The values of axis components and their attributes may be obtained
(i.e. read) freely via identifiers which refer to NDF sections (see
§).
In fact, in the case of axis character components there is no
difference between using NDF sections and base NDFs for this purpose.
With axis arrays, however, it is necessary that the appropriate
part of each array be selected so that it correctly matches the NDF pixels
to which the section refers.
This operation is performed automatically by the NDF_ system.
In contrast, the writing or modification of axis component values
and attributes has to be handled very differently when NDF sections are
involved, in order to adhere to the principles described in
§.
Accordingly, the following major restriction is placed on such operations:
No changes to axis component values or attributes may be made via an NDF section
This restriction is necessary so that an application which is applied to an NDF section is prevented from modifying axis values which may affect the interpretation of NDF pixels lying outside the section in question.
Applications should, nevertheless, still be able to operate on NDF sections with the possible limitation that axis modifications may be lost. To allow this, the NDF_ system permits attempts to modify axis values or attributes via NDF sections to proceed without error. However, all the relevant routines will simply return without action under these circumstances, so the attempted changes to the axis components will be disregarded, leaving the components unaffected.