Custom sprite multitask

Gruntfile.js

  var NUM_SPRITE_COLS = 6;
  var gm = require('gm');

  grunt.registerMultiTask('sprite', 'Sprite images', function() {
    var images = this.data;
    var numRows = Math.ceil(images.length / NUM_SPRITE_COLS);
    var sprite = gm('');
    sprite.tile(NUM_SPRITE_COLS + 'x' + numRows);
    sprite.quality('100');
    sprite.background('transparent');

    images.forEach(function(image) {
      sprite.geometry('+0+0 ', '');
      sprite.gravity('NorthEast');
      sprite._out.push(image);
    });

    sprite.subCommand('montage');

    var cb = this.async();
    // Write out the file asynchronously.
    sprite.write(this.target, function (err) {
      if (err) {
        console.error(err);
      }
      cb(err);
    });
  });