The NDF_ system provides support for the concept of bad pixels (sometimes called ``magic values'' or ``flagged values'') which may be present in the array components of an NDF. Bad pixels are pixels whose actual value may be unknown, and which have therefore been assigned a special ``null'' or bad value. They can arise in a wide variety of ways, for instance:
Many other examples exist where pixel values cannot be meaningfully assigned, and the bad pixel concept is designed to handle this general problem.