docs: function (pngFile, outputFile) {
          var command = format('%s/advpng/bin/advpng -z -4 -f "%s"', pwd, outputFile.name);
  
          // Before using of 'advpng' a raw file has to be copied
          // This algorithm can not compress a file and write a result to another file
          return qfs.copy(pngFile.name, outputFile.name)
              .then(function () {
                  return _compress(command, outputFile);
              });
      }