protected function outputUri( $mtime )
    {
        return Thumbnail::THUMBNAIL_BASEURI
             . '/' . $this->path .
               '/' . $this->method .
               '/' . ( empty( $this->crop ) ? '' :
                         $this->cropLeft . 'x' .
                         $this->cropTop . '-' .
                         $this->cropWidth . 'x' .
                         $this->cropHeight . '-' ) .
                     $this->width .
               'x' . $this->height .
               '/' . $this->bgColor .
               '/' . ( empty( $this->filters ) ? 'none' :
                         implode( '-', (array) $this->filters ) ) .
               '/' . $mtime .
               '/' . $this->file;
    }