Pixel coordinates in green -
pixel grid (15x15), in grey
Regular arrangement of image data values offers the possibility of
defining rules for computing the coordinates of each datum.
Image array data are addressed via integral array indices -
Array indices are generalized to floating-point
This generalization is based on the interpolability of
adjacent image data values.
If the data values are considered to be delta functions, then when
displayed as finite-sized pixels, it makes most sense to assign
integral pixel coordinates to the centre of the pixel.
Henceforth, we consider only two image axes, pixel coordinates
Additional FITS keywords: