next up previous
Next: Restrictions on Mapped Access to Sections
Up: NDF SECTIONS
Previous: Changing Dimensionality

Restrictions on the Use of Sections   

In general, an identifier for an NDF section can be passed without error to any routine which will accept an equivalent identifier referring to a base NDF. In certain cases, however, the behaviour of the routine may differ slightly when an NDF section is supplied in order to adhere to two guiding principles:

1.
It should be straightforward to write applications which process NDFs without having to know whether the NDFs concerned are base NDFs or NDF sections. Having written such an application, it should be able to process either without modification.
2.
Applications which access NDF sections should not generally cause changes to the values of NDF pixels which lie outside the pixel-index bounds (or more accurately the transfer window) of the sections they are processing.

The set of operations affected by these principles is rather small because most NDF components (e.g. label, units, title, history and extensions) are regarded as global and are equally accessible via identifiers referring to NDF sections and base NDFs. It is mainly operations on array components which behave differently when applied to NDF sections, and notably those which affect the attributes of these components. For instance, the numeric type of an NDF array component cannot be changed using NDF_STYPE[*]) via a section identifier; instead, this routine will simply return without action. Neither may an NDF be deleted (§[*]) via a section identifier.

These, and other differences, are noted in the appropriate routine descriptions in Appendix [*] and at other relevant points in this document.



next up previous
Next: Restrictions on Mapped Access to Sections
Up: NDF SECTIONS
Previous: Changing Dimensionality


Starlink User Note 33
R.F. Warren-Smith
11th January 2000
E-mail:rfws@star.rl.ac.uk

Copyright © 2000 Council for the Central Laboratory of the Research Councils