let fs = require("fs"),
  PNG = require("../lib/png").PNG; // note require('pngjs') outside this project

fs.createReadStream("test/in/basi0g01.png")
  .pipe(new PNG({}))
  .on("parsed", function () {
    for (let y = 0; y < this.height; y++) {
      for (let x = 0; x < this.width; x++) {
        let idx = (this.width * y + x) << 2;

        // invert color
        this.data[idx] = 255 - this.data[idx];
        this.data[idx + 1] = 255 - this.data[idx + 1];
        this.data[idx + 2] = 255 - this.data[idx + 2];

        // and reduce opacity
        this.data[idx + 3] = this.data[idx + 3] >> 1;
      }
    }

    this.pack().pipe(fs.createWriteStream("out.png"));
  });