Pixel coordinates in green 
pixel grid (15x15), in grey

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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 
storage coordinates.

Array indices are generalized to floatingpoint
pixel coordinates.

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 finitesized 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
(I, J).

Additional FITS keywords:
none.
