt.Color
import java.awt.image.BufferedImage
import java.io.File
import javax.imageio.ImageIO
import kotlin.system.exitProcess


fun main(args: Array<String>) {
    var inf: File? = null
    var outf: File? = null
    var doinvpremul = false
    var i = 0
    while (i in args.indices) {
        val arg = args[i]
        val hasNext = i + 1 in args.indices
        if (arg == "--inverse-premultiplication") {
            doinvpremul = true
        } else if (arg == "--in" && hasNext) {
            inf = File(args[++i])
        } else if (arg == "--out" && hasNext) {
            outf = File(args[++i])
        }
        i += 1
    }
    if (inf == null || outf == null) {
        println("Usage: --in <infile> --out <outfile> [--inverse-premultiplication]")
        exitProcess(1)
    }

    val img = ImageIO.read((inf))
    val target = BufferedImage(img.width, img.height, BufferedImage.TYPE_INT_ARGB)
    for (x in 0 until img.width) {
        for (y in 0 until img.height) {
            val rgb = Color(img.getRGB(x, y))
            val hsl = Color.RGBtoHSB(rgb.red, rgb.green, rgb.blue, null)
            val lum = hsl[2]
            fun invpremul(x: Int) = if (doinvpremul) (x / lum).toInt().coerceAtMost(255) else x
            target.setRGB(
                x, y,
                (invpremul(rgb.red) shl 16)
                        or (invpremul(rgb.blue) shl 0)
                        or (invpremul(rgb.green) shl 8)
                        or ((lum * 255).toInt() shl 24)
            )
        }
    }
    ImageIO.write(target, "PNG", (outf))
}