Flips every item in a PNG spritesheet horizontally, maintaining position

flip.js

#!/usr/bin/env node

/**
 * Flips each item in a spritesheet horizontally, maintaining position
 * @author Thomas Hunter II <me@thomashunter.name>
 * 
 * 1) Execute `npm install lwip async`
 * 2) Delete the following conditional in the lwip node_module:
 * https://github.com/EyalAr/lwip/blob/1138482318e79ee6c69706b544ed5d6ffe11391c/lib/ImagePrototypeInit.js#L467
 */

var lwip = require('lwip');
var async = require('async');

// TODO: These should be CLI arguments as well
var W = 16;
var H = 16;
var FLIP = 'x'; // x, y, xy

var filename = process.argv[2];
var output = process.argv[3];

var ops = [];

if (!filename || !output) {
  console.log('USAGE: flip.js old.png new.png');
  process.exit(-1);
}

lwip.open(filename, function(err, source) {
  if (err) throw err;

  var WIDTH = source.width();
  var HEIGHT = source.height();

  var PER_ROW = WIDTH / W;
  var ROWS = HEIGHT / H;

  console.log('width', WIDTH, 'height', HEIGHT);
  console.log('per row', PER_ROW, 'rows', ROWS);

  for (var y = 0; y < ROWS; y++) {
    for (var x = 0; x < PER_ROW; x++) {
      ops.push({
        t: y * H,
        r: (x + 1) * W,
        b: (y + 1) * H,
        l: x * W
      });
    }
  }

  var final_destination;

  lwip.create(WIDTH, HEIGHT, function(err, first_dest) {
    if (err) { return done(err); }

    final_destination = first_dest;

    async.mapSeries(ops, function(data, done) {
      console.log('PROCESSING...', data);

      source.extract(data.l, data.t, data.r, data.b, function(err, cropped) {
        if (err) { return done(err); }

        cropped.flip(FLIP, function(err, flipped) {
          if (err) { return done(err); }

          console.log('paste', data.l, data.t);
          console.log('dimen', final_destination.width());
          final_destination.paste(data.l, data.t, flipped, function(err, destination) {
            if (err) { return done(err); }

            final_destination = destination;

            done(null);
          });
        });
      });
    }, function(err) {
      if (err) { throw err; }

      final_destination.writeFile(output, function(err) {
        if (err) { throw err; }

        console.log('fin.');
      });
    });
  });
});