An important facility provided by the NDF_ system is the ability to select a region from an NDF which differs in shape from the original NDF, and to process it as if it were a complete NDF itself. Such regions are termed NDF sections. As a simple example, consider a routine which plots a contour map of an NDF's data component. By accessing an appropriate NDF section, the same routine could also be used to contour only a subset of the data without having to make any change to the routine itself.
This ability to concentrate on a subset of an NDF can clearly improve
efficiency in many circumstances, but NDF sections are also capable of
referring to super-sets of NDFs; i.e. they may extend beyond the
bounds of the NDF from which they are derived.
This gives them a number of further uses through their ability to match the
shapes of NDFs of otherwise unequal extent by effectively trimming or
padding them with bad pixels.
Their use for this type of operation is discussed more fully in