A control point: a position in the source image and where it belongs in the target.
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Source image coordinates (pixel (i, j) has its center at (i + 0.5, j + 0.5)).
(i, j)
(i + 0.5, j + 0.5)
Target coordinates (map coordinates, or pixels of the corrected image).
A control point: a position in the source image and where it belongs in the target.