Overview of an NDF's Axis Components  

Information about an NDF's axis coordinate system is stored in its axis components, which are conveniently categorised as follows:

Axis character components: LABEL -- Axis labels

UNITS -- Axis units

Axis array components: CENTRE -- Pixel centre coordinates

WIDTH -- Pixel width values

VARIANCE -- Variance estimates for pixel positions

As with the main components of an NDF, the names of these axis components are significant, since they are used by the NDF_ routines to identify the component(s) to which certain operations should be applied. Axis component names are specified in the same way as those of the main components of an NDF, including the use of abbreviations, mixed case and comma-separated lists where appropriate (see § for details).

Access to an axis component must also specify the number of the NDF axis to be used. This is normally an integer lying between 1 and the number of NDF dimensions, but many routines will also accept a value of zero, indicating that an operation is to be applied to all of an NDF's axes. This additional item of information means that a separate set of routines must be provided for accessing axis components. Nevertheless, many of the principles described in earlier sections for accessing other NDF components are also applicable here, so the descriptions given below are relatively brief. References to more complete descriptions are given where appropriate.

The following describes the purpose and interpretation of each axis component in slightly more detail.

Axis Character Components:

LABEL -

This is a character string, whose value is intended for general use for such things as labelling the axes of graphs or as a heading for columns in tabulated output; e.g. `Scanner X offset'. There is a separate axis label value for each NDF dimension.

UNITS -

This is a character string, whose value describes the physical units of the quantity measured along an NDF's axis; e.g. `micron'. There is a separate axis units value for each NDF dimension.

Axis Array Components:

CENTRE -

This is a 1-dimensional array which holds the coordinates of the pixel centres as described in §. The values in this array should either increase or decrease monotonically with position in the array. There is a separate 1-dimensional axis centre array for each dimension of an NDF, whose size matches the size of the corresponding NDF dimension.

WIDTH -

This is a 1-dimensional array which holds a set of non-negative width values for the NDF's pixels as described in §. There is a separate 1-dimensional axis width array for each dimension of an NDF, whose size matches the size of the corresponding NDF dimension. These width values should be such that no point can lie inside more than two NDF pixels simultaneously (i.e. although pixels are allowed to overlap with their neighbours, they may not overlap with more distant pixels).

VARIANCE -

This is a 1-dimensional array which holds a set of non-negative variance estimates representing any statistical uncertainty in the value of the corresponding pixel centre coordinate, as described in §. There is a separate 1-dimensional axis variance array for each dimension of an NDF, whose size matches the size of the corresponding NDF dimension.