code: protected function parseInput($input, array $options = array())
      {
          $matches = null;
          // search for @MyNamespace/path/to/asset
          if (preg_match("|^\@([a-z_][_a-z0-9]*)/|i", $input, $matches)) {
              $ns = $matches[1];
              if (!array_key_exists($ns, $this->namespaces)) {
                  throw new \RuntimeException("$ns : unknown namespace !");
              }
              $input = $this->namespaces[$ns] . substr($input, strlen($ns) + 1);
          }
          return parent::parseInput($input, $options);
      }
docs: Will detect inputs that begin with @MyNamespace/... and replace the namespace with the corresponding path.
  
  @see \Assetic\Factory\AssetFactory::parseInput()