Simple linear coordinates in yellow 
pixel coordinates in grey

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The first coordinate rules for FITS date from 1981 (Wells,
Greisen, & Harten).

Simple linear world
coordinates:

Basic mathematical requirement is ...
X = X_{0} + d_{X} I,
Y = Y_{0} + d_{Y} J,

... but the actual implementation was
X = X_{0} +
d_{X} (I  I_{0}),
Y = Y_{0} +
d_{Y} (J  J_{0}).

While not strictly necessary in a mathematical sense, introduction
of the reference point
(I_{0}, J_{0}) as a
convenience was to prove important later.

Can only handle separable, linear systems, i.e.
X = X(I),
Y = Y(J).

Additional FITS keywords:
CRPIXj (I_{0}),
CDELTi (d_{X}),
CRVALi (X_{0}).
