protected function prepOutputDir()
    {
        if (!is_dir($this->output_dir . $this->prefix)) {
            $this->filesystem->mkdir($this->output_dir . $this->prefix);
        }
    }
protected function prepOutputDir()
    {
        if (!is_dir($this->output_dir . $this->prefix)) {
            $this->filesystem->mkdir($this->output_dir . $this->prefix);
        }
    }
protected function prepOutputDir ( ) { if ( ! is_dir ( $ this -> output_dir . $ this -> prefix ) ) { $ this -> filesystem -> mkdir ( $ this -> output_dir . $ this -> prefix ) ; } }
Prepare the output directory.

This makes sure we have somewhere to put the thumbnails once we've generated them.
Prepare the output directory .