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The method by which NDF axis arrays are processed should also
reflect their meanings, as defined above.
By way of illustration, suppose that a transformation of axis values
is to be performed, so that each axis coordinate is converted to a
new value by means of some non-linear function.
Rather than simply applying this function to calculate new pixel
centre locations from the old ones, the correct procedure is to
transform
the pixel edge locations and to derive new centre positions from
these, as follows:
- 1.
- Obtain the relevant pixel centre and width arrays,
accepting their default values if necessary.
- 2.
- From these, calculate the positions of the edges of each pixel.
- 3.
- Transform the edge positions using the non-linear transformation
function.
- 4.
- Calculate new pixel centre positions (mid-way between the new
edge positions) and store them in the NDF's axis centre array.
- 5.
- Calculate associated pixel width values (from the difference
in the pixel edge positions).
Since the pixel centres will now be non-uniformly separated, these
new width values should also be stored in the NDF's axis
width array.
- 6.
- If axis variance array values are available, then these should
be propagated through the transformation function using the usual
error-propagation formulae.
This procedure is necessary to ensure that the pixels remain contiguous
(or non-contiguous, if appropriate) and that the new centre
positions lie mid-way between the new pixel edge locations.
Furthermore, the operations above can all be reversed if necessary to
recover the original axis array values.
Next: Axis Normalisation
Up: THE AXIS COORDINATE SYSTEM
Previous: Contiguous and Non-Contiguous Pixels
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