helper.js

var exec = require('child_process').exec

module.exports = (function (exec) {
  function generateCmd (options) {
    var filters = {
      blur : ' -channel RGBA -blur 0x',
      gaussian : ' -filter Gaussian -define filter:sigma=',
      sharpen : ' -sharpen 0x',
      unsharp : ' -unsharp 0x',
      threshold : ' -threshold ',
      oilpaint : ' -paint ',
      sketch : ' -sketch ',
      metal : ' -colorspace Gray -emboss 0x',
      edge : ' -negate -colorspace Gray -edge '
    }

    return  "convert " + options.image
        + " " + filters[options.filter] + (options.level || 5)
        + " -resize " + (options.size || 100) + "%"
        + " " + ( options.to || options.image );
  }
  
  function executeCommand (cmd, callback) {
    exec(cmd, function (error) {
      callback(error);
    });
  }
  
  return {
    applyEffect : function (effect, options, callback) {
      options.filter = effect;
      var cmd = generateCmd(options);
      executeCommand(cmd, callback);
    }
  }
}) (exec);