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When using NDF sections, the possibility also exists of reading axis
array values from sections which refer to super-sets of the associated base
NDF (§
).
Although the section's axis arrays may be in a defined state (values
having previously been assigned via the base NDF), parts of them will still
lie outside the bounds of the base NDF, and so cannot have any values.
In such cases, axis arrays must be extrapolated in order to
return useful values.
This process uses a set of axis extrapolation rules which are an
extension of the rules normally used to generate the default axis
values (§
), as follows:
- Centre:
- Axis centre arrays are linearly extrapolated from the
centre values of the two nearest base-NDF pixels on the same axis.
If only one of these pixels exists, then the extrapolation assumes unit
spacing between pixel centres.
- Width:
- If an axis width array is in a defined state, it is
extrapolated by duplicating the nearest base-NDF width value on the
same axis.
If it is in an undefined state, its new values are derived from the
corresponding extrapolated centre values by employing the usual
method for generating default axis width values
(§
)--this normally gives rise to width
values which match the pixel centre spacing of the two nearest
base-NDF pixels on the same axis.
- Variance:
- Axis variance arrays are always extrapolated by
padding with the value zero.
There is also the possibility that an NDF section may have more dimensions
than its associated base NDF (see §
), in
which case associated axis components will also exist.
These ``virtual'' axis components may be accessed in the same way as
those associated with other dimensions, but they are always regarded as
being in an undefined state.
Default values will be supplied for them, if necessary, using the normal
defaulting rules (see §
), but any new values assigned
will simply be discarded.
Next: CONNECTING WITH THE DATA SYSTEM
Up: AXIS COMPONENTS
Previous: Accessing Axis Components via NDF Sections
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