Skew linear coordinates in yellow -
pixel coordinates in grey
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The 1981 prescription of linear coordinates reserved keyword
CROTAj to define a rotation,
but usage was not formally defined.
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General linear transformation
matrix introduced in 1988
(Hanisch &
Wells):
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Form without scales (CDij
matrix):
X = X0 +
c11 (I - I0) +
c12 (J - J0),
Y = Y0 +
c21 (I - I0) +
c22 (J - J0).
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Form which retains scales
(PCij matrix):
X = X0 + dX [
p11 (I - I0) +
p12 (J - J0) ],
Y = Y0 + dY [
p21 (I - I0) +
p22 (J - J0) ].
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Allows general affine transformations to be defined -
scale, rotation, skew.
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Additional FITS keywords:
CDij (cij),
PCij (pij).
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