index.js

var queue = require('queue-async');
var glob = require('glob');
var cwise = require('cwise');
var ndarray = require('ndarray');
var getPixels = require('get-pixels');
var savePixels = require('save-pixels');
var q = queue(10);

['data/20130815_1658.png',
'data/20130815_1738.png',
'data/20130815_1718.png'
].forEach(function(f) {
    q.defer(getPixels, f);
});

var justGrey = cwise({
  args: ["array", "index", "array", "array", "array"],
  body: function grey(out, i, r, g, b) {
    if (i % 3 == 0) out = r
    else if (i % 3 == 1) out = g
    else if (i % 3 == 2) out = b
    else out = 255
  }
});

q.awaitAll(function(err, results) {
    var a = results[0],
        b = results[1],
        c = results[2];
    var in_shape = a.shape.slice(0);
    var result = ndarray(new Uint8Array(a.size), in_shape);
    justGrey(result, a.pick(undefined, undefined, 0), b.pick(undefined, undefined, 1), c.pick(undefined, undefined, 2));
    savePixels(result, 'png').pipe(process.stdout);
});