static public function commandLineDefaults() {
		if ( ! defined( 'PB_CONVERT_COMMAND' ) ) {
			define( 'PB_CONVERT_COMMAND', '/usr/bin/convert' );
		}
		if ( ! defined( 'PB_GS_COMMAND' ) ) {
			define( 'PB_GS_COMMAND', '/usr/bin/gs' );
		}
		if ( ! defined( 'PB_PDFINFO_COMMAND' ) ) {
			define( 'PB_PDFINFO_COMMAND', '/usr/bin/pdfinfo' );
		}
		if ( ! defined( 'PB_PDFTOPPM_COMMAND' ) ) {
			define( 'PB_PDFTOPPM_COMMAND', '/usr/bin/pdftoppm' );
		}
		if ( ! defined( 'PB_PRINCE_COMMAND' ) ) {
			define( 'PB_PRINCE_COMMAND', '/usr/bin/prince' );
		}
	}