A printer includes a processor that operates in accordance with firmware containing instructions to check page description data for compliance with a predetermined format. In particular, the firmware contains instructions to check that the page description data contains a predetermined signature. In a preferred embodiment the processor is also programmed to check that black and contone layer scale factors embedded in the page description data meet constraints dictated by page width and page height parameters. In the event that the constraints are not met then the processor clips the black and contone layers appropriately.