The Need for Bad Pixels  

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:

• In an image-processing algorithm, arithmetic problems, such as attempts to divide by zero, might prevent values being calculated for certain output pixels. These missing pixels could be designated as bad.
• In an interactive editing application, deleted pixel values might be denoted as bad, indicating that they no longer have a useful value.
• Bad pixels might be introduced into array components during the process of type conversion (e.g. using the routine NDF_STYPE - see §) if values are encountered which cannot be represented using the component's new type. The same situation can also arise if type conversion takes place implicitly when an array component is accessed by mapping and unmapping it (see §).
• If the shape of an NDF is altered (see §), then new pixels may be introduced by changes in its pixel-index bounds. Such pixels will usually be marked as bad, since they will never have had a value assigned to them.

Many other examples exist where pixel values cannot be meaningfully assigned, and the bad pixel concept is designed to handle this general problem.