An important feature of each axis array is a set of default values
which serve to define the axis coordinate system in the absence of
complete information.
In the simplest case (i.e. no information), this reduces to the pixel
coordinate system discussed in §.
The following describes how default values are obtained for each axis array:
2.5, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5, 9.5
This means that the default pixel widths match the local average
spacing between pixel centres, which is usually appropriate.
Note, however, that this does not guarantee that the pixels will be
contiguous (i.e. that their edges will meet exactly) except in cases
where the pixel centres are uniformly spaced (see
§).